看Android开发笔记之:消息循环与Looper的详解有感

看完Android开发笔记之:消息循环与Looper的详解后的总结，原文和原文作者没有找到，找到的都是跟这个类似的转载。

先上代码

package com.example.gulei.myviewdraghelperdemo;

import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.os.SystemClock;
import android.text.TextUtils;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

/**
* Created by gulei on 2016/2/14.
*/
public class LooperDemoActivity extends Activity {
private WorkerThread mWorkerThread;
private TextView mStatusLine;
private Handler mMainHandler;

@Override
public void onCreate(Bundle icicle) {
super.onCreate(icicle);
setContentView(R.layout.activity_looper);
mMainHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
String text = (String) msg.obj;
if (TextUtils.isEmpty(text)) {
return;
}
mStatusLine.setText(text);
}
};

mWorkerThread = new WorkerThread();
final Button action = (Button) findViewById(R.id.looper_demo_action);
action.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
mWorkerThread.executeTask("please do me a favor");
}
});
final Button end = (Button) findViewById(R.id.looper_demo_quit);
end.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
mWorkerThread.exit();
}
});
mStatusLine = (TextView) findViewById(R.id.looper_demo_displayer);
mStatusLine.setText("Press 'do me a favor' to execute a task, press 'end of service' to stop looper thread");
}

@Override
public void onDestroy() {
super.onDestroy();
mWorkerThread.exit();
mWorkerThread = null;
}

private class WorkerThread extends Thread {
protected static final String TAG = "WorkerThread";
private Handler mHandler;
private Looper mLooper;

public WorkerThread() {
start();
}

public void run() {
// Attention: if you obtain looper before Looper#prepare(), you can still use the looper
// to process message even after you call Looper#quit(), which means the looper does not
//really quit.
Looper.prepare();
// So we should call Looper#myLooper() after Looper#prepare(). Anyway, we should put all stuff between Looper#prepare()
// and Looper#loop().
// In this case, you will receive "Handler{4051e4a0} sending message to a Handler on a dead thread
// 05-09 08:37:52.118: W/MessageQueue(436): java.lang.RuntimeException: Handler{4051e4a0} sending message
// to a Handler on a dead thread", when try to send a message to a looper which Looper#quit() had called,
// because the thread attaching the Looper and Handler dies once Looper#quit() gets called.
mLooper = Looper.myLooper();
// either new Handler() and new Handler(mLooper) will work
mHandler = new Handler(mLooper) {
@Override
public void handleMessage(Message msg) {
/*
* Attention: object Message is not reusable, you must obtain a new one for each time you want to use it.
* Otherwise you got "android.util.AndroidRuntimeException: { what=1000 when=-15
4000
ms obj=it is my please
* to serve you, please be patient to wait!........ } This message is already in use."
*/
//      Message newMsg = Message.obtain();
StringBuilder sb = new StringBuilder();
sb.append("it is my please to serve you, please be patient to wait!\n");
Log.e(TAG, "workerthread, it is my please to serve you, please be patient to wait!");
for (int i = 1; i < 100; i++) {
sb.append(".");
Message newMsg = Message.obtain();
newMsg.obj = sb.toString();
mMainHandler.sendMessage(newMsg);
Log.e(TAG, "workthread, working" + sb.toString());
SystemClock.sleep(100);
}
Log.e(TAG, "workerthread, your work is done.");
sb.append("\nyour work is done");
Message newMsg = Message.obtain();
newMsg.obj = sb.toString();
mMainHandler.sendMessage(newMsg);
}
};
Looper.loop();
System.out.println("调用Looper.loop()之后");
}

public void exit() {
if (mLooper != null) {
mLooper.quit();
mLooper = null;
}
}

// This method returns immediately, it just push an Message into Thread's MessageQueue.
// You can also call this method continuously, the task will be executed one by one in the
// order of which they are pushed into MessageQueue(they are called).
public void executeTask(String text) {
if (mLooper == null || mHandler == null) {
Message msg = Message.obtain();
msg.obj = "Sorry man, it is out of service";
mMainHandler.sendMessage(msg);
return;
}
System.out.println("开始执行任务");
Message msg = Message.obtain();
msg.obj = text;
mHandler.sendMessage(msg);
}
}
}

这个代码来自上面引用的文章，我只是添加了两处打印

先说一下这个代码的运行结果，在action的点击事件响应后，手机屏幕开始输出”.......“之后输出结束的文字，end按钮的点击事件响应后，等待mHandler的handlemessage方法执行完毕，控制台输出“调用Looper.loop()之后”，如果不点击end，则不会输出，也就是looper的quite的方法没有执行之前，looper.loop()之后的方法是不会被执行的。多次点击action，并不会影响当前正在执行的mHandler.handlermessage()方法，而是按顺序执行多次handlemessage方法。而只要点了end，那么当前mHandler.handlemessage方法执行完毕后即输出“调用Looper.loop()之后”，而不再执行后续的mHanlder.handlemessage()方法，估计原因是looper已经释放。

另外，当我把线程中Looper拿掉后，报错，提醒我handler在子线程中未调用Looper.prepare之前不能调用（大概的意思，原话不是这个），也就是说looper在这里是为了维护子线程的消息队列。

最后，在实际项目中，我对handler的调用基本都是在activity中初始化，然后传到thread中，然后发送message，而这个原理其实跟上面中的并没有什么差别，因为最后负责ui显示的handler还是得在activity中初始化，然后传到thread，在thread中发送message，所以还是没搞明白looper在实际应用的中有哪些场景...有人能看到我的疑问的话还请劳烦告知一下，谢谢