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 positionchild = 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 positionedsetupChild（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 possiblechild = obtainView（position, mIsScrap）;// This needs to be positioned and measuredsetupChild（child, position, y, flow, childrenLeft, selected, mIsScrap[0]）;//返回该childViewreturn 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 MismatchedReadAndWriteOfArrayfinal 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TransientStateViewscrapView = 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mActiveViewspruneScrapViews（）;}
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