Adapter的The content of the adapter has changed问题分析

为了更好的理解这个问题出现的真正原因，建议首先看看下面两篇文章：

浅析notifyDataSetChanged内部工作流程

ListView中requestLayout执行流程解析

综合上面两篇文章，我们可以把整个执行过程用下图展示出来。这个图非常的关键，一定要保证在看下文的时候已经理解了上图的过程。

下面看看错误提示。
07-28 17:22:02.162: E/AndroidRuntime(16779): java.lang.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. Make sure your adapter calls notifyDataSetChanged() when its content changes. [in ListView(2131034604, class android.widget.ListView) with Adapter(class com.nodin.sarah.HeartListAdapter)]

这个问题就是当我们调用notifyDataSetChanged()的过程中，数据源可能发生了改变，这样出现前后数据源不一致而报错，可能这样说还是不好理解，下面我们可以从源码的角度来进行分析。

从浅析notifyDataSetChanged内部工作流程文章中我们需要理出以下两点：

当执行notifyDataSetChanged()，它内部做了两件事情,具体的代码在AdapterDataSetObserver的onChanged方法里面：
1、mDataChanged = true;
mOldItemCount = mItemCount;
mItemCount = getAdapter().getCount();
2、requestLayout();

可以看到做完这两件事情后，它进入了requestLayout()方法中，requestLayout方法中做了什么事情，这里就不细说，看看ListView中requestLayout执行流程解析这篇文章应该就知道了，从上图可以知道，requestLayout内部其实就是执行了performTraversals(),performTraversals()内部执行的就是measure,layout,draw方法，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大部分的功能其实都是在onLayout()方法中进行的了,所以我重点关注到onLayout()方法，从图知道它执行了layoutChildren()方法。

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,我们来跟跟源码：

@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();    }    if (mFastScroller != null && (mItemCount != mOldItemCount || mDataChanged)) {        mFastScroller.onItemCountChanged(mItemCount);    }    //这个方法是重点    layoutChildren();    mInLayout = false;    mOverscrollMax = (b - t) / OVERSCROLL_LIMIT_DIVISOR;}

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。重点调用了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。

@Overrideprotected void layoutChildren() {    final boolean blockLayoutRequests = mBlockLayoutRequests;    if (blockLayoutRequests) {        return;    }    mBlockLayoutRequests = true;    try {        super.layoutChildren();        invalidate();        if (mAdapter == null) {            resetList();            invokeOnItemScrollListener();            return;        }        final int childrenTop = mListPadding.top;        final int childrenBottom = mBottom - mTop - mListPadding.bottom;        final int childCount = getChildCount();        int index = 0;        int delta = 0;        View sel;        View oldSel = null;        View oldFirst = null;        View newSel = 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. Make sure your adapter calls notifyDataSetChanged() "                    + "when its content changes. [in ListView(" + getId() + ", " + getClass()                    + ") with Adapter(" + mAdapter.getClass() + ")]");        }        setSelectedPositionInt(mNextSelectedPosition);        // Remember which child, if any, had accessibility focus.        final int accessibilityFocusPosition;        final View accessFocusedChild = getAccessibilityFocusedChild();        if (accessFocusedChild != null) {            accessibilityFocusPosition = getPositionForView(accessFocusedChild);            accessFocusedChild.setHasTransientState(true);        } else {            accessibilityFocusPosition = INVALID_POSITION;        }        // Ensure the child containing focus, if any, has transient state.        // If the list data hasn't changed, or if the adapter has stable        // IDs, this will maintain focus.        final View focusedChild = getFocusedChild();        if (focusedChild != null) {            focusedChild.setHasTransientState(true);        }        // Pull all children into the RecycleBin.        // These views will be reused if possible        final int firstPosition = mFirstPosition;        final RecycleBin recycleBin = mRecycler;        if (dataChanged) {            for (int i = 0; i < childCount; i++) {                recycleBin.addScrapView(getChildAt(i), firstPosition+i);            }        } else {            recycleBin.fillActiveViews(childCount, firstPosition);        }        // Clear out old views        detachAllViewsFromParent();        recycleBin.removeSkippedScrap();        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) {            final boolean shouldPlaceFocus = mItemsCanFocus && hasFocus();            final boolean maintainedFocus = focusedChild != null && focusedChild.hasFocus();            if (shouldPlaceFocus && !maintainedFocus && !sel.hasFocus()) {                if (sel.requestFocus()) {                    // Successfully placed focus, clear selection.                    sel.setSelected(false);                    mSelectorRect.setEmpty();                } else {                    // Failed to place focus, clear current (invalid) focus.                    final View focused = getFocusedChild();                    if (focused != null) {                        focused.clearFocus();                    }                    positionSelector(INVALID_POSITION, sel);                }            } else {                positionSelector(INVALID_POSITION, sel);            }            mSelectedTop = sel.getTop();        } else {            // If the user's finger is down, select the motion position.            // Otherwise, clear selection.            if (mTouchMode == TOUCH_MODE_TAP || mTouchMode == TOUCH_MODE_DONE_WAITING) {                final View child = getChildAt(mMotionPosition - mFirstPosition);                if (child != null)  {                    positionSelector(mMotionPosition, child);                }            } else {                mSelectedTop = 0;                mSelectorRect.setEmpty();            }        }        if (accessFocusedChild != null) {            accessFocusedChild.setHasTransientState(false);            // If we failed to maintain accessibility focus on the previous            // view, attempt to restore it to the previous position.            if (!accessFocusedChild.isAccessibilityFocused()                && accessibilityFocusPosition != INVALID_POSITION) {                // Bound the position within the visible children.                final int position = MathUtils.constrain(                        accessibilityFocusPosition - mFirstPosition, 0, getChildCount() - 1);                final View restoreView = getChildAt(position);                if (restoreView != null) {                    restoreView.requestAccessibilityFocus();                }            }        }        if (focusedChild != null) {            focusedChild.setHasTransientState(false);        }        mLayoutMode = LAYOUT_NORMAL;        mDataChanged = false;        if (mPositionScrollAfterLayout != null) {            post(mPositionScrollAfterLayout);            mPositionScrollAfterLayout = null;        }        mNeedSync = false;        setNextSelectedPositionInt(mSelectedPosition);        updateScrollIndicators();        if (mItemCount > 0) {            checkSelectionChanged();        }        invokeOnItemScrollListener();    } finally {        if (!blockLayoutRequests) {            mBlockLayoutRequests = false;        }    }}

在上面代码中，我们一眼就看出了报错的地方：

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. Make sure your adapter calls notifyDataSetChanged() "                + "when its content changes. [in ListView(" + getId() + ", " + getClass()                + ") with Adapter(" + mAdapter.getClass() + ")]");    }

原来错误是从这里报处理的，那什么时候会报错呢？我们看看上面的判断条件：
mItemCount != mAdapter.getCount()。

在上面我们说过notifyDataSetChanged干了两件事，其中的第一件事就是给mItemCount赋值。可以回过头去看看mItemCount = getAdapter().getCount();

现在我们应该已经知道了，当我们调用了notifyDataSetChanged之后，说明数据源已经发生了变化，所以它会重新获取到Adapter里面的count赋值给mItemCount,接着就是执行重新布局，如果我们在给mItemCount赋值之后到执行上面的这个判断之间再一次修改了数据源，那么当执行到上面的这个判断的时候，就会出现mAdapter.getCount()获取的count数是新的数据源的count,跟之前存取mItemCount不一致，这样就会抛出这个异常。也就是说当我们mItemCount赋值之后，如果在执行if (mItemCount != mAdapter.getCount())之前又修改了数据源，就会出现异常。

这个异常一般出现的具体场合，参考下面文章：
关于Adapter的The content of the adapter has changed问题分析

ListView/Adapter IllegalStateException

参考文章：
Android ListView工作原理完全解析，带你从源码的角度彻底理解