Question

Well, please don’t ask me why, but I need to start a synchronous and blocking activity so that the program flow won’t continue until it’s finished. I know how to do a synchronous Dialog, but how can I do a synchronous activity?

Following are two approaches I tried but failed:

// In the 1st activity, start the 2nd activity in the usual way
startActivity(intent);
Looper.loop();        // but pause the program here
// Program continuse running afer Looper.loop() returns
....

// Then, in the second activity's onBackPressed method:
public void onBackPressed() {
    // I was hoping the following quit() will terminate the loop() call in
    // the first activity. But it caused an exception "Main thread not allowed
    // to quit." Understandable.
    new Hanlder().getLooper().quit();
}

I also tried to use another thread to achieve this:

// In the first activity, start a thread
SecondThread m2ndThread = new SecondThread(this);
Thread th = new Thread(m2ndThread, "Second Thread");
th.run();
synchronized(m2ndThread) {
    m2ndThread.wait();
}

class SecondThread implements Runnable {

    Activity m1stActivity;

    SecondThread(Activity a) {
        m1stActivity = a;
    }

    public void run() {
        Looper.prepare();
        Handler h = new Handler() {
            public void handleMessage() {
                Intent intent = new Intent(m1stActivity, SecondActivity.class);
                m1stActivity.startActivity(intent); // !!!!!!!!!!!!!!
            }
        }
        h.sendEmptyMessage(10); // let it run
        Looper.quit();
    }
}

However, this approach doesn’t work because when I’m using the 1st activity to start the 2nd activity, the main thread is already in wait state and doing nothing. So the 2nd activity didn’t even get created. (This is ironical: You have to use an activity to start an activity, but how can you do that when it’s already in wait state?)

Answer 1

You can’t.

That is all there is to it. You just can’t do this. Activities execute on the main thread, always. Your main thread must be actively running its event loop. It is ALWAYS broken to block the main thread’s event loop. Always.