Update UI from Thread in Android

I want to update my UI from a Thread which updates a Progressbar.开发者_开发知识库 Unfortunately, when updating the progressbar's drawable from the "runnable" the progressbar disappears! Changing the progressbars's drawable in onCreate() on the otherside works!

Any Suggestions?

public void onCreate(Bundle savedInstanceState) {
    res = getResources();
    super.onCreate(savedInstanceState);
    setContentView(R.layout.gameone);
    pB.setProgressDrawable(getResources().getDrawable(R.drawable.green)); //**Works**/
    handler.postDelayed(runnable, 1);       
}

private Runnable runnable = new Runnable() {
    public void run() {  
        runOnUiThread(new Runnable() { 
            public void run() 
            { 
                //* The Complete ProgressBar does not appear**/                         
                pB.setProgressDrawable(getResources().getDrawable(R.drawable.green)); 
            } 
        }); 
    }
}

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You should do this with the help of AsyncTask (an intelligent backround thread) and ProgressDialog

AsyncTask enables proper and easy use of the UI thread. This class allows to perform background operations and publish results on the UI thread without having to manipulate threads and/or handlers.

An asynchronous task is defined by a computation that runs on a background thread and whose result is published on the UI thread. An asynchronous task is defined by 3 generic types, called Params, Progress and Result, and 4 steps, called begin, doInBackground, processProgress and end.

The 4 steps

When an asynchronous task is executed, the task goes through 4 steps:

onPreExecute(), invoked on the UI thread immediately after the task is executed. This step is normally used to setup the task, for instance by showing a progress bar in the user interface.

doInBackground(Params...), invoked on the background thread immediately after onPreExecute() finishes executing. This step is used to perform background computation that can take a long time. The parameters of the asynchronous task are passed to this step. The result of the computation must be returned by this step and will be passed back to the last step. This step can also use publishProgress(Progress...) to publish one or more units of progress. These values are published on the UI thread, in the onProgressUpdate(Progress...) step.

onProgressUpdate(Progress...), invoked on the UI thread after a call to publishProgress(Progress...). The timing of the execution is undefined. This method is used to display any form of progress in the user interface while the background computation is still executing. For instance, it can be used to animate a progress bar or show logs in a text field.

onPostExecute(Result), invoked on the UI thread after the background computation finishes. The result of the background computation is passed to this step as a parameter. Threading rules

There are a few threading rules that must be followed for this class to work properly:

The task instance must be created on the UI thread. execute(Params...) must be invoked on the UI thread. Do not call onPreExecute(), onPostExecute(Result), doInBackground(Params...), onProgressUpdate(Progress...) manually. The task can be executed only once (an exception will be thrown if a second execution is attempted.)

Example code
What the adapter does in this example is not important, more important to understand that you need to use AsyncTask to display a dialog for the progress.

private class PrepareAdapter1 extends AsyncTask<Void,Void,ContactsListCursorAdapter > {
    ProgressDialog dialog;
    @Override
    protected void onPreExecute() {
        dialog = new ProgressDialog(viewContacts.this);
        dialog.setMessage(getString(R.string.please_wait_while_loading));
        dialog.setIndeterminate(true);
        dialog.setCancelable(false);
        dialog.show();
    }
    /* (non-Javadoc)
     * @see android.os.AsyncTask#doInBackground(Params[])
     */
    @Override
    protected ContactsListCursorAdapter doInBackground(Void... params) {
        cur1 = objItem.getContacts();
        startManagingCursor(cur1);

        adapter1 = new ContactsListCursorAdapter (viewContacts.this,
                R.layout.contact_for_listitem, cur1, new String[] {}, new int[] {});

        return adapter1;
    }

    protected void onPostExecute(ContactsListCursorAdapter result) {
        list.setAdapter(result);
        dialog.dismiss();
    }
}

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The most simplest solution I have seen to supply a short execution to the UI thread is via the post() method of a view. This is needed since UI methods are not re-entrant. The method for this is:

package android.view;

public class View;

public boolean post(Runnable action);

The post() method corresponds to the SwingUtilities.invokeLater(). Unfortunately I didn't find something simple that corresponds to the SwingUtilities.invokeAndWait(), but one can build the later based on the former with a monitor and a flag.

So what you save by this is creating a handler. You simply need to find your view and then post on it. You can find your view via findViewById() if you tend to work with id-ed resources. The resulting code is very simple:

/* inside your non-UI thread */

view.post(new Runnable() {
        public void run() {
            /* the desired UI update */
        }
    });
}

Note: Compared to SwingUtilities.invokeLater() the method View.post() does return a boolean, indicating whether the view has an associated event queue. Since I used the invokeLater() resp. post() anyway only for fire and forget, I did not check the result value. Basically you should call post() only after onAttachedToWindow() has been called on the view.

Best Regards

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If you use Handler (I see you do and hopefully you created its instance on the UI thread), then don't use runOnUiThread() inside of your runnable. runOnUiThread() is used when you do smth from a non-UI thread, however Handler will already execute your runnable on UI thread.

Try to do smth like this:

private Handler mHandler = new Handler();
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.gameone);
    res = getResources();
    // pB.setProgressDrawable(getResources().getDrawable(R.drawable.green)); **//Works**
    mHandler.postDelayed(runnable, 1);
}

private Runnable runnable = new Runnable() {
    public void run() {
        pB.setProgressDrawable(getResources().getDrawable(R.drawable.green));
        pB.invalidate(); // maybe this will even not needed - try to comment out
    }
};

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Use the AsyncTask class (instead of Runnable). It has a method called onProgressUpdate which can affect the UI (it's invoked in the UI thread).

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You need to create a Handler in the UI thread and then use it to post or send a message from your other thread to update the UI

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If you don't like the AsyncTask you could use the observer pattern. In that example use the ResponseHandler as an inner class in your activity then have a string message that will set the progress bars percentage... You would need to make sure that any alterations to the UI are performed within the ResponseHandler to avoid freezing up the UI, then your worker thread (EventSource in the example) can perform the tasks required.

I would use the AsyncTask tho, however the observer pattern can be good for customization reasons, plus its easier to understand. Also im not sure if this way is widely accepted or will 100% work. Im downloading and the android plugin now to test it

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As recommended by official documentation, you can use AsyncTask to handle work items shorter than 5ms in duration. If your task take more time, lookout for other alternatives.

HandlerThread is one alternative to Thread or AsyncTask. If you need to update UI from HandlerThread, post a message on UI Thread Looper and UI Thread Handler can handle UI updates.

Example code:

Android: Toast in a thread