为什么我可以在非UI线程中更改Textview？

Question

public class MainActivity extends Activity {
TextView  textview;
HandlerThread myThread = new HandlerThread("my");
Handler myHandler;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    textview = (TextView)findViewById(R.id.text);
    myThread.start();
    myHandler = new Handler(myThread.getLooper());
    Toast.makeText(MainActivity.this,"in act = " + Thread.currentThread().getId(), Toast.LENGTH_SHORT).show();
    myHandler.post(new Runnable() {
        @Override
        public void run() {
            textview.setText("gzw " + Thread.currentThread().getId() + Thread.currentThread().getName());
        }
    });
    new Thread(){
        @Override
        public void run() {
            String s = textview.getText().toString();
            textview.setText(s + "\n gzw2 " + Thread.currentThread().getId() + Thread.currentThread().getName());
        }
    }.start();
}

在两个run（）方法中工作正常！为什么不从callwrongthreadexception调用？ textview可以在两个线程中更改 第一次敬酒“in act = 1” textview是： gzw 516my gzw 517Thread-517