有人可以解释descendantFocusability = afterDescendants吗？

Question

我很难理解descendantFocusability。特别是afterDescendants。

有人可以告诉我什么时候这会有用吗？

Answer 1

定义ViewGroup及其后代之间的关系，当寻找View来关注。

必须是以下常数值之一。

 
+------------------------------------------------------------------------------------------+ 
|  Constant   Value   Description         | 
+------------------------------------------------------------------------------------------+ 
| afterDescendants   1   The ViewGroup will get focus only if    | 
|          none of its descendants want it.     | 
+------------------------------------------------------------------------------------------+ 
| beforeDescendants   0   The ViewGroup will get focus before    | 
|          any of its descendants.       | 
+------------------------------------------------------------------------------------------+ 
| blocksDescendants   2   The ViewGroup will block its descendants from  | 
|          receiving focus.         | 
+------------------------------------------------------------------------------------------+ 

您可以查看完整的示例here。

的片段是：

public void onItemSelected(AdapterView<?> parent, View view, 
     int position, long id) { 
    ListView listView = getListView(); 
    Log.d(TAG, "onItemSelected gave us " + view.toString()); 
    Button b = (Button) view.findViewById(R.id.button); 
    EditText et = (EditText) view.findViewById(R.id.editor); 
    if (b != null || et != null) { 
     // Use afterDescendants to keep ListView from getting focus 
     listView.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS); 
     if(et!=null) et.requestFocus(); 
     else if(b!=null) b.requestFocus(); 
    } else { 
     if (!listView.isFocused()) { 
      // Use beforeDescendants so that previous selections don't re-take focus 
      listView.setDescendantFocusability(ViewGroup.FOCUS_BEFORE_DESCENDANTS); 
      listView.requestFocus(); 
     } 
    } 

} 

按照上面的片段，afterDescendants用于防止listview从得到焦点，因此，要么EditText或Button可以请求焦点。