Android Looper and Toast from WorkerThread

Have you ever tried to launch Android Toast message from worker thread? Probably you are wondering why the heck it is giving this error-

java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()

In this article we are going to explore reason behind the above exception and try to understand how Looper works in Android. At the end, I am going to explain one approach to run Toast from a worker thread, but before that we need to understand how Looper works. If you already know in/out of Looper, you can skip below section and directly move to the solution part.

Looper is a very basic wrapper class which attach a MessageQueue to a Thread and manage this queue. MessageQueue is a structure to sequentialize simultaneous processing requests of a Thread.  In Android, message/request processing classes like Handler uses Looper to manage their respective MessageQueue.
Looper = Thread + MessageQueue

Android Looper Life Cycle:

As you can see in the above figure, Looper Life cycle starts when we call prepare(), this static method creates instance of Looper class and store this in a ThreadLocal variable. Below code snippet from Looper source.

private static void prepare(boolean quitAllowed) {
    if (sThreadLocal.get() != null) {
        throw new RuntimeException("Only one Looper may be created per thread");
    }
    sThreadLocal.set(new Looper(quitAllowed));
}

private Looper(boolean quitAllowed) {
    mQueue = new MessageQueue(quitAllowed);
    mRun = true;
    mThread = Thread.currentThread();
}

Once Looper instance is created we have to call loop() which starts an infinite Loop and process requests accumulated in the Message Queue. Below code from Looper.java

public static void loop() {

// Looper instance and Thread verification

    final MessageQueue queue = me.mQueue;

    for (;;) {
        Message msg = queue.next(); // might block
        if (msg == null) {
            // No message indicates that the message queue is quitting.
            return;
        }
        // Process Messages
    }
}

Due to above Infinite Loop, Looper blocks the current Thread execution until we call Quit. To get a hook to know when Looper is done with Message processing or when Message Queue is empty, we need to register MessageQueue.IdleHandler listener to the MessageQueue associated to the respective Looper. This can be achieved as-

// Prepare looper
Looper.prepare();
                 
// Register Queue listener hook
MessageQueue queue = Looper.myQueue();
queue.addIdleHandler(new IdleHandler() {
@Override
     public boolean queueIdle() {
          // TODO Auto-generated method stub
          return false;
     }
});
                 
// Start looping Message Queue
Looper.loop();

Running Toast from Worker Thread
Now, lets explain how we can run Toast in a thread other than UI or Main Thread. If we look inside of Toast.java, when we call makeText() internally it initialize Handler on the Calling thread. So, if the calling thread has not yet been attached to a MessageQueue Toast fails to create its instance. The only way to attach MessageQueue to a Thread is to use Looper, as a result Toast.java throws below exception-

java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()

Toast.java -> Toast.TN.java -> Handler.java

public Toast(Context context) {
        mContext = context;
        mTN = new TN();
        mTN.mY = context.getResources().getDimensionPixelSize(
                com.android.internal.R.dimen.toast_y_offset);
}

public Handler() {
        // .......
        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = null;
}

So, I hope we now understand the real meaning of this famous error message. Lets now try an alternate approach to execute Toast in a separate Thread.

We need to initialize Looper and attach MessageQueue to our calling Thread prior to launch Toast. When we call show() method of Toast, it sends Handler message to process display logic of Toast, then it waits for defined time (LONG/SHORT) and then calls hide() which again sends Handler message to remove Toast View. Thus Looper associated to Toast is requesting for MessageQueue twice, so we'll keep a counter to track number of MessageQueue requests and when it is 2, we'll quit the Looper. It is really important to quit the Looper, as if we don't do so, it will block all further operations, since Looper.loop() is a blocking call.

Here is the code snippet to run Toast in a separate Thread-

public void aboutLooper() {
 Thread th = new Thread() {
      public void run() {
          System.out.println("Start Looper...");
          // Prepare looper
          Looper.prepare();

          // Register Queue listener hook
          MessageQueue queue = Looper.myQueue();
          queue.addIdleHandler(new IdleHandler() {
               int mReqCount = 0;

               @Override
               public boolean queueIdle() {
                   if (++mReqCount == 2) {
                        // Quit looper
                        Looper.myLooper().quit();
                        return false;
                   } else
                        return true;
               }
          });

          // Show Toast- will be called when Looper.loop() starts
          Toast.makeText(MainActivity.this, "Hey there!!",
                   Toast.LENGTH_LONG).show();
          // Start looping Message Queue- Blocking call
          Looper.loop();
          System.out.println("It appears after Looper.myLooper().quit()");
      }
 };
 th.start();
}