Android的消息机制(4)-优快云博客

本文详细阐述了Android中消息机制的应用，包括主线程与子线程间的消息传递，通过实例展示了如何利用Handler和Looper实现跨线程消息交互。

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上面的文章我们演示了如何把一个Message由子线程发送给主线程，但是如何将一个Message从主线程发送给子线程呢？子线程在默认的情况下是没有Looper的，也就没有可能操作子线程的消息队列。我们通过查API文档可以看到：

Class used to run a message loop for a thread. Threads by default do not have a message loop associated with them; to create one, call prepare() in the thread that is to run the loop, and then loop() to have it process messages until the loop is stopped.

Most interaction with a message loop is through the Handler class.

大概意思是，线程在默认情况下是没有Looper的，但是可以通过调用Prepare()方法来运行一个loop，然后使用loop()方法来处理消息直到loop结束。通常的使用方式是：

^? Download zuiniuwang.java

 class LooperThread extends Thread {
	public Handler mHandler;
 
	public void run() {
		Looper.prepare();
		mHandler = new Handler() {
			public void handleMessage(Message msg) { // process incoming
														// messages here
			}
		};
		Looper.loop();
	}
 
}

例程如下，基本过程是这样的，启动主线程，然后启动子线程，子线程注册Looper，主线程发送一个Message给子线程，然后子线程的handler处理此消息，再把此消息发送给主线程的消息队列，主线程空间显示此消息~

^? Download zuiniuwang.java

/**
 * MessageExample4.java
 * com.test.Message
 *
 * Function： TODO
 *
 *   ver     date      		author
 * ──────────────────────────────────
 *   		 2011-3-23 		Leon
 *
 * Copyright (c) 2011, TNT All Rights Reserved.
 */
 
package com.test.Message;
 
import android.app.Activity;
import android.graphics.Color;
import android.os.Bundle;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.TextView;
 
/**
 * ClassName:MessageExample4 主线程如何传递消息给子线程 Function: TODO ADD FUNCTION Reason:
 * TODO ADD REASON
 *
 * @author Leon
 * @version
 * @since Ver 1.1
 * @Date 2011-3-23
 */
public class MessageExample4 extends Activity implements OnClickListener {
 
	private final int WC = LinearLayout.LayoutParams.WRAP_CONTENT;
	private final int FP = LinearLayout.LayoutParams.FILL_PARENT;
	public TextView tv;
	private Button btn, btn2;
	private Handler h;
 
	public void onCreate(Bundle icicle) {
		super.onCreate(icicle);
		LinearLayout layout = new LinearLayout(this);
		layout.setOrientation(LinearLayout.VERTICAL);
 
		btn = new Button(this);
		btn.setId(101);
 
		btn.setText("test looper");
		btn.setOnClickListener(this);
		LinearLayout.LayoutParams param = new LinearLayout.LayoutParams(100, 50);
		param.topMargin = 10;
		layout.addView(btn, param);
 
		btn2 = new Button(this);
		btn2.setId(102);
 
		btn2.setText("exit");
		btn2.setOnClickListener(this);
		layout.addView(btn2, param);
 
		tv = new TextView(this);
		tv.setTextColor(Color.WHITE);
		tv.setText("");
		LinearLayout.LayoutParams param2 = new LinearLayout.LayoutParams(FP, WC);
		param2.topMargin = 10;
		layout.addView(tv, param2);
 
		setContentView(layout);
		// 启动子线程
		new myThread().start();
	}
 
	public void onClick(View v) {
		switch (v.getId()) {
		case 101:
			String obj = "mainThread";
			Message m = h.obtainMessage(1, 1, 1, obj);
			h.sendMessage(m);
			break;
		case 102:
			finish();
			break;
		}
 
	}
 
	// ------------------------------------------------------
 
	class EHandler extends Handler {
		public EHandler(Looper looper) {
			super(looper);
		}
 
		@Override
		public void handleMessage(Message msg) {
			tv.setText((String) msg.obj);
		}
	}
 
	// ------------------------------------------------------
	class myThread extends Thread {
		private EHandler mHandler;
 
		public void run() {
			Looper.prepare();
			h = new Handler() {
				public void handleMessage(Message msg) {
					EHandler ha = new EHandler(Looper.getMainLooper());
					String obj = (String) msg.obj + ", myThread";
					Message m = ha.obtainMessage(1, 1, 1, obj);
					ha.sendMessage(m);
				}
			};
			Looper.loop();
 
		}
	}
 
}