android - 获取第一组Expandable ListView的View句柄

Question

在 Activity 中，如何访问 Expandable ListView 的第一个组 View ？

这是我正在做的:

public int getFirstVisibleGroup() {
        LogUtil.i(TAG, "getFirstVisibleGroup called");
        int firstVis = listView.getFirstVisiblePosition();
        LogUtil.i(TAG, "firstVis = " + firstVis);
        long packedPosition = listView.getExpandableListPosition(firstVis);
        LogUtil.i(TAG, "packedPosition = " + packedPosition);
        LogUtil.i(TAG, "firstVisibleGroup = " + ExpandableListView.getPackedPositionGroup(packedPosition));
        return ExpandableListView.getPackedPositionGroup(packedPosition);
    }

    public View getGroupView(ExpandableListView listView, int groupPosition) {
        LogUtil.i(TAG, "getGroupView called");
        int flatPosition = listView.getFlatListPosition(groupPosition);
        LogUtil.i(TAG, "flatPosition = " + flatPosition);
        int first = getFirstVisibleGroup();
        LogUtil.i(TAG, "first = " + first);
        LogUtil.i(TAG, "returning child at position " + (flatPosition - first));
        return listView.getChildAt(flatPosition - first);
    }

我这样调用它:

View view = getGroupView(listView, 0);

最终它变成了listView.getChildAt(0)。返回的 view 为 null。

正确的做法是什么？

Answer 1

所有基于适配器的 View (ListView、GridView、RecyclerView)仅在 View 布局到屏幕后才将 View 添加到自身。这样他们就可以计算出适当的大小并查询足够的 subview 。

因此，在 onCreate 期间，您将永远不会有任何 View 。这意味着如果您想与其某些 subview 进行交互，则必须稍后发生。

一种合适的方法是使用OnPreDraw 监听器。那是在系统调用 View 上的 draw(canvas) 之前。示例:

public MyActivity extends Activity implements ViewTreeObserver.OnPreDrawListener {

    @Override
    public void onCreate(bundle){
        ... build your layout and your listview

        // during onCreate you add a PreDrawListener
        rootView.getViewTreeObserver().addOnPreDrawListener(this);
    }

    @Override
    public void onPreDraw() {

        ... do your logic here !!!


        rootView.getViewTreeObserver().removeOnPreDrawListener(this); // remove itself, you only need the fist pass
        return true; // must return true, or else the system won't draw anything.
    }

}