Handler在Android项目中的运行原理是什么

public class MainActivity extends AppCompatActivity {  private static final int MESSAGE_TEXT_VIEW = 0;    private TextView mTextView;  private Handler mHandler = new Handler() {    @Override    public void handleMessage(Message msg) {      switch (msg.what) {        case MESSAGE_TEXT_VIEW:          mTextView.setText("UI成功更新");        default:          super.handleMessage(msg);      }    }  };  @Override  protected void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    setContentView(R.layout.activity_main);    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);    setSupportActionBar(toolbar);    mTextView = (TextView) findViewById(R.id.text_view);    new Thread(new Runnable() {      @Override      public void run() {        try {          Thread.sleep(3000);        } catch (InterruptedException e) {          e.printStackTrace();        }        mHandler.obtainMessage(MESSAGE_TEXT_VIEW).sendToTarget();      }    }).start();  }}