在android 中我们一般用 Handler 做主线程 和 子线程 之间的通信 。

现在有了一种更为简洁的写法，就是 Activity 里面的 runOnUiThread（ Runnable ）方法。

利用Activity.runOnUiThread(Runnable)把更新ui的代码创建在Runnable中，然后在需要更新ui时，把这个Runnable对象传给Activity.runOnUiThread(Runnable)。

package com.app;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //创建一个线程
        new Thread(new Runnable() {
            @Override
            public void run() {
                //延迟两秒
                try {
                    Thread.sleep( 2000 );
                } catch (InterruptedException e) {
                    e.printStackTrace();
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        Toast.makeText(MainActivity.this, "hah", Toast.LENGTH_SHORT).show();
        }).start();
 Activity的runOnUiThread(Runnable)
     * Runs the specified action on the UI thread. If the current thread is the UI
     * thread, then the action is executed immediately. If the current thread is
     * not the UI thread, the action is posted to the event queue of the UI thread.
     * @param action the action to run on the UI thread
    public final void runOnUiThread(Runnable action) {
        if (Thread.currentThread() != mUiThread) {
            mHandler.post(action);
        } else {
            action.run();