Android ListView工作原理完全解析，带你从源码的角度彻底理解

在Android所有常用的原生控件当中，用法最复杂的应该就是ListView了，它专门用于处理那种内容元素很多，手机屏幕无法展示出所有内容的情况。ListView可以使用列表的形式来展示内容，超出屏幕部分的内容只需要通过手指滑动就可以移动到屏幕内了。

另外ListView还有一个非常神奇的功能，我相信大家应该都体验过，即使在ListView中加载非常非常多的数据，比如达到成百上千条甚至更多，ListView都不会发生OOM或者崩溃，而且随着我们手指滑动来浏览更多数据时，程序所占用的内存竟然都不会跟着增长。那么ListView是怎么实现这么神奇的功能的呢？

首先我们先来看一下ListView的继承结构，如下图所示：

可以看到，ListView的继承结构还是相当复杂的，它是直接继承自的AbsListView，而AbsListView有两个子实现类，一个是ListView，另一个就是GridView，因此我们从这一点就可以猜出来，ListView和GridView在工作原理和实现上都是有很多共同点的。然后AbsListView又继承自AdapterView，AdapterView继承自ViewGroup，后面就是我们所熟知的了。先把ListView的继承结构了解一下，待会儿有助于我们更加清晰地分析代码。

Adapter的作用

Adapter相信大家都不会陌生，我们平时使用ListView的时候一定都会用到它。那么话说回来大家有没有仔细想过，为什么需要Adapter这个东西呢？总感觉正因为有了Adapter，ListView的使用变得要比其它控件复杂得多。那么这里我们就先来学习一下Adapter到底起到了什么样的一个作用。

其实说到底，控件就是为了交互和展示数据用的，只不过ListView更加特殊，它是为了展示很多很多数据用的，但是ListView只承担交互和展示工作而已，至于这些数据来自哪里，ListView是不关心的。因此，我们能设想到的最基本的ListView工作模式就是要有一个ListView控件和一个数据源。

不过如果真的让ListView和数据源直接打交道的话，那ListView所要做的适配工作就非常繁杂了。因为数据源这个概念太模糊了，我们只知道它包含了很多数据而已，至于这个数据源到底是什么样类型，并没有严格的定义，有可能是数组，也有可能是集合，甚至有可能是数据库表中查询出来的游标。所以说如果ListView真的去为每一种数据源都进行适配操作的话，一是扩展性会比较差，内置了几种适配就只有几种适配，不能动态进行添加。二是超出了它本身应该负责的工作范围，不再是仅仅承担交互和展示工作就可以了，这样ListView就会变得比较臃肿。

那么显然Android开发团队是不会允许这种事情发生的，于是就有了Adapter这样一个机制的出现。顾名思义，Adapter是适配器的意思，它在ListView和数据源之间起到了一个桥梁的作用，ListView并不会直接和数据源打交道，而是会借助Adapter这个桥梁来去访问真正的数据源，与之前不同的是，Adapter的接口都是统一的，因此ListView不用再去担心任何适配方面的问题。而Adapter又是一个接口(interface)，它可以去实现各种各样的子类，每个子类都能通过自己的逻辑来去完成特定的功能，以及与特定数据源的适配操作，比如说ArrayAdapter可以用于数组和List类型的数据源适配，SimpleCursorAdapter可以用于游标类型的数据源适配，这样就非常巧妙地把数据源适配困难的问题解决掉了，并且还拥有相当不错的扩展性。简单的原理示意图如下所示：

当然Adapter的作用不仅仅只有数据源适配这一点，还有一个非常非常重要的方法也需要我们在Adapter当中去重写，就是getView()方法，这个在下面的文章中还会详细讲到。

RecycleBin机制

那么在开始分析ListView的源码之前，还有一个东西是我们提前需要了解的，就是RecycleBin机制，这个机制也是ListView能够实现成百上千条数据都不会OOM最重要的一个原因。其实RecycleBin的代码并不多，只有300行左右，它是写在AbsListView中的一个内部类，所以所有继承自AbsListView的子类，也就是ListView和GridView，都可以使用这个机制。那我们来看一下RecycleBin中的主要代码，如下所示：

/** * The RecycleBin facilitates reuse of views across layouts. The RecycleBin * has two levels of storage: ActiveViews and ScrapViews. ActiveViews are * those views which were onscreen at the start of a layout. By * construction, they are displaying current information. At the end of * layout, all views in ActiveViews are demoted to ScrapViews. ScrapViews * are old views that could potentially be used by the adapter to avoid * allocating views unnecessarily. *  * @see android.widget.AbsListView#setRecyclerListener(android.widget.AbsListView.RecyclerListener) * @see android.widget.AbsListView.RecyclerListener */class RecycleBin {    private RecyclerListener mRecyclerListener;    /**     * The position of the first view stored in mActiveViews.     */    private int mFirstActivePosition;    /**     * Views that were on screen at the start of layout. This array is     * populated at the start of layout, and at the end of layout all view     * in mActiveViews are moved to mScrapViews. Views in mActiveViews     * represent a contiguous range of Views, with position of the first     * view store in mFirstActivePosition.     */     //存储View    private View[] mActiveViews = new View[0];    /**     * Unsorted views that can be used by the adapter as a convert view.     */     //mScrapViews和mCurrentScrap这两个List来存储废弃View    private ArrayList<View>[] mScrapViews;    private int mViewTypeCount;    //mScrapViews和mCurrentScrap这两个List来存储废弃View    private ArrayList<View> mCurrentScrap;    /**     * Fill ActiveViews with all of the children of the AbsListView.     *      * @param childCount     *            The minimum number of views mActiveViews should hold     * @param firstActivePosition     *            The position of the first view that will be stored in     *            mActiveViews     */     //第一个参数表示要存储的view的数量，第二个参数表示ListView中第一个可见元素的position值    void fillActiveViews(int childCount, int firstActivePosition) {        if (mActiveViews.length < childCount) {            mActiveViews = new View[childCount];        }        mFirstActivePosition = firstActivePosition;        final View[] activeViews = mActiveViews;        for (int i = 0; i < childCount; i++) {            View child = getChildAt(i);            AbsListView.LayoutParams lp = (AbsListView.LayoutParams) child.getLayoutParams();            // Don't put header or footer views into the scrap heap            if (lp != null && lp.viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {                // Note: We do place AdapterView.ITEM_VIEW_TYPE_IGNORE in                // active views.                // However, we will NOT place them into scrap views.                activeViews[i] = child;            }        }    }    /**     * Get the view corresponding to the specified position. The view will     * be removed from mActiveViews if it is found.     *      * @param position     *            The position to look up in mActiveViews     * @return The view if it is found, null otherwise     */     //用于从mActiveViews数组当中获取数据。该方法接收一个position参数，表示元素在ListView当中的位置，方法内部会自动将position值转换成mActiveViews数组对应的下标值。需要注意的是，mActiveViews当中所存储的View，一旦被获取了之后就会从mActiveViews当中移除，下次获取同样位置的View将会返回null，也就是说mActiveViews不能被重复利用。    View getActiveView(int position) {        int index = position - mFirstActivePosition;        final View[] activeViews = mActiveViews;        if (index >= 0 && index < activeViews.length) {            final View match = activeViews[index];            activeViews[index] = null;            return match;        }        return null;    }    /**     * Put a view into the ScapViews list. These views are unordered.     *      * @param scrap     *            The view to add     */     //用于将一个废弃的View进行缓存，该方法接收一个View参数，当有某个View确定要废弃掉的时候(比如滚动出了屏幕)，就应该调用这个方法来对View进行缓存，RecycleBin当中使用mScrapViews和mCurrentScrap这两个List来存储废弃View。    void addScrapView(View scrap) {        AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams();        if (lp == null) {            return;        }        // Don't put header or footer views or views that should be ignored        // into the scrap heap        int viewType = lp.viewType;        if (!shouldRecycleViewType(viewType)) {            if (viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {                removeDetachedView(scrap, false);            }            return;        }        if (mViewTypeCount == 1) {            dispatchFinishTemporaryDetach(scrap);            mCurrentScrap.add(scrap);        } else {            dispatchFinishTemporaryDetach(scrap);            mScrapViews[viewType].add(scrap);        }        if (mRecyclerListener != null) {            mRecyclerListener.onMovedToScrapHeap(scrap);        }    }    /**     * @return A view from the ScrapViews collection. These are unordered.     */     //getScrapView 用于从废弃缓存中取出一个View    View getScrapView(int position) {        ArrayList<View> scrapViews;        if (mViewTypeCount == 1) {            scrapViews = mCurrentScrap;            int size = scrapViews.size();            if (size > 0) {                return scrapViews.remove(size - 1);            } else {                return null;            }        } else {            int whichScrap = mAdapter.getItemViewType(position);            if (whichScrap >= 0 && whichScrap < mScrapViews.length) {                scrapViews = mScrapViews[whichScrap];                int size = scrapViews.size();                if (size > 0) {                    return scrapViews.remove(size - 1);                }            }        }        return null;    }    //为每种类型的数据项都单独启用一个RecycleBin缓存机制    public void setViewTypeCount(int viewTypeCount) {        if (viewTypeCount < 1) {            throw new IllegalArgumentException("Can't have a viewTypeCount < 1");        }        // noinspection unchecked        ArrayList<View>[] scrapViews = new ArrayList[viewTypeCount];        for (int i = 0; i < viewTypeCount; i++) {            scrapViews[i] = new ArrayList<View>();        }        mViewTypeCount = viewTypeCount;        mCurrentScrap = scrapViews[0];        mScrapViews = scrapViews;    }}

这里的RecycleBin代码并不全，我只是把最主要的几个方法提了出来。那么我们先来对这几个方法进行简单解读，这对后面分析ListView的工作原理将会有很大的帮助。

• fillActiveViews() 这个方法接收两个参数，第一个参数表示要存储的view的数量，第二个参数表示ListView中第一个可见元素的position值。RecycleBin当中使用mActiveViews这个数组来存储View，调用这个方法后就会根据传入的参数来将ListView中的指定元素存储到mActiveViews数组当中。

• getActiveView() 这个方法和fillActiveViews()是对应的，用于从mActiveViews数组当中获取数据。该方法接收一个position参数，表示元素在ListView当中的位置，方法内部会自动将position值转换成mActiveViews数组对应的下标值。需要注意的是，mActiveViews当中所存储的View，一旦被获取了之后就会从mActiveViews当中移除，下次获取同样位置的View将会返回null，也就是说mActiveViews不能被重复利用。

• addScrapView() 用于将一个废弃的View进行缓存，该方法接收一个View参数，当有某个View确定要废弃掉的时候(比如滚动出了屏幕)，就应该调用这个方法来对View进行缓存，RecycleBin当中使用mScrapViews和mCurrentScrap这两个List来存储废弃View。

• getScrapView 用于从废弃缓存中取出一个View，这些废弃缓存中的View是没有顺序可言的，因此getScrapView()方法中的算法也非常简单，就是直接从mCurrentScrap当中获取尾部的一个scrap view进行返回。

• setViewTypeCount() 我们都知道Adapter当中可以重写一个getViewTypeCount()来表示ListView中有几种类型的数据项，而setViewTypeCount()方法的作用就是为每种类型的数据项都单独启用一个RecycleBin缓存机制。实际上，getViewTypeCount()方法通常情况下使用的并不是很多，所以我们只要知道RecycleBin当中有这样一个功能就行了。

了解了RecycleBin中的主要方法以及它们的用处之后，下面就可以开始来分析ListView的工作原理了，这里我将还是按照以前分析源码的方式来进行，即跟着主线执行流程来逐步阅读并点到即止，不然的话要是把ListView所有的代码都贴出来，那么本篇文章将会很长很长了.

第一次Layout

不管怎么说，ListView即使再特殊最终还是继承自View的，因此它的执行流程还将会按照View的规则来执行。

View的执行流程无非就分为三步，onMeasure()用于测量View的大小，onLayout()用于确定View的布局，onDraw()用于将View绘制到界面上。而在ListView当中，onMeasure()并没有什么特殊的地方，因为它终归是一个View，占用的空间最多并且通常也就是整个屏幕。onDraw()在ListView当中也没有什么意义，因为ListView本身并不负责绘制，而是由ListView当中的子元素来进行绘制的。那么ListView大部分的神奇功能其实都是在onLayout()方法中进行的了，因此我们本篇文章也是主要分析的这个方法里的内容。

如果你到ListView源码中去找一找，你会发现ListView中是没有onLayout()这个方法的，这是因为这个方法是在ListView的父类AbsListView中实现的，代码如下所示：

/** * Subclasses should NOT override this method but {@link #layoutChildren()} * instead. */@Overrideprotected void onLayout(boolean changed, int l, int t, int r, int b) {    super.onLayout(changed, l, t, r, b);    mInLayout = true;    if (changed) {        int childCount = getChildCount();        for (int i = 0; i < childCount; i++) {            getChildAt(i).forceLayout();        }        mRecycler.markChildrenDirty();    }    layoutChildren();    mInLayout = false;}

可以看到，onLayout()方法中并没有做什么复杂的逻辑操作，主要就是一个判断，如果ListView的大小或者位置发生了变化，那么changed变量就会变成true，此时会要求所有的子布局都强制进行重绘。除此之外倒没有什么难理解的地方了，不过我们注意到，在第16行调用了layoutChildren()这个方法，从方法名上我们就可以猜出这个方法是用来进行子元素布局的，不过进入到这个方法当中你会发现这是个空方法，没有一行代码。这当然是可以理解的了，因为子元素的布局应该是由具体的实现类来负责完成的，而不是由父类完成。那么进入ListView的layoutChildren()方法，代码如下所示：

@Overrideprotected void layoutChildren() {    final boolean blockLayoutRequests = mBlockLayoutRequests;    if (!blockLayoutRequests) {        mBlockLayoutRequests = true;    } else {        return;    }    try {        super.layoutChildren();        invalidate();        if (mAdapter == null) {            resetList();            invokeOnItemScrollListener();            return;        }        int childrenTop = mListPadding.top;        int childrenBottom = getBottom() - getTop() - mListPadding.bottom;        int childCount = getChildCount();        int index = 0;        int delta = 0;        View sel;        View oldSel = null;        View oldFirst = null;        View newSel = null;        View focusLayoutRestoreView = null;        // Remember stuff we will need down below        switch (mLayoutMode) {        case LAYOUT_SET_SELECTION:            index = mNextSelectedPosition - mFirstPosition;            if (index >= 0 && index < childCount) {                newSel = getChildAt(index);            }            break;        case LAYOUT_FORCE_TOP:        case LAYOUT_FORCE_BOTTOM:        case LAYOUT_SPECIFIC:        case LAYOUT_SYNC:            break;        case LAYOUT_MOVE_SELECTION:        default:            // Remember the previously selected view            index = mSelectedPosition - mFirstPosition;            if (index >= 0 && index < childCount) {                oldSel = getChildAt(index);            }            // Remember the previous first child            oldFirst = getChildAt(0);            if (mNextSelectedPosition >= 0) {                delta = mNextSelectedPosition - mSelectedPosition;            }            // Caution: newSel might be null            newSel = getChildAt(index + delta);        }        boolean dataChanged = mDataChanged;        if (dataChanged) {            handleDataChanged();        }        // Handle the empty set by removing all views that are visible        // and calling it a day        if (mItemCount == 0) {            resetList();            invokeOnItemScrollListener();            return;        } else if (mItemCount != mAdapter.getCount()) {            throw new IllegalStateException("The content of the adapter has changed but "                    + "ListView did not receive a notification. Make sure the content of "                    + "your adapter is not modified from a background thread, but only "                    + "from the UI thread. [in ListView(" + getId() + ", " + getClass()                     + ") with Adapter(" + mAdapter.getClass() + ")]");        }        setSelectedPositionInt(mNextSelectedPosition);        // Pull all children into the RecycleBin.        // These views will be reused if possible        final int firstPosition = mFirstPosition;        final RecycleBin recycleBin = mRecycler;        // reset the focus restoration        View focusLayoutRestoreDirectChild = null;        // Don't put header or footer views into the Recycler. Those are        // already cached in mHeaderViews;        if (dataChanged) {            for (int i = 0; i < childCount; i++) {                recycleBin.addScrapView(getChildAt(i));                if (ViewDebug.TRACE_RECYCLER) {                    ViewDebug.trace(getChildAt(i),                            ViewDebug.RecyclerTraceType.MOVE_TO_SCRAP_HEAP, index, i);                }            }        } else {            recycleBin.fillActiveViews(childCount, firstPosition);        }        // take focus back to us temporarily to avoid the eventual        // call to clear focus when removing the focused child below        // from messing things up when ViewRoot assigns focus back        // to someone else        final View focusedChild = getFocusedChild();        if (focusedChild != null) {            // TODO: in some cases focusedChild.getParent() == null            // we can remember the focused view to restore after relayout if the            // data hasn't changed, or if the focused position is a header or footer            if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)) {                focusLayoutRestoreDirectChild = focusedChild;                // remember the specific view that had focus                focusLayoutRestoreView = findFocus();                if (focusLayoutRestoreView != null) {                    // tell it we are going to mess with it                    focusLayoutRestoreView.onStartTemporaryDetach();                }            }            requestFocus();        }        // Clear out old views        detachAllViewsFromParent();        switch (mLayoutMode) {        case LAYOUT_SET_SELECTION:            if (newSel != null) {                sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);            } else {                sel = fillFromMiddle(childrenTop, childrenBottom);            }            break;        case LAYOUT_SYNC:            sel = fillSpecific(mSyncPosition, mSpecificTop);            break;        case LAYOUT_FORCE_BOTTOM:            sel = fillUp(mItemCount - 1, childrenBottom);            adjustViewsUpOrDown();            break;        case LAYOUT_FORCE_TOP:            mFirstPosition = 0;            sel = fillFromTop(childrenTop);            adjustViewsUpOrDown();            break;        case LAYOUT_SPECIFIC:            sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop);            break;        case LAYOUT_MOVE_SELECTION:            sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);            break;        default:            if (childCount == 0) {                if (!mStackFromBottom) {                    final int position = lookForSelectablePosition(0, true);                    setSelectedPositionInt(position);                    sel = fillFromTop(childrenTop);                } else {                    final int position = lookForSelectablePosition(mItemCount - 1, false);                    setSelectedPositionInt(position);                    sel = fillUp(mItemCount - 1, childrenBottom);                }            } else {                if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {                    sel = fillSpecific(mSelectedPosition,                            oldSel == null ? childrenTop : oldSel.getTop());                } else if (mFirstPosition < mItemCount) {                    sel = fillSpecific(mFirstPosition,                            oldFirst == null ? childrenTop : oldFirst.getTop());                } else {                    sel = fillSpecific(0, childrenTop);                }            }            break;        }        // Flush any cached views that did not get reused above        recycleBin.scrapActiveViews();        if (sel != null) {            // the current selected item should get focus if items            // are focusable            if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {                final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&                        focusLayoutRestoreView.requestFocus()) || sel.requestFocus();                if (!focusWasTaken) {                    // selected item didn't take focus, fine, but still want                    // to make sure something else outside of the selected view                    // has focus                    final View focused = getFocusedChild();                    if (focused != null) {                        focused.clearFocus();                    }                    positionSelector(sel);                } else {                    sel.setSelected(false);                    mSelectorRect.setEmpty();                }            } else {                positionSelector(sel);            }            mSelectedTop = sel.getTop();        } else {            if (mTouchMode > TOUCH_MODE_DOWN && mTouchMode < TOUCH_MODE_SCROLL) {                View child = getChildAt(mMotionPosition - mFirstPosition);                if (child != null) positionSelector(child);            } else {                mSelectedTop = 0;                mSelectorRect.setEmpty();            }            // even if there is not selected position, we may need to restore            // focus (i.e. something focusable in touch mode)            if (hasFocus() && focusLayoutRestoreView != null) {                focusLayoutRestoreView.requestFocus();            }        }        // tell focus view we are done mucking with it, if it is still in        // our view hierarchy.        if (focusLayoutRestoreView != null                && focusLayoutRestoreView.getWindowToken() != null) {            focusLayoutRestoreView.onFinishTemporaryDetach();        }        mLayoutMode = LAYOUT_NORMAL;        mDataChanged = false;        mNeedSync = false;        setNextSelectedPositionInt(mSelectedPosition);        updateScrollIndicators();        if (mItemCount > 0) {            checkSelectionChanged();        }        invokeOnItemScrollListener();    } finally {        if (!blockLayoutRequests) {            mBlockLayoutRequests = false;        }    }}

这段代码比较长，我们挑重点的看。首先可以确定的是，ListView当中目前还没有任何子View，数据都还是由Adapter管理的，并没有展示到界面上，因此第19行getChildCount()方法得到的值肯定是0。接着在第81行会根据dataChanged这个布尔型的值来判断执行逻辑，dataChanged只有在数据源发生改变的情况下才会变成true，其它情况都是false，因此这里会进入到第90行的执行逻辑，调用RecycleBin的fillActiveViews()方法。按理来说，调用fillActiveViews()方法是为了将ListView的子View进行缓存的，可是目前ListView中还没有任何的子View，因此这一行暂时还起不了任何作用。

接下来在第114行会根据mLayoutMode的值来决定布局模式，默认情况下都是普通模式LAYOUT_NORMAL，因此会进入到第140行的default语句当中。而下面又会紧接着进行两次if判断，childCount目前是等于0的，并且默认的布局顺序是从上往下，因此会进入到第145行的fillFromTop()方法，我们跟进去瞧一瞧：

/** * Fills the list from top to bottom, starting with mFirstPosition * * @param nextTop The location where the top of the first item should be *        drawn * * @return The view that is currently selected */private View fillFromTop(int nextTop) {    mFirstPosition = Math.min(mFirstPosition, mSelectedPosition);    mFirstPosition = Math.min(mFirstPosition, mItemCount - 1);    if (mFirstPosition < 0) {        mFirstPosition = 0;    }    return fillDown(mFirstPosition, nextTop);}

从这个方法的注释中可以看出，它所负责的主要任务就是从mFirstPosition开始，自顶至底去填充ListView。而这个方法本身并没有什么逻辑，就是判断了一下mFirstPosition值的合法性，然后调用fillDown()方法，那么我们就有理由可以猜测，填充ListView的操作是在fillDown()方法中完成的。进入fillDown()方法，代码如下所示：

/** * Fills the list from pos down to the end of the list view. * * @param pos The first position to put in the list * * @param nextTop The location where the top of the item associated with pos *        should be drawn * * @return The view that is currently selected, if it happens to be in the *         range that we draw. */private View fillDown(int pos, int nextTop) {    View selectedView = null;    int end = (getBottom() - getTop()) - mListPadding.bottom;    while (nextTop < end && pos < mItemCount) {        // is this the selected item?        boolean selected = pos == mSelectedPosition;        View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected);        nextTop = child.getBottom() + mDividerHeight;        if (selected) {            selectedView = child;        }        pos++;    }    return selectedView;}

可以看到，这里使用了一个while循环来执行重复逻辑，一开始nextTop的值是第一个子元素顶部距离整个ListView顶部的像素值，pos则是刚刚传入的mFirstPosition的值，而end是ListView底部减去顶部所得的像素值，mItemCount则是Adapter中的元素数量。因此一开始的情况下nextTop必定是小于end值的，并且pos也是小于mItemCount值的。那么每执行一次while循环，pos的值都会加1，并且nextTop也会增加，当nextTop大于等于end时，也就是子元素已经超出当前屏幕了，或者pos大于等于mItemCount时，也就是所有Adapter中的元素都被遍历结束了，就会跳出while循环。

那么while循环当中又做了什么事情呢？值得让人留意的就是第18行调用的makeAndAddView()方法，进入到这个方法当中，代码如下所示：

/** * Obtain the view and add it to our list of children. The view can be made * fresh, converted from an unused view, or used as is if it was in the * recycle bin. * * @param position Logical position in the list * @param y Top or bottom edge of the view to add * @param flow If flow is true, align top edge to y. If false, align bottom *        edge to y. * @param childrenLeft Left edge where children should be positioned * @param selected Is this position selected? * @return View that was added */private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,        boolean selected) {    View child;    if (!mDataChanged) {        // Try to use an exsiting view for this position        child = mRecycler.getActiveView(position);        if (child != null) {            // Found it -- we're using an existing child            // This just needs to be positioned            setupChild(child, position, y, flow, childrenLeft, selected, true);            return child;        }    }    // Make a new view for this position, or convert an unused view if possible    child = obtainView(position, mIsScrap);    // This needs to be positioned and measured    setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);    return child;}

这里在第19行尝试从RecycleBin当中快速获取一个active view，不过很遗憾的是目前RecycleBin当中还没有缓存任何的View，所以这里得到的值肯定是null。那么取得了null之后就会继续向下运行，到第28行会调用obtainView()方法来再次尝试获取一个View，这次的obtainView()方法是可以保证一定返回一个View的，于是下面立刻将获取到的View传入到了setupChild()方法当中。那么obtainView()内部到底是怎么工作的呢？我们先进入到这个方法里面看一下：

/** * Get a view and have it show the data associated with the specified * position. This is called when we have already discovered that the view is * not available for reuse in the recycle bin. The only choices left are * converting an old view or making a new one. *  * @param position *            The position to display * @param isScrap *            Array of at least 1 boolean, the first entry will become true *            if the returned view was taken from the scrap heap, false if *            otherwise. *  * @return A view displaying the data associated with the specified position */View obtainView(int position, boolean[] isScrap) {    isScrap[0] = false;    View scrapView;    scrapView = mRecycler.getScrapView(position);    View child;    if (scrapView != null) {        child = mAdapter.getView(position, scrapView, this);        if (child != scrapView) {            mRecycler.addScrapView(scrapView);            if (mCacheColorHint != 0) {                child.setDrawingCacheBackgroundColor(mCacheColorHint);            }        } else {            isScrap[0] = true;            dispatchFinishTemporaryDetach(child);        }    } else {        child = mAdapter.getView(position, null, this);        if (mCacheColorHint != 0) {            child.setDrawingCacheBackgroundColor(mCacheColorHint);        }    }    return child;}

obtainView()方法中的代码并不多，但却包含了非常非常重要的逻辑，不夸张的说，整个ListView中最重要的内容可能就在这个方法里了。那么我们还是按照执行流程来看，在第19行代码中调用了RecycleBin的getScrapView()方法来尝试获取一个废弃缓存中的View，同样的道理，这里肯定是获取不到的，getScrapView()方法会返回一个null。这时该怎么办呢？没有关系，代码会执行到第33行，调用mAdapter的getView()方法来去获取一个View。那么mAdapter是什么呢？当然就是当前ListView关联的适配器了。而getView()方法又是什么呢？还用说吗，这个就是我们平时使用ListView时最最经常重写的一个方法了，这里getView()方法中传入了三个参数，分别是position，null和this。

那么我们平时写ListView的Adapter时，getView()方法通常会怎么写呢？这里我举个简单的例子：

@Overridepublic View getView(int position, View convertView, ViewGroup parent) {    Fruit fruit = getItem(position);    View view;    if (convertView == null) {        view = LayoutInflater.from(getContext()).inflate(resourceId, null);    } else {        view = convertView;    }    ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);    TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);    fruitImage.setImageResource(fruit.getImageId());    fruitName.setText(fruit.getName());    return view;}

getView()方法接受的三个参数，第一个参数position代表当前子元素的的位置，我们可以通过具体的位置来获取与其相关的数据。第二个参数convertView，刚才传入的是null，说明没有convertView可以利用，因此我们会调用LayoutInflater的inflate()方法来去加载一个布局。接下来会对这个view进行一些属性和值的设定，最后将view返回。

那么这个View也会作为obtainView()的结果进行返回，并最终传入到setupChild()方法当中。其实也就是说，第一次layout过程当中，所有的子View都是调用LayoutInflater的inflate()方法加载出来的，这样就会相对比较耗时，但是不用担心，后面就不会再有这种情况了，那么我们继续往下看：

/** * Add a view as a child and make sure it is measured (if necessary) and * positioned properly. * * @param child The view to add * @param position The position of this child * @param y The y position relative to which this view will be positioned * @param flowDown If true, align top edge to y. If false, align bottom *        edge to y. * @param childrenLeft Left edge where children should be positioned * @param selected Is this position selected? * @param recycled Has this view been pulled from the recycle bin? If so it *        does not need to be remeasured. */private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,        boolean selected, boolean recycled) {    final boolean isSelected = selected && shouldShowSelector();    final boolean updateChildSelected = isSelected != child.isSelected();    final int mode = mTouchMode;    final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL &&            mMotionPosition == position;    final boolean updateChildPressed = isPressed != child.isPressed();    final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested();    // Respect layout params that are already in the view. Otherwise make some up...    // noinspection unchecked    AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();    if (p == null) {        p = new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,                ViewGroup.LayoutParams.WRAP_CONTENT, 0);    }    p.viewType = mAdapter.getItemViewType(position);    if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter &&            p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {        attachViewToParent(child, flowDown ? -1 : 0, p);    } else {        p.forceAdd = false;        if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {            p.recycledHeaderFooter = true;        }        addViewInLayout(child, flowDown ? -1 : 0, p, true);    }    if (updateChildSelected) {        child.setSelected(isSelected);    }    if (updateChildPressed) {        child.setPressed(isPressed);    }    if (needToMeasure) {        int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,                mListPadding.left + mListPadding.right, p.width);        int lpHeight = p.height;        int childHeightSpec;        if (lpHeight > 0) {            childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);        } else {            childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);        }        child.measure(childWidthSpec, childHeightSpec);    } else {        cleanupLayoutState(child);    }    final int w = child.getMeasuredWidth();    final int h = child.getMeasuredHeight();    final int childTop = flowDown ? y : y - h;    if (needToMeasure) {        final int childRight = childrenLeft + w;        final int childBottom = childTop + h;        child.layout(childrenLeft, childTop, childRight, childBottom);    } else {        child.offsetLeftAndRight(childrenLeft - child.getLeft());        child.offsetTopAndBottom(childTop - child.getTop());    }    if (mCachingStarted && !child.isDrawingCacheEnabled()) {        child.setDrawingCacheEnabled(true);    }}

setupChild()方法当中的代码虽然比较多，但是我们只看核心代码的话就非常简单了，刚才调用obtainView()方法获取到的子元素View，这里在第40行调用了addViewInLayout()方法将它添加到了ListView当中。那么根据fillDown()方法中的while循环，会让子元素View将整个ListView控件填满然后就跳出，也就是说即使我们的Adapter中有一千条数据，ListView也只会加载第一屏的数据，剩下的数据反正目前在屏幕上也看不到，所以不会去做多余的加载工作，这样就可以保证ListView中的内容能够迅速展示到屏幕上。

那么到此为止，第一次Layout过程结束。

第二次Layout

虽然我在源码中并没有找出具体的原因，但如果你自己做一下实验的话就会发现，即使是一个再简单的View，在展示到界面上之前都会经历至少两次onMeasure()和两次onLayout()的过程。其实这只是一个很小的细节，平时对我们影响并不大，因为不管是onMeasure()或者onLayout()几次，反正都是执行的相同的逻辑，我们并不需要进行过多关心。但是在ListView中情况就不一样了，因为这就意味着layoutChildren()过程会执行两次，而这个过程当中涉及到向ListView中添加子元素，如果相同的逻辑执行两遍的话，那么ListView中就会存在一份重复的数据了。因此ListView在layoutChildren()过程当中做了第二次Layout的逻辑处理，非常巧妙地解决了这个问题，下面我们就来分析一下第二次Layout的过程。

其实第二次Layout和第一次Layout的基本流程是差不多的，那么我们还是从layoutChildren()方法开始看起：

@Overrideprotected void layoutChildren() {    final boolean blockLayoutRequests = mBlockLayoutRequests;    if (!blockLayoutRequests) {        mBlockLayoutRequests = true;    } else {        return;    }    try {        super.layoutChildren();        invalidate();        if (mAdapter == null) {            resetList();            invokeOnItemScrollListener();            return;        }        int childrenTop = mListPadding.top;        int childrenBottom = getBottom() - getTop() - mListPadding.bottom;        int childCount = getChildCount();        int index = 0;        int delta = 0;        View sel;        View oldSel = null;        View oldFirst = null;        View newSel = null;        View focusLayoutRestoreView = null;        // Remember stuff we will need down below        switch (mLayoutMode) {        case LAYOUT_SET_SELECTION:            index = mNextSelectedPosition - mFirstPosition;            if (index >= 0 && index < childCount) {                newSel = getChildAt(index);            }            break;        case LAYOUT_FORCE_TOP:        case LAYOUT_FORCE_BOTTOM:        case LAYOUT_SPECIFIC:        case LAYOUT_SYNC:            break;        case LAYOUT_MOVE_SELECTION:        default:            // Remember the previously selected view            index = mSelectedPosition - mFirstPosition;            if (index >= 0 && index < childCount) {                oldSel = getChildAt(index);            }            // Remember the previous first child            oldFirst = getChildAt(0);            if (mNextSelectedPosition >= 0) {                delta = mNextSelectedPosition - mSelectedPosition;            }            // Caution: newSel might be null            newSel = getChildAt(index + delta);        }        boolean dataChanged = mDataChanged;        if (dataChanged) {            handleDataChanged();        }        // Handle the empty set by removing all views that are visible        // and calling it a day        if (mItemCount == 0) {            resetList();            invokeOnItemScrollListener();            return;        } else if (mItemCount != mAdapter.getCount()) {            throw new IllegalStateException("The content of the adapter has changed but "                    + "ListView did not receive a notification. Make sure the content of "                    + "your adapter is not modified from a background thread, but only "                    + "from the UI thread. [in ListView(" + getId() + ", " + getClass()                     + ") with Adapter(" + mAdapter.getClass() + ")]");        }        setSelectedPositionInt(mNextSelectedPosition);        // Pull all children into the RecycleBin.        // These views will be reused if possible        final int firstPosition = mFirstPosition;        final RecycleBin recycleBin = mRecycler;        // reset the focus restoration        View focusLayoutRestoreDirectChild = null;        // Don't put header or footer views into the Recycler. Those are        // already cached in mHeaderViews;        if (dataChanged) {            for (int i = 0; i < childCount; i++) {                recycleBin.addScrapView(getChildAt(i));                if (ViewDebug.TRACE_RECYCLER) {                    ViewDebug.trace(getChildAt(i),                            ViewDebug.RecyclerTraceType.MOVE_TO_SCRAP_HEAP, index, i);                }            }        } else {            recycleBin.fillActiveViews(childCount, firstPosition);        }        // take focus back to us temporarily to avoid the eventual        // call to clear focus when removing the focused child below        // from messing things up when ViewRoot assigns focus back        // to someone else        final View focusedChild = getFocusedChild();        if (focusedChild != null) {            // TODO: in some cases focusedChild.getParent() == null            // we can remember the focused view to restore after relayout if the            // data hasn't changed, or if the focused position is a header or footer            if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)) {                focusLayoutRestoreDirectChild = focusedChild;                // remember the specific view that had focus                focusLayoutRestoreView = findFocus();                if (focusLayoutRestoreView != null) {                    // tell it we are going to mess with it                    focusLayoutRestoreView.onStartTemporaryDetach();                }            }            requestFocus();        }        // Clear out old views        detachAllViewsFromParent();        switch (mLayoutMode) {        case LAYOUT_SET_SELECTION:            if (newSel != null) {                sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);            } else {                sel = fillFromMiddle(childrenTop, childrenBottom);            }            break;        case LAYOUT_SYNC:            sel = fillSpecific(mSyncPosition, mSpecificTop);            break;        case LAYOUT_FORCE_BOTTOM:            sel = fillUp(mItemCount - 1, childrenBottom);            adjustViewsUpOrDown();            break;        case LAYOUT_FORCE_TOP:            mFirstPosition = 0;            sel = fillFromTop(childrenTop);            adjustViewsUpOrDown();            break;        case LAYOUT_SPECIFIC:            sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop);            break;        case LAYOUT_MOVE_SELECTION:            sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);            break;        default:            if (childCount == 0) {                if (!mStackFromBottom) {                    final int position = lookForSelectablePosition(0, true);                    setSelectedPositionInt(position);                    sel = fillFromTop(childrenTop);                } else {                    final int position = lookForSelectablePosition(mItemCount - 1, false);                    setSelectedPositionInt(position);                    sel = fillUp(mItemCount - 1, childrenBottom);                }            } else {                if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {                    sel = fillSpecific(mSelectedPosition,                            oldSel == null ? childrenTop : oldSel.getTop());                } else if (mFirstPosition < mItemCount) {                    sel = fillSpecific(mFirstPosition,                            oldFirst == null ? childrenTop : oldFirst.getTop());                } else {                    sel = fillSpecific(0, childrenTop);                }            }            break;        }        // Flush any cached views that did not get reused above        recycleBin.scrapActiveViews();        if (sel != null) {            // the current selected item should get focus if items            // are focusable            if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {                final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&                        focusLayoutRestoreView.requestFocus()) || sel.requestFocus();                if (!focusWasTaken) {                    // selected item didn't take focus, fine, but still want                    // to make sure something else outside of the selected view                    // has focus                    final View focused = getFocusedChild();                    if (focused != null) {                        focused.clearFocus();                    }                    positionSelector(sel);                } else {                    sel.setSelected(false);                    mSelectorRect.setEmpty();                }            } else {                positionSelector(sel);            }            mSelectedTop = sel.getTop();        } else {            if (mTouchMode > TOUCH_MODE_DOWN && mTouchMode < TOUCH_MODE_SCROLL) {                View child = getChildAt(mMotionPosition - mFirstPosition);                if (child != null) positionSelector(child);            } else {                mSelectedTop = 0;                mSelectorRect.setEmpty();            }            // even if there is not selected position, we may need to restore            // focus (i.e. something focusable in touch mode)            if (hasFocus() && focusLayoutRestoreView != null) {                focusLayoutRestoreView.requestFocus();            }        }        // tell focus view we are done mucking with it, if it is still in        // our view hierarchy.        if (focusLayoutRestoreView != null                && focusLayoutRestoreView.getWindowToken() != null) {            focusLayoutRestoreView.onFinishTemporaryDetach();        }        mLayoutMode = LAYOUT_NORMAL;        mDataChanged = false;        mNeedSync = false;        setNextSelectedPositionInt(mSelectedPosition);        updateScrollIndicators();        if (mItemCount > 0) {            checkSelectionChanged();        }        invokeOnItemScrollListener();    } finally {        if (!blockLayoutRequests) {            mBlockLayoutRequests = false;        }    }}

同样还是在第19行，调用getChildCount()方法来获取子View的数量，只不过现在得到的值不会再是0了，而是ListView中一屏可以显示的子View数量，因为我们刚刚在第一次Layout过程当中向ListView添加了这么多的子View。下面在第90行调用了RecycleBin的fillActiveViews()方法，这次效果可就不一样了，因为目前ListView中已经有子View了，这样所有的子View都会被缓存到RecycleBin的mActiveViews数组当中，后面将会用到它们。

接下来将会是非常非常重要的一个操作，在第113行调用了detachAllViewsFromParent()方法。这个方法会将所有ListView当中的子View全部清除掉，从而保证第二次Layout过程不会产生一份重复的数据。那有的朋友可能会问了，这样把已经加载好的View又清除掉，待会还要再重新加载一遍，这不是严重影响效率吗？不用担心，还记得我们刚刚调用了RecycleBin的fillActiveViews()方法来缓存子View吗，待会儿将会直接使用这些缓存好的View来进行加载，而并不会重新执行一遍inflate过程，因此效率方面并不会有什么明显的影响。

那么我们接着看，在第141行的判断逻辑当中，由于不再等于0了，因此会进入到else语句当中。而else语句中又有三个逻辑判断，第一个逻辑判断不成立，因为默认情况下我们没有选中任何子元素，mSelectedPosition应该等于-1。第二个逻辑判断通常是成立的，因为mFirstPosition的值一开始是等于0的，只要adapter中的数据大于0条件就成立。那么进入到fillSpecific()方法当中，代码如下所示：

/** * Put a specific item at a specific location on the screen and then build * up and down from there. * * @param position The reference view to use as the starting point * @param top Pixel offset from the top of this view to the top of the *        reference view. * * @return The selected view, or null if the selected view is outside the *         visible area. */private View fillSpecific(int position, int top) {    boolean tempIsSelected = position == mSelectedPosition;    View temp = makeAndAddView(position, top, true, mListPadding.left, tempIsSelected);    // Possibly changed again in fillUp if we add rows above this one.    mFirstPosition = position;    View above;    View below;    final int dividerHeight = mDividerHeight;    if (!mStackFromBottom) {        above = fillUp(position - 1, temp.getTop() - dividerHeight);        // This will correct for the top of the first view not touching the top of the list        adjustViewsUpOrDown();        below = fillDown(position + 1, temp.getBottom() + dividerHeight);        int childCount = getChildCount();        if (childCount > 0) {            correctTooHigh(childCount);        }    } else {        below = fillDown(position + 1, temp.getBottom() + dividerHeight);        // This will correct for the bottom of the last view not touching the bottom of the list        adjustViewsUpOrDown();        above = fillUp(position - 1, temp.getTop() - dividerHeight);        int childCount = getChildCount();        if (childCount > 0) {             correctTooLow(childCount);        }    }    if (tempIsSelected) {        return temp;    } else if (above != null) {        return above;    } else {        return below;    }}

fillSpecific()这算是一个新方法了，不过其实它和fillUp()、fillDown()方法功能也是差不多的，主要的区别在于，fillSpecific()方法会优先将指定位置的子View先加载到屏幕上，然后再加载该子View往上以及往下的其它子View。那么由于这里我们传入的position就是第一个子View的位置，于是fillSpecific()方法的作用就基本上和fillDown()方法是差不多的了，这里我们就不去关注太多它的细节，而是将精力放在makeAndAddView()方法上面。再次回到makeAndAddView()方法，代码如下所示：

/** * Obtain the view and add it to our list of children. The view can be made * fresh, converted from an unused view, or used as is if it was in the * recycle bin. * * @param position Logical position in the list * @param y Top or bottom edge of the view to add * @param flow If flow is true, align top edge to y. If false, align bottom *        edge to y. * @param childrenLeft Left edge where children should be positioned * @param selected Is this position selected? * @return View that was added */private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,        boolean selected) {    View child;    if (!mDataChanged) {        // Try to use an exsiting view for this position        child = mRecycler.getActiveView(position);        if (child != null) {            // Found it -- we're using an existing child            // This just needs to be positioned            setupChild(child, position, y, flow, childrenLeft, selected, true);            return child;        }    }    // Make a new view for this position, or convert an unused view if possible    child = obtainView(position, mIsScrap);    // This needs to be positioned and measured    setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);    return child;}

仍然还是在第19行尝试从RecycleBin当中获取Active View，然而这次就一定可以获取到了，因为前面我们调用了RecycleBin的fillActiveViews()方法来缓存子View。那么既然如此，就不会再进入到第28行的obtainView()方法，而是会直接进入setupChild()方法当中，这样也省去了很多时间，因为如果在obtainView()方法中又要去infalte布局的话，那么ListView的初始加载效率就大大降低了。

注意在第23行，setupChild()方法的最后一个参数传入的是true，这个参数表明当前的View是之前被回收过的，那么我们再次回到setupChild()方法当中：

/** * Add a view as a child and make sure it is measured (if necessary) and * positioned properly. * * @param child The view to add * @param position The position of this child * @param y The y position relative to which this view will be positioned * @param flowDown If true, align top edge to y. If false, align bottom *        edge to y. * @param childrenLeft Left edge where children should be positioned * @param selected Is this position selected? * @param recycled Has this view been pulled from the recycle bin? If so it *        does not need to be remeasured. */private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,        boolean selected, boolean recycled) {    final boolean isSelected = selected && shouldShowSelector();    final boolean updateChildSelected = isSelected != child.isSelected();    final int mode = mTouchMode;    final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL &&            mMotionPosition == position;    final boolean updateChildPressed = isPressed != child.isPressed();    final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested();    // Respect layout params that are already in the view. Otherwise make some up...    // noinspection unchecked    AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();    if (p == null) {        p = new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,                ViewGroup.LayoutParams.WRAP_CONTENT, 0);    }    p.viewType = mAdapter.getItemViewType(position);    if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter &&            p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {        attachViewToParent(child, flowDown ? -1 : 0, p);    } else {        p.forceAdd = false;        if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {            p.recycledHeaderFooter = true;        }        addViewInLayout(child, flowDown ? -1 : 0, p, true);    }    if (updateChildSelected) {        child.setSelected(isSelected);    }    if (updateChildPressed) {        child.setPressed(isPressed);    }    if (needToMeasure) {        int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,                mListPadding.left + mListPadding.right, p.width);        int lpHeight = p.height;        int childHeightSpec;        if (lpHeight > 0) {            childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);        } else {            childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);        }        child.measure(childWidthSpec, childHeightSpec);    } else {        cleanupLayoutState(child);    }    final int w = child.getMeasuredWidth();    final int h = child.getMeasuredHeight();    final int childTop = flowDown ? y : y - h;    if (needToMeasure) {        final int childRight = childrenLeft + w;        final int childBottom = childTop + h;        child.layout(childrenLeft, childTop, childRight, childBottom);    } else {        child.offsetLeftAndRight(childrenLeft - child.getLeft());        child.offsetTopAndBottom(childTop - child.getTop());    }    if (mCachingStarted && !child.isDrawingCacheEnabled()) {        child.setDrawingCacheEnabled(true);    }}

可以看到，setupChild()方法的最后一个参数是recycled，然后在第32行会对这个变量进行判断，由于recycled现在是true，所以会执行attachViewToParent()方法，而第一次Layout过程则是执行的else语句中的addViewInLayout()方法。这两个方法最大的区别在于，如果我们需要向ViewGroup中添加一个新的子View，应该调用addViewInLayout()方法，而如果是想要将一个之前detach的View重新attach到ViewGroup上，就应该调用attachViewToParent()方法。那么由于前面在layoutChildren()方法当中调用了detachAllViewsFromParent()方法，这样ListView中所有的子View都是处于detach状态的，所以这里attachViewToParent()方法是正确的选择。

经历了这样一个detach又attach的过程，ListView中所有的子View又都可以正常显示出来了，那么第二次Layout过程结束。

滑动加载更多数据

经历了两次Layout过程，虽说我们已经可以在ListView中看到内容了，然而关于ListView最神奇的部分我们却还没有接触到，因为目前ListView中只是加载并显示了第一屏的数据而已。比如说我们的Adapter当中有1000条数据，但是第一屏只显示了10条，ListView中也只有10个子View而已，那么剩下的990是怎样工作并显示到界面上的呢？这就要看一下ListView滑动部分的源码了，因为我们是通过手指滑动来显示更多数据的。

由于滑动部分的机制是属于通用型的，即ListView和GridView都会使用同样的机制，因此这部分代码就肯定是写在AbsListView当中的了。那么监听触控事件是在onTouchEvent()方法当中进行的，我们就来看一下AbsListView中的这个方法：

@Overridepublic boolean onTouchEvent(MotionEvent ev) {    if (!isEnabled()) {        // A disabled view that is clickable still consumes the touch        // events, it just doesn't respond to them.        return isClickable() || isLongClickable();    }    final int action = ev.getAction();    View v;    int deltaY;    if (mVelocityTracker == null) {        mVelocityTracker = VelocityTracker.obtain();    }    mVelocityTracker.addMovement(ev);    switch (action & MotionEvent.ACTION_MASK) {    case MotionEvent.ACTION_DOWN: {        mActivePointerId = ev.getPointerId(0);        final int x = (int) ev.getX();        final int y = (int) ev.getY();        int motionPosition = pointToPosition(x, y);        if (!mDataChanged) {            if ((mTouchMode != TOUCH_MODE_FLING) && (motionPosition >= 0)                    && (getAdapter().isEnabled(motionPosition))) {                // User clicked on an actual view (and was not stopping a                // fling). It might be a                // click or a scroll. Assume it is a click until proven                // otherwise                mTouchMode = TOUCH_MODE_DOWN;                // FIXME Debounce                if (mPendingCheckForTap == null) {                    mPendingCheckForTap = new CheckForTap();                }                postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());            } else {                if (ev.getEdgeFlags() != 0 && motionPosition < 0) {                    // If we couldn't find a view to click on, but the down                    // event was touching                    // the edge, we will bail out and try again. This allows                    // the edge correcting                    // code in ViewRoot to try to find a nearby view to                    // select                    return false;                }                if (mTouchMode == TOUCH_MODE_FLING) {                    // Stopped a fling. It is a scroll.                    createScrollingCache();                    mTouchMode = TOUCH_MODE_SCROLL;                    mMotionCorrection = 0;                    motionPosition = findMotionRow(y);                    reportScrollStateChange(OnScrollListener.SCROLL_STATE_TOUCH_SCROLL);                }            }        }        if (motionPosition >= 0) {            // Remember where the motion event started            v = getChildAt(motionPosition - mFirstPosition);            mMotionViewOriginalTop = v.getTop();        }        mMotionX = x;        mMotionY = y;        mMotionPosition = motionPosition;        mLastY = Integer.MIN_VALUE;        break;    }    case MotionEvent.ACTION_MOVE: {        final int pointerIndex = ev.findPointerIndex(mActivePointerId);        final int y = (int) ev.getY(pointerIndex);        deltaY = y - mMotionY;        switch (mTouchMode) {        case TOUCH_MODE_DOWN:        case TOUCH_MODE_TAP:        case TOUCH_MODE_DONE_WAITING:            // Check if we have moved far enough that it looks more like a            // scroll than a tap            startScrollIfNeeded(deltaY);            break;        case TOUCH_MODE_SCROLL:            if (PROFILE_SCROLLING) {                if (!mScrollProfilingStarted) {                    Debug.startMethodTracing("AbsListViewScroll");                    mScrollProfilingStarted = true;                }            }            if (y != mLastY) {                deltaY -= mMotionCorrection;                int incrementalDeltaY = mLastY != Integer.MIN_VALUE ? y - mLastY : deltaY;                // No need to do all this work if we're not going to move                // anyway                boolean atEdge = false;                if (incrementalDeltaY != 0) {                    atEdge = trackMotionScroll(deltaY, incrementalDeltaY);                }                // Check to see if we have bumped into the scroll limit                if (atEdge && getChildCount() > 0) {                    // Treat this like we're starting a new scroll from the                    // current                    // position. This will let the user start scrolling back                    // into                    // content immediately rather than needing to scroll                    // back to the                    // point where they hit the limit first.                    int motionPosition = findMotionRow(y);                    if (motionPosition >= 0) {                        final View motionView = getChildAt(motionPosition - mFirstPosition);                        mMotionViewOriginalTop = motionView.getTop();                    }                    mMotionY = y;                    mMotionPosition = motionPosition;                    invalidate();                }                mLastY = y;            }            break;        }        break;    }    case MotionEvent.ACTION_UP: {        switch (mTouchMode) {        case TOUCH_MODE_DOWN:        case TOUCH_MODE_TAP:        case TOUCH_MODE_DONE_WAITING:            final int motionPosition = mMotionPosition;            final View child = getChildAt(motionPosition - mFirstPosition);            if (child != null && !child.hasFocusable()) {                if (mTouchMode != TOUCH_MODE_DOWN) {                    child.setPressed(false);                }                if (mPerformClick == null) {                    mPerformClick = new PerformClick();                }                final AbsListView.PerformClick performClick = mPerformClick;                performClick.mChild = child;                performClick.mClickMotionPosition = motionPosition;                performClick.rememberWindowAttachCount();                mResurrectToPosition = motionPosition;                if (mTouchMode == TOUCH_MODE_DOWN || mTouchMode == TOUCH_MODE_TAP) {                    final Handler handler = getHandler();                    if (handler != null) {                        handler.removeCallbacks(mTouchMode == TOUCH_MODE_DOWN ? mPendingCheckForTap                                : mPendingCheckForLongPress);                    }                    mLayoutMode = LAYOUT_NORMAL;                    if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {                        mTouchMode = TOUCH_MODE_TAP;                        setSelectedPositionInt(mMotionPosition);                        layoutChildren();                        child.setPressed(true);                        positionSelector(child);                        setPressed(true);                        if (mSelector != null) {                            Drawable d = mSelector.getCurrent();                            if (d != null && d instanceof TransitionDrawable) {                                ((TransitionDrawable) d).resetTransition();                            }                        }                        postDelayed(new Runnable() {                            public void run() {                                child.setPressed(false);                                setPressed(false);                                if (!mDataChanged) {                                    post(performClick);                                }                                mTouchMode = TOUCH_MODE_REST;                            }                        }, ViewConfiguration.getPressedStateDuration());                    } else {                        mTouchMode = TOUCH_MODE_REST;                    }                    return true;                } else if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {                    post(performClick);                }            }            mTouchMode = TOUCH_MODE_REST;            break;        case TOUCH_MODE_SCROLL:            final int childCount = getChildCount();            if (childCount > 0) {                if (mFirstPosition == 0                        && getChildAt(0).getTop() >= mListPadding.top                        && mFirstPosition + childCount < mItemCount                        && getChildAt(childCount - 1).getBottom() <= getHeight()                                - mListPadding.bottom) {                    mTouchMode = TOUCH_MODE_REST;                    reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);                } else {                    final VelocityTracker velocityTracker = mVelocityTracker;                    velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);                    final int initialVelocity = (int) velocityTracker                            .getYVelocity(mActivePointerId);                    if (Math.abs(initialVelocity) > mMinimumVelocity) {                        if (mFlingRunnable == null) {                            mFlingRunnable = new FlingRunnable();                        }                        reportScrollStateChange(OnScrollListener.SCROLL_STATE_FLING);                        mFlingRunnable.start(-initialVelocity);                    } else {                        mTouchMode = TOUCH_MODE_REST;                        reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);                    }                }            } else {                mTouchMode = TOUCH_MODE_REST;                reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);            }            break;        }        setPressed(false);        // Need to redraw since we probably aren't drawing the selector        // anymore        invalidate();        final Handler handler = getHandler();        if (handler != null) {            handler.removeCallbacks(mPendingCheckForLongPress);        }        if (mVelocityTracker != null) {            mVelocityTracker.recycle();            mVelocityTracker = null;        }        mActivePointerId = INVALID_POINTER;        if (PROFILE_SCROLLING) {            if (mScrollProfilingStarted) {                Debug.stopMethodTracing();                mScrollProfilingStarted = false;            }        }        break;    }    case MotionEvent.ACTION_CANCEL: {        mTouchMode = TOUCH_MODE_REST;        setPressed(false);        View motionView = this.getChildAt(mMotionPosition - mFirstPosition);        if (motionView != null) {            motionView.setPressed(false);        }        clearScrollingCache();        final Handler handler = getHandler();        if (handler != null) {            handler.removeCallbacks(mPendingCheckForLongPress);        }        if (mVelocityTracker != null) {            mVelocityTracker.recycle();            mVelocityTracker = null;        }        mActivePointerId = INVALID_POINTER;        break;    }    case MotionEvent.ACTION_POINTER_UP: {        onSecondaryPointerUp(ev);        final int x = mMotionX;        final int y = mMotionY;        final int motionPosition = pointToPosition(x, y);        if (motionPosition >= 0) {            // Remember where the motion event started            v = getChildAt(motionPosition - mFirstPosition);            mMotionViewOriginalTop = v.getTop();            mMotionPosition = motionPosition;        }        mLastY = y;        break;    }    }    return true;}

这个方法中的代码就非常多了，因为它所处理的逻辑也非常多，要监听各种各样的触屏事件。但是我们目前所关心的就只有手指在屏幕上滑动这一个事件而已，对应的是ACTION_MOVE这个动作，那么我们就只看这部分代码就可以了。

可以看到，ACTION_MOVE这个case里面又嵌套了一个switch语句，是根据当前的TouchMode来选择的。那这里我可以直接告诉大家，当手指在屏幕上滑动时，TouchMode是等于TOUCH_MODE_SCROLL这个值的，至于为什么那又要牵扯到另外的好几个方法，这里限于篇幅原因就不再展开讲解了，喜欢寻根究底的朋友们可以自己去源码里找一找原因。

这样的话，代码就应该会走到第78行的这个case里面去了，在这个case当中并没有什么太多需要注意的东西，唯一一点非常重要的就是第92行调用的trackMotionScroll()方法，相当于我们手指只要在屏幕上稍微有一点点移动，这个方法就会被调用，而如果是正常在屏幕上滑动的话，那么这个方法就会被调用很多次。那么我们进入到这个方法中瞧一瞧，代码如下所示：

boolean trackMotionScroll(int deltaY, int incrementalDeltaY) {    final int childCount = getChildCount();    if (childCount == 0) {        return true;    }    final int firstTop = getChildAt(0).getTop();    final int lastBottom = getChildAt(childCount - 1).getBottom();    final Rect listPadding = mListPadding;    final int spaceAbove = listPadding.top - firstTop;    final int end = getHeight() - listPadding.bottom;    final int spaceBelow = lastBottom - end;    final int height = getHeight() - getPaddingBottom() - getPaddingTop();    if (deltaY < 0) {        deltaY = Math.max(-(height - 1), deltaY);    } else {        deltaY = Math.min(height - 1, deltaY);    }    if (incrementalDeltaY < 0) {        incrementalDeltaY = Math.max(-(height - 1), incrementalDeltaY);    } else {        incrementalDeltaY = Math.min(height - 1, incrementalDeltaY);    }    final int firstPosition = mFirstPosition;    if (firstPosition == 0 && firstTop >= listPadding.top && deltaY >= 0) {        // Don't need to move views down if the top of the first position        // is already visible        return true;    }    if (firstPosition + childCount == mItemCount && lastBottom <= end && deltaY <= 0) {        // Don't need to move views up if the bottom of the last position        // is already visible        return true;    }    final boolean down = incrementalDeltaY < 0;    final boolean inTouchMode = isInTouchMode();    if (inTouchMode) {        hideSelector();    }    final int headerViewsCount = getHeaderViewsCount();    final int footerViewsStart = mItemCount - getFooterViewsCount();    int start = 0;    int count = 0;    if (down) {        final int top = listPadding.top - incrementalDeltaY;        for (int i = 0; i < childCount; i++) {            final View child = getChildAt(i);            if (child.getBottom() >= top) {                break;            } else {                count++;                int position = firstPosition + i;                if (position >= headerViewsCount && position < footerViewsStart) {                    mRecycler.addScrapView(child);                }            }        }    } else {        final int bottom = getHeight() - listPadding.bottom - incrementalDeltaY;        for (int i = childCount - 1; i >= 0; i--) {            final View child = getChildAt(i);            if (child.getTop() <= bottom) {                break;            } else {                start = i;                count++;                int position = firstPosition + i;                if (position >= headerViewsCount && position < footerViewsStart) {                    mRecycler.addScrapView(child);                }            }        }    }    mMotionViewNewTop = mMotionViewOriginalTop + deltaY;    mBlockLayoutRequests = true;    if (count > 0) {        detachViewsFromParent(start, count);    }    offsetChildrenTopAndBottom(incrementalDeltaY);    if (down) {        mFirstPosition += count;    }    invalidate();    final int absIncrementalDeltaY = Math.abs(incrementalDeltaY);    if (spaceAbove < absIncrementalDeltaY || spaceBelow < absIncrementalDeltaY) {        fillGap(down);    }    if (!inTouchMode && mSelectedPosition != INVALID_POSITION) {        final int childIndex = mSelectedPosition - mFirstPosition;        if (childIndex >= 0 && childIndex < getChildCount()) {            positionSelector(getChildAt(childIndex));        }    }    mBlockLayoutRequests = false;    invokeOnItemScrollListener();    awakenScrollBars();    return false;}

这个方法接收两个参数，deltaY表示从手指按下时的位置到当前手指位置的距离，incrementalDeltaY则表示据上次触发event事件手指在Y方向上位置的改变量，那么其实我们就可以通过incrementalDeltaY的正负值情况来判断用户是向上还是向下滑动的了。如第34行代码所示，如果incrementalDeltaY小于0，说明是向下滑动，否则就是向上滑动。

下面将会进行一个边界值检测的过程，可以看到，从第43行开始，当ListView向下滑动的时候，就会进入一个for循环当中，从上往下依次获取子View，第47行当中，如果该子View的bottom值已经小于top值了，就说明这个子View已经移出屏幕了，所以会调用RecycleBin的addScrapView()方法将这个View加入到废弃缓存当中，并将count计数器加1，计数器用于记录有多少个子View被移出了屏幕。那么如果是ListView向上滑动的话，其实过程是基本相同的，只不过变成了从下往上依次获取子View，然后判断该子View的top值是不是大于bottom值了，如果大于的话说明子View已经移出了屏幕，同样把它加入到废弃缓存中，并将计数器加1。

接下来在第76行，会根据当前计数器的值来进行一个detach操作，它的作用就是把所有移出屏幕的子View全部detach掉，在ListView的概念当中，所有看不到的View就没有必要为它进行保存，因为屏幕外还有成百上千条数据等着显示呢，一个好的回收策略才能保证ListView的高性能和高效率。紧接着在第78行调用了offsetChildrenTopAndBottom()方法，并将incrementalDeltaY作为参数传入，这个方法的作用是让ListView中所有的子View都按照传入的参数值进行相应的偏移，这样就实现了随着手指的拖动，ListView的内容也会随着滚动的效果。

然后在第84行会进行判断，如果ListView中最后一个View的底部已经移入了屏幕，或者ListView中第一个View的顶部移入了屏幕，就会调用fillGap()方法，那么因此我们就可以猜出fillGap()方法是用来加载屏幕外数据的，进入到这个方法中瞧一瞧，如下所示：

/** * Fills the gap left open by a touch-scroll. During a touch scroll, * children that remain on screen are shifted and the other ones are * discarded. The role of this method is to fill the gap thus created by * performing a partial layout in the empty space. *  * @param down *            true if the scroll is going down, false if it is going up */abstract void fillGap(boolean down);

OK，AbsListView中的fillGap()是一个抽象方法，那么我们立刻就能够想到，它的具体实现肯定是在ListView中完成的了。回到ListView当中，fillGap()方法的代码如下所示：

void fillGap(boolean down) {    final int count = getChildCount();    if (down) {        final int startOffset = count > 0 ? getChildAt(count - 1).getBottom() + mDividerHeight :                getListPaddingTop();        fillDown(mFirstPosition + count, startOffset);        correctTooHigh(getChildCount());    } else {        final int startOffset = count > 0 ? getChildAt(0).getTop() - mDividerHeight :                getHeight() - getListPaddingBottom();        fillUp(mFirstPosition - 1, startOffset);        correctTooLow(getChildCount());    }}

down参数用于表示ListView是向下滑动还是向上滑动的，可以看到，如果是向下滑动的话就会调用fillDown()方法，而如果是向上滑动的话就会调用fillUp()方法。那么这两个方法我们都已经非常熟悉了，内部都是通过一个循环来去对ListView进行填充，所以这两个方法我们就不看了，但是填充ListView会通过调用makeAndAddView()方法来完成，又是makeAndAddView()方法，但这次的逻辑再次不同了，所以我们还是回到这个方法瞧一瞧：

/** * Obtain the view and add it to our list of children. The view can be made * fresh, converted from an unused view, or used as is if it was in the * recycle bin. * * @param position Logical position in the list * @param y Top or bottom edge of the view to add * @param flow If flow is true, align top edge to y. If false, align bottom *        edge to y. * @param childrenLeft Left edge where children should be positioned * @param selected Is this position selected? * @return View that was added */private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,        boolean selected) {    View child;    if (!mDataChanged) {        // Try to use an exsiting view for this position        child = mRecycler.getActiveView(position);        if (child != null) {            // Found it -- we're using an existing child            // This just needs to be positioned            setupChild(child, position, y, flow, childrenLeft, selected, true);            return child;        }    }    // Make a new view for this position, or convert an unused view if possible    child = obtainView(position, mIsScrap);    // This needs to be positioned and measured    setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);    return child;}

不管怎么说，这里首先仍然是会尝试调用RecycleBin的getActiveView()方法来获取子布局，只不过肯定是获取不到的了，因为在第二次Layout过程中我们已经从mActiveViews中获取过了数据，而根据RecycleBin的机制，mActiveViews是不能够重复利用的，因此这里返回的值肯定是null。

既然getActiveView()方法返回的值是null，那么就还是会走到第28行的obtainView()方法当中，代码如下所示：

/** * Get a view and have it show the data associated with the specified * position. This is called when we have already discovered that the view is * not available for reuse in the recycle bin. The only choices left are * converting an old view or making a new one. *  * @param position *            The position to display * @param isScrap *            Array of at least 1 boolean, the first entry will become true *            if the returned view was taken from the scrap heap, false if *            otherwise. *  * @return A view displaying the data associated with the specified position */View obtainView(int position, boolean[] isScrap) {    isScrap[0] = false;    View scrapView;    scrapView = mRecycler.getScrapView(position);    View child;    if (scrapView != null) {        child = mAdapter.getView(position, scrapView, this);        if (child != scrapView) {            mRecycler.addScrapView(scrapView);            if (mCacheColorHint != 0) {                child.setDrawingCacheBackgroundColor(mCacheColorHint);            }        } else {            isScrap[0] = true;            dispatchFinishTemporaryDetach(child);        }    } else {        child = mAdapter.getView(position, null, this);        if (mCacheColorHint != 0) {            child.setDrawingCacheBackgroundColor(mCacheColorHint);        }    }    return child;}

这里在第19行会调用RecyleBin的getScrapView()方法来尝试从废弃缓存中获取一个View，那么废弃缓存有没有View呢？当然有，因为刚才在trackMotionScroll()方法中我们就已经看到了，一旦有任何子View被移出了屏幕，就会将它加入到废弃缓存中，而从obtainView()方法中的逻辑来看，一旦有新的数据需要显示到屏幕上，就会尝试从废弃缓存中获取View。所以它们之间就形成了一个生产者和消费者的模式，那么ListView神奇的地方也就在这里体现出来了，不管你有任意多条数据需要显示，ListView中的子View其实来来回回就那么几个，移出屏幕的子View会很快被移入屏幕的数据重新利用起来，因而不管我们加载多少数据都不会出现OOM的情况，甚至内存都不会有所增加。

那么另外还有一点是需要大家留意的，这里获取到了一个scrapView，然后我们在第22行将它作为第二个参数传入到了Adapter的getView()方法当中。那么第二个参数是什么意思呢？我们再次看一下一个简单的getView()方法示例：

@Overridepublic View getView(int position, View convertView, ViewGroup parent) {    Fruit fruit = getItem(position);    View view;    if (convertView == null) {        view = LayoutInflater.from(getContext()).inflate(resourceId, null);    } else {        view = convertView;    }    ImageView fruitImage = (ImageView) view.findViewById(R.id.fruit_image);    TextView fruitName = (TextView) view.findViewById(R.id.fruit_name);    fruitImage.setImageResource(fruit.getImageId());    fruitName.setText(fruit.getName());    return view;}

第二个参数就是我们最熟悉的convertView呀，难怪平时我们在写getView()方法是要判断一下convertView是不是等于null，如果等于null才调用inflate()方法来加载布局，不等于null就可以直接利用convertView，因为convertView就是我们之间利用过的View，只不过被移出屏幕后进入到了废弃缓存中，现在又重新拿出来使用而已。然后我们只需要把convertView中的数据更新成当前位置上应该显示的数据，那么看起来就好像是全新加载出来的一个布局一样，这背后的道理你是不是已经完全搞明白了？

之后的代码又都是我们熟悉的流程了，从缓存中拿到子View之后再调用setupChild()方法将它重新attach到ListView当中，因为缓存中的View也是之前从ListView中detach掉的，这部分代码就不再重复进行分析了。

为了方便大家理解，这里我再附上一张图解说明：