How is it possible to do UI operations outside the UI thread?

Question

I tried to set a text inside a normal thread outside of the UI thread and it worked fine.

    private TextView mTextView;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mTextView = (TextView) findViewById(R.id.my_text);
        Log.d("**** ONCREATE****", Thread.currentThread().getId()+"");
        new BackgroundTestThread().start();
    }

    class BackgroundTestThread extends Thread{
    @Override
    public void run() {
         Log.d("**** THREAD****", Thread.currentThread().getId()+"");
         mTextView.setText("Text was set successfully", TextView.BufferType.EDITABLE);
         }
    }

I was pretty much surprised to see this happen. I thought a worker thread can never update main thread because only main thread can render the UI elements (TextView, EditText etc.) and if we try to update, for sure we are going to get an exception.

Why did this happen?

what happens if you also set the text in the oncreate before you start the thread? — Blundell, Dec 07 '17 at 07:34
https://stackoverflow.com/questions/7700881/why-cant-a-thread-that-is-not-the-ui-thread-access-the-view refer this — Saranya Subramanian, Dec 07 '17 at 07:34
@ADM: I checked the link, but my question was different. I wanted to know why this code worked fine. I know there are different ways to do it. But according to what I have understood to date, the above code should never work. — Adarsh Mathew Thomas, Dec 07 '17 at 07:34
@SaranyaSubramanian: Thanks for the link. My question was a different one. I wanted to know why this code worked. — Adarsh Mathew Thomas, Dec 07 '17 at 07:39
@HemanthSTobi: Oh yes, not doubt about it. Wanted clarity in the use of threads. Got surprised to see UI action done from a non ui thread. Just wanted to know how that was possible. — Adarsh Mathew Thomas, Dec 07 '17 at 12:43

Anandu R · Accepted Answer · 2017-12-07T17:53:05.433

3

It won't work sometimes as it is not thread safe. Thus, it is recommended to use handler or runonuithead.

edited Dec 07 '17 at 17:53

answered Dec 07 '17 at 07:38

Anandu R

56
5

score 1 · Answer 2 · answered Dec 07 '17 at 16:15

1

Android doesnt check every where whether its UI thread or Not so in some cases it might work .but its never thread safe.The preferred way is to display in UIThread.

answered Dec 07 '17 at 16:15

Rockin

723
4
25
51

score 0 · Answer 3 · answered Dec 07 '17 at 08:48

Another way to do UI operations safety outside main UI thread.

    runOnUiThread(new Runnable() {
        @Override
        public void run() {
           mTextView.setText("Text was set successfully", TextView.BufferType.EDITABLE);
        }
    });

How is it possible to do UI operations outside the UI thread?

3 Answers3