关于Android Activity 调用onDestroy 后可以接收KeyEvent 吗？为什么 mapLayout 为空？答案

【问题标题】：Can receive KeyEvent after onDestroy called about Android Activity? Why mapLayout is null?关于Android Activity 调用onDestroy 后可以接收KeyEvent 吗？为什么 mapLayout 为空？
【发布时间】：2017-04-04 16:13:08
【问题描述】：

mapLayout 在onCreate 初始化并在onDestroy 释放。为什么mapLayout 是null 而onNewIntent 和dispatchKeyEvent？

onNewIntent有NullPointException

java.lang.NullPointerException: Attempt to invoke virtual method 'void MapLayout.clearFocusedOverlay()' on a null object reference
at com.xxx.activity.MainActivity.onNewIntent(MainActivity.java:486)
    at android.app.Instrumentation.callActivityOnNewIntent(Instrumentation.java:1210)
    at android.app.ActivityThread.deliverNewIntents(ActivityThread.java:2438)

这是堆栈跟踪。

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean MapLayout.hasFocusedOverlay()' on a null object reference
        at com.xxx.activity.MainActivity.dispatchKeyEvent(MainActivity.java:794)
        at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:2389)
        at ...

这是我的代码

public class MainActivity extends Activity {

  private static MapLayout mapLayout;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);    
     mapLayout = (MapLayout) findViewById(R.id.map_container);
  }
  @Override
  protected void onNewIntent(Intent intent) {
    super.onNewIntent(intent);
    setIntent(intent);
    mapLayout.clearFocusedOverlay()
  }
  @Override
  protected void onDestroy() {
    Log.i(TAG, "onDestroy");
    mapLayout = null;
    super.onDestroy();
  }

  @Override
  public boolean dispatchKeyEvent(KeyEvent event) {
    Log.i(TAG, "dispatchKeyEvent: "+event);
    if (event.getAction() == KeyEvent.ACTION_DOWN 
          && event.getKeyCode() == KeyEvent.KEYCODE_BACK) {
        if (mapLayout.hasFocusedOverlay()) {
                mapLayout.clearFocusedOverlay();
                mapLayout.mapView.invalidate();
        } else {
                Intent i = new Intent();
                i.setAction(Intent.ACTION_MAIN);
                i.addCategory(Intent.CATEGORY_HOME);
                startActivity(i);
        }
        return true;
    }
    return super.dispatchKeyEvent(event);
  }
}

【问题讨论】：

标签： android android-activity nullpointerexception

【解决方案1】：

因为 私有静态 MapLayout mapLayout; 当 'MainActivity' 有多个实例时。如果 'MainActivity' 的一个实例销毁，则 'mapLayout' 变为 'null'。

所以当 'MainActivity' 的其他实例使用 'mapLayout' 时会导致 NullPointException。

【讨论】：