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)。 

Runnable对像就能在ui程序中被调用。如果当前线程是UI线程，那么行动是立即执行。如果当前线程不是UI线程,操作是发布到事件队列的UI线程

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();
    }
}