AsyncTask - Why can I access layout widgets in the doInBackground method?

Issue

From my understanding, the doInBackground method should not have access to the layout views. So why does the following code work? As you can see, I access the ProgressBar from doInBackground

private class TestAsyncTask extends AsyncTask<Integer, Integer, String> {
    @Override
    protected String doInBackground(Integer... integers) {
        progressBar.setMax(integers[0]);

        for (int i = 1; i <= integers[0]; i++) {
            try {
                Thread.sleep(1000);
                publishProgress(i);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        return "Finished!";
    }

    @Override
    protected void onPostExecute(String s) {
        super.onPostExecute(s);
        Toast.makeText(MainActivity.this, s, Toast.LENGTH_SHORT).show();
        progressBar.setProgress(0);
    }

    @Override
    protected void onProgressUpdate(Integer... values) {
        super.onProgressUpdate(values);
        progressBar.setProgress(values[0]);
    }
}

Solution

Because in the widget code it checks the thread, if not UI it creates a Runnable and posts it to the UI Thread :

private synchronized void refreshProgress(int id, int progress, boolean fromUser,
            boolean animate) {
        if (mUiThreadId == Thread.currentThread().getId()) {
            doRefreshProgress(id, progress, fromUser, true, animate);
        } else {
            if (mRefreshProgressRunnable == null) {
                mRefreshProgressRunnable = new RefreshProgressRunnable();
            }
            final RefreshData rd = RefreshData.obtain(id, progress, fromUser, animate);
            mRefreshData.add(rd);
            if (mAttached && !mRefreshIsPosted) {
                post(mRefreshProgressRunnable);
                mRefreshIsPosted = true;
            }
        }
    }

RefreshProgressRunnable : Causes the Runnable to be added to the message queue. The runnable will be run on the user interface thread.

Answered By - Mark