ListView 缓存机制源码浅析

来源：互联网发布：同和行知职业技术学校编辑：程序博客网时间：2024/06/08 07:33

ListView的缓存机制是通过RecycleBin实现对View的缓存实现的。缓存机制大大的提高了View的复用率，这也是为什么ListView可以加载大量的Item也不会OOM的原因。当然了，为了进一步提高性能我们都会自定义一个ViewHolder来避免不必要的findViewById操作，这也是RecycleView与ListView的区别之一<RecycleView缓存的是ViewHolder>。RecycleBin是ListView/GridView的父类AbsListView的内部类，这说明ListView/GridView的缓存机制大致是相同的。

一、概览

当item view 在屏幕之外，RecycleBin将其缓存起来，当需要新的itemView，除非RecycleBin中没有缓存，否则就直接从缓存中取，还记得在getView中有一个参数convertView就是从RecycleBin中取出来的。

二、RecycleBin

由上图我们可以看大，RecycleBin主要通过两个集合：View[] mActiveViews和ArrayList<View>[] mScrapViews来实现的。

下面看一下RecycleBin中需要重点关注的部分：

private View[] mActiveViews：缓存当前屏幕可以看到的item view;这个数组是为了防止多次layout带来的性能即数据重复问题而存在的。因为多次layout会导致ListView重复多次加载当前的屏幕显示的item views，为了避免数据重复以及多次实例化item views带来的资源浪费，有必要将其缓存起来。
private int mViewTypeCount：item view的种类；
private ArrayList<View>[] mScrapViews:注意到这是一个ArrayList<View>数组，之所以是一个数组，是因为item view的种类可能大于一，而mScrapViews:就是缓存每种item View的。还记得BaseAdapter里面有一个getViewTypeCount（）方法吗？这个方法的返回值就是mViewTypeCount。
private ArrayList<View> mCurrentScrap：当前种类的item view的缓存。

public void setViewTypeCount(int viewTypeCount)：设置列表item的种类；
void fillActiveViews(int childCount, int firstActivePosition)：将当前屏幕可以看到的item view添加到mActiveViews中；
View getActiveView(int position)：根据position获取mActiveViews中的item view;
void addScrapView(View scrap, int position):将废弃的（通常是滚出屏幕外的）item view缓存起来；简单的看一下addSrapView的源码：

void addScrapView(View scrap, int position){                .......                if (mViewTypeCount == 1) {                    mCurrentScrap.add(scrap);                } else {                    mScrapViews[viewType].add(scrap);                }                if (mRecyclerListener != null) {                    mRecyclerListener.onMovedToScrapHeap(scrap);                }            }

这验证了我们对mScrapViews的分析；
View getScrapView(int position):根据position的值获取一个废弃的item view用于重用；

三、详细缓存流程

ListView是ViewGroup的子类，遵循自定义View的基本步骤，measure、layout、draw。缓存部分我们重点分析layout过程。下面看一下ListView的layoutChildren部分源码：

  // 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();

第一次layout的时候，由于数据还没有显示，仍然有Adapter管理，并没有填充到ListView中，因此，childCount等于0。datasetchanged只有在数据源发生变化的情况下才会是true，其他的都是false。因此，会执行recycleBin.fillActiveViews(childCount, firstPosition);，但是此时childCount等于0，因此并不会向mActiveViews添加任何东西。

继续向下看，mLayoutMode代表了布局模式，默认情况下是LAYOUT_NORMAL，因此会走default语句中。因为childCount是0，并且默认的顺序从上到下的，因此会执行fillFromTop -- 执行该函数就可以将Adapter中的数据填充到ListView中了，填充完成之后，childCount就大于0了。下面看一下fillFromTop的源码：

 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);    }

可以看到fillFromTop调用了fillDown方法。

   private View fillDown(int pos, int nextTop) {        View selectedView = null;        int end = (mBottom - mTop);        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {            end -= 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++;        }        setVisibleRangeHint(mFirstPosition, mFirstPosition + getChildCount() - 1);        return selectedView;    }

可以看到View的产生是通过makeAndAddView进行的。

  private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,            boolean selected) {        View child;        if (!mDataChanged) {            // Try to use an existing 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;    }

由于mRecycler.getActiveView返回null，因此，最终makeAndAddView又调用了obtainView生成View的。下面看一下obtainView的部分源码：

......        final View scrapView = mRecycler.getScrapView(position);        final View child = mAdapter.getView(position, scrapView, this);        if (scrapView != null) {            if (child != scrapView) {                // Failed to re-bind the data, return scrap to the heap.                mRecycler.addScrapView(scrapView, position);            } else {                isScrap[0] = true;                // Finish the temporary detach started in addScrapView().                child.dispatchFinishTemporaryDetach();            }        }......return child;

看到这一句是不是很熟悉，final View child = mAdapter.getView(position, scrapView, this);没错，这就是我们自定义adapter的时候，重写的getView方法。由于第一次layout的时候，mRecycleView还没有缓存，因此scrapView是null，也就是说此时我们填充ListView都是通过getView产生的。

下面看一下第二次layout的时候，会发生什么

// 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();

第二次layout开始，会执行我们所说的fillActiveViews过程，因此此时childCount不再是0，所以当前屏幕显示的item view会被缓存到mActiveViews中。紧接着执行了detachAllViewsFromParent（）方法，该方法清空了所有的view，这是为了避免向下执行填充之后出现数据重复。之后的执行逻辑与前面分析的相似了，mActiveViews中缓存的View会在makeAndAddView中使用。
滑动过程中，会产生大量的Item View，RecycleBin也发挥了重要的作用。这也是ListView不会OOM的原因。

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