Question

我正在一个片段中实现一个dragListener，如下面的代码所示：

private class ChoiceDragListener implements View.OnDragListener {
    @Override
    public boolean onDrag(View v, DragEvent event) {
        View view = (View) event.getLocalState();
        TextView dropTarget = (TextView) v;
        if (view == null) {
            return false;
        }
        final TextView dropped = (TextView) view;
        switch (event.getAction()) {
            case DragEvent.ACTION_DRAG_STARTED:
                dropped.post(new Runnable() {
                    @Override
                    public void run() {
                        dropped.setAlpha(0);
                    }
                });
                //
                break;
// MORE CODE FOLLOWS

该实施在我的测试设备上以及大量大多数用户都可以使用。根据Google控制台，一小部分用户在行dropped.post(new Runnable(){上有一个NullPointException，但在我添加if (view == null) {return false;}之后消失了，这仍然是一个谜。

问题是，现在大约有相同数量的用户（在20.000个以上的用户中，少于10个）在View view = (View) event.getLocalState();上获得ClassCastException

对此我感到困惑，因为view只能是TextView。更不用说，如果这仅影响极少数用户，我怀疑那里还有其他东西，也许与我的代码无关。

还是，我想知道：

我的代码中有什么可以解释为什么（即使在极少数情况下）ClassCastException可能被抛出的原因？
有什么我可以添加的方法来帮助这几个用户体验？我有点不愿意使用try / catch而不了解发生了什么。

我检查了其他答案，例如 Android: classCastException on view holder Android 7.0 and 7.1 getApplication() ClassCastException，但它们似乎无关。

Logcat：

Samsung Galaxy S9 (starqltesq), Android 9

java.lang.ClassCastException: 
  at com.****.***.***.fragmentOne$ChoiceDragListener.onDrag (fragmentOne.java:1101)
  at android.view.View.callDragEventHandler (View.java:26053)
  at android.view.View.dispatchDragEvent (View.java:26044)
  at android.view.ViewGroup.notifyChildOfDragStart (ViewGroup.java:1827)
  at android.view.ViewGroup.dispatchDragEvent (ViewGroup.java:1674)
  at android.view.ViewGroup.notifyChildOfDragStart (ViewGroup.java:1827)
  at android.view.ViewGroup.dispatchDragEvent (ViewGroup.java:1674)
  at android.view.ViewGroup.notifyChildOfDragStart (ViewGroup.java:1827)
  at android.view.ViewGroup.dispatchDragEvent (ViewGroup.java:1674)
  at android.view.ViewGroup.notifyChildOfDragStart (ViewGroup.java:1827)
  at android.view.ViewGroup.dispatchDragEvent (ViewGroup.java:1674)
  at android.view.ViewGroup.notifyChildOfDragStart (ViewGroup.java:1827)
  at android.view.ViewGroup.dispatchDragEvent (ViewGroup.java:1674)
  at android.view.ViewGroup.notifyChildOfDragStart (ViewGroup.java:1827)
  at android.view.ViewGroup.dispatchDragEvent (ViewGroup.java:1674)
  at android.view.ViewGroup.notifyChildOfDragStart (ViewGroup.java:1827)
  at android.view.ViewGroup.dispatchDragEvent (ViewGroup.java:1674)
  at android.view.ViewGroup.notifyChildOfDragStart (ViewGroup.java:1827)
  at android.view.ViewGroup.dispatchDragEvent (ViewGroup.java:1674)
  at android.view.ViewGroup.notifyChildOfDragStart (ViewGroup.java:1827)
  at android.view.ViewGroup.dispatchDragEvent (ViewGroup.java:1674)
  at android.view.ViewGroup.notifyChildOfDragStart (ViewGroup.java:1827)
  at android.view.ViewGroup.dispatchDragEvent (ViewGroup.java:1674)
  at android.view.ViewGroup.notifyChildOfDragStart (ViewGroup.java:1827)
  at android.view.ViewGroup.dispatchDragEvent (ViewGroup.java:1674)
  at android.view.ViewRootImpl.handleDragEvent (ViewRootImpl.java:7423)
  at android.view.ViewRootImpl.access$2100 (ViewRootImpl.java:198)
  at android.view.ViewRootImpl$ViewRootHandler.handleMessage (ViewRootImpl.java:4984)
  at android.os.Handler.dispatchMessage (Handler.java:106)
  at android.os.Looper.loop (Looper.java:214)
  at android.app.ActivityThread.main (ActivityThread.java:6981)
  at java.lang.reflect.Method.invoke (Native Method)
  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run (RuntimeInit.java:493)
  at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:1445)

如果您需要更多详细信息或代码，我很乐意提供更多背景信息。

为什么event.getLocalState（）仅很少抛出ClassCastException？

0 个答案: