能够从后台线程更新UI

Question

As per my basic knowledge, We cannot update UI from worker threads. I tried this below code, it is able to update from async task to UI.

Is my understanding wrong. From this code i am getting impression that we can update UI from background thread. Only in case of toast it is failing. Remaining all UI operation i am able to do from background thread. Can someone clear me this ?

Main Activity

public class MainActivity extends AppCompatActivity {

TextView textView;
SeekBar progressBar;
ConstraintLayout constraintLayout;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    textView = findViewById(R.id.txtview);
    progressBar = findViewById(R.id.prg);
    constraintLayout = findViewById(R.id.con);

    AsyncTaskThread asyncTaskThread = new AsyncTaskThread();
    asyncTaskThread.execute();

}

public class AsyncTaskThread extends AsyncTask {

    @Override
    protected Object doInBackground(Object[] objects) {
        textView.setText("Hi Heloo");

        progressBar.setIndeterminate(false);
        progressBar.setMax(100);
        progressBar.setProgress(50);

        return null;
    }
}
}

Answer 1

In onCreate method, your views are not be lay out and visible on screen yet, they are just java objects in memory, at that time AsyncTask just set properties on them.

Now change your code to

public class MainActivity extends AppCompatActivity {

    TextView textView;
    SeekBar progressBar;
    ConstraintLayout constraintLayout;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textView = findViewById(R.id.txtview);
        progressBar = findViewById(R.id.prg);
        constraintLayout = findViewById(R.id.con);

        // TODO: Add this code
        textView.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
            @Override
            public void onGlobalLayout() {
                // At this time, this view is lay out and visible on screen.
                AsyncTaskThread asyncTaskThread = new AsyncTaskThread();
                asyncTaskThread.execute();
            }
        });
    }

    public class AsyncTaskThread extends AsyncTask {

        @Override
        protected Object doInBackground(Object[] objects) {
            textView.setText("Hi Heloo");

            progressBar.setIndeterminate(false);
            progressBar.setMax(100);
            progressBar.setProgress(50);

            return null;
        }
    }
}

As a result, your app is always crash with message

android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.