How to display Toast from a Service after main Act

问题:

UPDATE: I don't agree that this is a duplicate - because I am seeking for a way to exit the main app and still show a Toast from the service.

In a very simple test app I have 2 buttons:

Clicking any of the buttons will run a service with a corresponding action string ("open" or "flash") -

OpenActivity.java:

public class OpenActivity extends Activity {
    private Intent mServiceIntent;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_open);
        mServiceIntent = new Intent(this, RegionService.class);
   }

    public void openCar(View v) {
        mServiceIntent.setAction("open");
        startService(mServiceIntent);
    }

RegionService.java:

public class RegionService extends IntentService {
    private static final String TAG = "RegionService";

    @Override
    protected void onHandleIntent(Intent intent) {
        Log.d(TAG, "Received an intent: " + intent);
        String action = intent.getAction();
        Log.d(TAG, "Received an action: " + action);

        if(action.equals("open")) {
            Toast.makeText(this, 
                    getString(R.string.car_opened), 
                    Toast.LENGTH_SHORT).show();
        } 

Unfortunately my app crashes with:

D/RegionService(24506): Received an intent: Intent { act=open cmp=de.afarber.mynotification/.RegionService }

D/RegionService(24506): Received an action: open

W/MessageQueue(24506): Handler (android.os.Handler) {422768a8} sending message to a Handler on a dead thread
W/MessageQueue(24506): java.lang.RuntimeException: Handler (android.os.Handler) {422768a8} sending message to a Handler on a dead thread
W/MessageQueue(24506):  at android.os.MessageQueue.enqueueMessage(MessageQueue.java:320)
W/MessageQueue(24506):  at android.os.Handler.enqueueMessage(Handler.java:626)
W/MessageQueue(24506):  at android.os.Handler.sendMessageAtTime(Handler.java:595)
W/MessageQueue(24506):  at android.os.Handler.sendMessageDelayed(Handler.java:566)
W/MessageQueue(24506):  at android.os.Handler.post(Handler.java:326)
W/MessageQueue(24506):  at android.widget.Toast$TN.hide(Toast.java:370)
W/MessageQueue(24506):  at android.app.ITransientNotification$Stub.onTransact(ITransientNotification.java:54)
W/MessageQueue(24506):  at android.os.Binder.execTransact(Binder.java:412)
W/MessageQueue(24506):  at dalvik.system.NativeStart.run(Native Method)

Being an Android programming newbie I wonder How to display a Toast from Service in a correct way?

I think I've already seen Toasts at Android Home (i.e. there was no Activity on the device screen and still there were Toasts).

My background: I would like to monitor a beacon device from my service and show some text Toasts - even when my app has been closed.

回答1:

OnHandleIntent will run in a differant Thread so you are showing Toast in a thread which is not allowed in android

so change your code like this

Handler handler = new Handler(Looper.getMainLooper());
handler.post(new Runnable() {

    @Override
    public void run() {
         Toast.makeText(getApplicationContext(), 
                       getString(R.string.car_opened), 
                       Toast.LENGTH_SHORT).show();              
    }
});

From this dead thread in service

IntentService will create a thread to handle the new intent, and terminated it immediately once the task has done. So, the Toast will be out of controlled by a dead thread.

You should see some exceptions in the console when the toast showing on the screen.

回答2:

An IntentService has a few limitations:

It can't interact directly with your user interface. To put its results in the UI, you have to send them to an Activity.

Everything is happening in the background thread and not on the UI thread, so you need a different way as shown below:

@Override 
public void onCreate() { 
    super.onCreate(); 
    mHandler = new Handler(); 
} 

@Override 
protected void onHandleIntent(Intent intent) {
    mHandler.post(new Runnable() {            
        @Override 
        public void run() { 
            Toast.makeText(MyIntentService.this, "Hello Toast!", Toast.LENGTH_LONG).show();                
        } 
    }); 
} 

Source: Toast created in an IntentService never goes away

回答3:

OnHandleIntent is called on a background thread, and any attempt to touch the UI will result in a crash. Use an Handler to post a Runnable on the UI Thread to show your toast.

private class MyRunnable implements Runnable {

   final int mTextId = -1; 
   final Context mContext;
   public MyRunnable(Context c, int textId) {
       mTextId = textId;
       mContext = c;
   }

   @Override
   public void run() {
       Toast.makeText(mContext, 
           getString(mTextId), 
           Toast.LENGTH_SHORT).show();             
    }
}

   Handler handler = new Handler();
   handler.post(new MyRunnable(this, R.string.car_opened));

回答4:

use the following code:

  runOnUiThread(new Runnable(){
     public void run() {
          // UI code goes here
     }
    });

回答5:

This problem because of not running the Toast from the main_thread, and to overcome that, when creating the Activity save it's context from onCreate() method:

public static Context ctx;

// the method responsible for running the MainActivity
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        ctx = this;
}

then, in the service add a handler and run a thread from it(as the Handler is executed through the main Thread):

    Handler handler = new Handler(Looper.getMainLooper());
    handler.post(new Runnable() {
        @Override
        public void run() {
            Toast.makeText(OpenActivity.ctx, getString(R.string.car_opened),
                    Toast.LENGTH_SHORT).show();
        }
    });