如何防止 Activity 关闭时添加 Fragment

Question

我正在创建一个Activity，它与Service 通信以通过POST 方法从互联网下载一些数据。为此，我使用Messenger。这是我的代码，让您更清楚：

我的onCreated() 方法：

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_comments);

    CommentsHandler commentsHandler = new CommentsHandler(this, savedInstanceState);
    Messenger messenger = new Messenger(commentsHandler);

    Intent serviceIntent = new Intent(this, WindowService.class);
    serviceIntent.putExtra("messenger", messenger);
    serviceIntent.putExtra("state", 888);
    serviceIntent.putExtra("number", getIntent().getStringExtra("number"));
    startService(serviceIntent);
}

我的Service 线程中的代码通过Messenger 对象将结果数据发布到Activity：

    /** ... **/
    Messenger messenger = intent.getParcelableExtra("messenger");
    /** ... **/
    Message resultMsg = this.obtainMessage();
    resultMsg.obj = jParser.getArrayList(); //This is an ArrayList of my downloaded data.
    messenger.send(resultMsg);

Activity 中的代码用于处理来自Service 的Message：

public static class CommentsHandler extends Handler {
    Bundle mSavedInstanceState;
    ActionBarActivity activity;

    public CommentsHandler(Activity a, Bundle savedInstanceState) {
        activity = (ActionBarActivity) a;
        mSavedInstanceState = savedInstanceState;
    }

    @Override
    public void handleMessage(Message msg) {
        comments = (ArrayList<HashMap<String, String>>) msg.obj;
        if (mSavedInstanceState == null && msg.arg1 != 793) {
            activity.getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new CommentsFragment()).commit();
        } else if (msg.arg1 == 793) { //793 is my preferred code to determine
                                      //if the internet connection could not be
                                      //established when the Service was trying
                                      //to download the data.
            activity.finish();
        }
    }
}

问题是：如果我在下载数据之前打开Activity并关闭它，则此代码.add(R.id.container, new CommentsFragment()).commit();会给我错误Can not perform this action after onSaveInstanceState，因为此代码仅在我的Service中的数据之后执行通过Messenger 对象处理和发送，但当时Activity 已被用户关闭，因此无法添加Fragment。如何解决这个问题？如何在添加Fragment 之前检查Activity 是否未关闭/正在关闭？或者，更好的是，如何停止在Activity 的onDestroy() 方法上运行该代码的线程，以便在Activity 关闭时不会执行它？提前致谢！

Answer 1

在您的活动中，您应该创建一个布尔值来检查活动是否可见：

public ActionBarActivity extends Activity {
  private boolean isActivityVisible = false;

  @Override
  protected void onResume(){
     isActivityVisible = true;
  }

  @Override
  protected void onPause(){
     isActivityVisible = false;
  }

  public boolean isVisible(){
     return this.isActivityVisible;
  }
}

然后你修改你的 Handler 类定义：

public static class CommentsHandler extends Handler {
Bundle mSavedInstanceState;
ActionBarActivity activity;

public CommentsHandler(Activity a, Bundle savedInstanceState) {
    activity = (ActionBarActivity) a;
    mSavedInstanceState = savedInstanceState;
}

@Override
public void handleMessage(Message msg) {
    // here you check if your activity is no longer visible and then break up
    if(activity == null || !activity.isVisible())
        return;

    comments = (ArrayList<HashMap<String, String>>) msg.obj;
    if (mSavedInstanceState == null && msg.arg1 != 793) {
        activity.getSupportFragmentManager().beginTransaction()
            .add(R.id.container, new CommentsFragment()).commit();
    } else if (msg.arg1 == 793) { //793 is my preferred code to determine
                                  //if the internet connection could not be
                                  //established when the Service was trying
                                  //to download the data.
        activity.finish();
    }
}
}

Answer 2

最小的变化是在 Activity 中有一个布尔字段，在 onResume() 中将其设置为 true，在 onPause() 中设置为 false，并检查其在 handleMessage() 中的值（即，如果标志为目前为假）。

另一个选项，而不是使用 Messenger 和 handleMessage()，使用 BroadcastReceiver 执行此操作。在onResume() 中注册接收器，在onPause() 中注销它。这样，来自服务的广播将被简单地忽略。

无论如何，两种解决方案基本相同，但广播有点“更高级别”。

这假设如果活动暂停，您对服务的结果不感兴趣。如果您是（例如，如果您退出应用程序并重新进入，并且需要显示更新），那么您应该将接收到的数据放在一个字段中并在以下onResume() 上处理它。

Answer 3

您的处理方式与我的处理方式不同，但使用您所拥有的，我会做出这些调整：

public static class CommentsHandler extends Handler {
    Bundle mSavedInstanceState;
    ActionBarActivity activity;

    public CommentsHandler(Activity a, Bundle savedInstanceState) {
        activity = (ActionBarActivity) a;
        mSavedInstanceState = savedInstanceState;
    }

    public void setActivity(Activity a){
        activity = a;
    }

    @Override
    public void handleMessage(Message msg) {
        if(activity == null){
            return;
        }
        comments = (ArrayList<HashMap<String, String>>) msg.obj;
        if (mSavedInstanceState == null && msg.arg1 != 793) {
            activity.getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new CommentsFragment()).commit();
        } else if (msg.arg1 == 793) { //793 is my preferred code to determine
                                      //if the internet connection could not be
                                      //established when the Service was trying
                                      //to download the data.
            activity.finish();
        }
    }
}

然后我会使用你的活动 onPause()/onResume() 方法来喜欢这个：

@Override
protected void onPause() {
    super.onPause();
    commentsHandler.setActivity(null);  
}

@Override
protected void onResume() {
    super.onResume();
    commentsHandler.setActivity(this);  
}