在这个简单的android代码中我缺少什么？

Question

I was trying to do something else but kept on removing code thinking that simplifying things will help in identifying my mistake. This is what I am left with (after a lot of simplification) and still I am not getting it why it is not doing it is supposed to!

public class MyOwnTimerActivity extends Activity {

    int timeLeft = 5;
    TextView timerTextView;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        Button timerButton = (Button) findViewById(R.id.button1);
        timerTextView = (TextView) findViewById(R.id.timerText);

        Thread timer = new Thread() {
            public void run() {
                try {
                    sleep(3000);
                } 
                catch (InterruptedException e) {
                    e.printStackTrace();
                }
                finally{
                    timerTextView.setText("something");
                }
            }
        };
        timer.start();
    }
}

Error: The application crashed after 3 seconds ie after timer goes off.

Answer 1

Ah, yes)) This is because you try to modify UI from non UI-thread

Update:

There are several ways to modify UI from another thread:

Using Handler class with overriding handleMessage() method

Using Activity class with calling runOnUI() method, passing there Runnable object in which run() method you modify UI

Using View.post() or View.postDelayed() methods

The last variant is the most convenient in your case. Modify your code in such manner:

 @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); Button timerButton = (Button) findViewById(R.id.button1); timerTextView = (TextView) findViewById(R.id.timerText); timerTextView.postDelayed( new Runnable() { @Override run() { timerTextView.setText("something"); } }, 3000); } 

Answer 2

You're not supposed to change UI from worker threads. Use a Handler, initialized on the main thread, with an ad-hoc Runnable to execute the setText() on the main thread.

Handler h = new Handler();
Thread timer = new Thread() { 
//...
    finally
    {
         h.post(new Runnable(){
             public void run()
             {
                 timerTextView.setText("something"); 
             }
         });
     }

In fact, the whole snippet can be rewritten using Handler.postDelayed(). But you'll probably want to do more interesting things with a thread eventually.

Answer 3

Have you tried using runOnUI to set the Text from the UI thread? You can do so like this:

runOnUiThread(new Runnable() { 
    public void run() 
    { 
        timerTextView.setText("something");
    } 
 }); 

Keep in mind, your reference to timerTextView and whatever text you want displayed would have to be modified as a final variable in order to reference it from another thread.