獲取第一組Expandable ListView視圖的句柄

Question

在Activity中，如何獲得第一組視圖Expandable ListView？

下面是我在做什麼：

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); 
    } 

而且我把它叫做：

View view = getGroupView(listView, 0); 

最終成爲listView.getChildAt(0)。並且返回的view爲空。

什麼是正確的做法？

Answer 1

所有基於適配器的視圖（ListView，GridView，RecyclerView）僅在屏幕上佈置完成後纔將視圖添加到自身。以便他們可以計算適當的大小並查詢足夠的子視圖。

因此，在onCreate期間，您永遠不會有任何意見。這意味着如果您想與其子視圖進行交互，那麼必須稍後再進行。

一個合適的方法是使用OnPreDraw偵聽器。這是在系統調用draw(canvas)之前的觀點。例如：

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. 
    } 

}