探究ListView 的缓存机制

概述

ListView 是继承AbListView，AbListView是所有列表类控件的基类。

ListView的数据加载

在ListView数据加载中最关键的一个函数就是makeAndAddView(),这个函数的作用就获得一个ChildView并把该ChildView添加到List中，具体见源码分析：

private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
        boolean selected) {
    View child;//即ChildView

	//如果数据没有发生改变
    if (!mDataChanged) {
        //优先从循环器中获取该位置的视图
        // Try to use an existing view for this position
        child = mRecycler.getActiveView(position);
        if (child != null) {
            // Found it -- we're using an existing child
            //如果找到了就直接添加到List中
            // This just needs to be positioned
            setupChild(child, position, y, flow, childrenLeft, selected, true);

            return child;
        }
    }

	//如果数据发生了改变，则在该位置上新建一个视图，或者如果可能的话转换一个已经没有用的视图(可能是当整个ListView其他位置发生了变化，但是该位置的ChildView并未发生任何变化)
    // 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]);

	//返回该childView
    return child;
}

ListView的缓存机制

当ListView发生滑动操作时，若干已经加载的ChildView会被因滑动而被暂时隐藏掉，为了避免下次显示再重新加载，这时ListView的缓存机制就会被触发，即运行layoutChildren()函数(其实任何触碰事件都会触发，即onTouchEvent() -。-)。

那么ListView的缓存机制是依靠什么来缓存的呢？答案就是AbListView中 的内部类RecycleBin。关于RecycleBin的具体作用，源码中的注释已经解释的非常清楚了，在此就不在赘述。

 /**
 * 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.
 *... ...
 */

当需要缓存ActiveViews时会调用fillActiveViews()函数，该函数会把ListView中的所有ActiveViews 一次性都缓存起来。

	/**
     * Fill ActiveViews with all of the children of the AbsListView.
     * ... ...
     */
    void fillActiveViews(int childCount, int firstActivePosition) {
        if (mActiveViews.length < childCount) {
            mActiveViews = new View[childCount];
        }
        mFirstActivePosition = firstActivePosition;

        //noinspection MismatchedReadAndWriteOfArray
        final View[] activeViews = mActiveViews;
		... ...
    }

而对于ScrapViews则是调用的addScrapView()函数。

	/**
     * Puts a view into the list of scrap views.
     * <p>
     * If the list data hasn't changed or the adapter has stable IDs, views
     * with transient state will be preserved for later retrieval.
     *
     * @param scrap The view to add
     * @param position The view's position within its parent
     */
    void addScrapView(View scrap, int position) {
    ... ...
    // Don't scrap views that have transient state.
        final boolean scrapHasTransientState = scrap.hasTransientState();
        if (scrapHasTransientState) {
        //Transient状态
		... ...
		}else{
		//scrap状态
		... ...
		}
		... ...
    }

该函数中又分为两个不同的level，一个是transient瞬时态，另一个就是一般的普通态，关于这两个状态的区分我个人的想法是为了更加快速的获取ScrapViews，因为处于瞬时状态的view最有可能是接下来将要在界面上显示的View，毕竟你向上或向下滑动列表时目的就是这个，这一点在obtainView()函数中得到了体现：

View obtainView(int position, boolean[] isScrap) {
... ...
//优先获取TransientStateView
scrapView = mRecycler.getTransientStateView(position);
    if (scrapView == null) {
        scrapView = mRecycler.getScrapView(position);
    }
... ...
}

还有一个比较重要的函数就是scrapActiveViews()函数，它的作用是将目前所有的ActiveViews降级为ScrapViews，并将之前的所有ScrapViews清除。该函数在每次调用layoutChildern()函数时必定会被调用执行，目的就是为清空所有当前的ActiveViews，为新产生的ActiveViews做好准备。

    /**
     * Move all views remaining in mActiveViews to mScrapViews.
     */
    void scrapActiveViews() {
    ... ...
    //该函数确保mScrapViews的大小不会超过mActiveViews
    pruneScrapViews();
    }

结语

以上是阅读了ListView以及AbListView源码后的一些心得总结，毕竟阅读Android源码也才刚刚起步，还有很多地方理解的不是很透彻，上文若有理解不当之处欢迎各位指正。

查看原文:http://www.xyczero.com/blog/article/14/.