1.活用Android线程间通信的Message机制
1.1.Message
代码在frameworks\base\core\java\android\Os\Message.java中。
Message.obtain函数:有多个obtain函数,主要功能一样,只是参数不一样。作用是从Message Pool中取出一个Message,如果Message Pool中已经没有Message可取则新建一个Message返回,同时用对应的参数给得到的Message对象赋值。
Message Pool:大小为10个;通过Message.mPool->(Message并且Message.next)-> (Message并且Message.next)-> (Message并且Message.next)...构造一个Message Pool。Message Pool的第一个元素直接new出来,然后把Message.mPool(static类的static变量)指向它。其他的元素都是使用完的 Message通过Message的recycle函数清理后放到Message Pool(通过Message Pool最后一个Message的next指向需要回收的Message的方式实现)。下图为Message Pool的结构:
1.2.MessageQueue
MessageQueue里面有一个收到的Message的对列:
MessageQueue.mMessages(static变量)->( Message并且Message.next)-> ( Message并且Message.next)->...,下图为接收消息的消息队列:
上层代码通过Handler的sendMessage等函数放入一个message到MessageQueue里面时最终会调用MessageQueue的 enqueueMessage函数。enqueueMessage根据上面的接收的Message的队列的构造把接收到的Message放入队列中。
MessageQueue的removeMessages函数根据上面的接收的Message的队列的构造把接收到的Message从队列中删除,并且调用对应Message对象的recycle函数把不用的Message放入Message Pool中。
1.3.Looper
Looper对象的创建是通过prepare函数,而且每一个Looper对象会和一个线程关联
Looper对象创建时会创建一个MessageQueue,主线程默认会创建一个Looper从而有MessageQueue,其他线程默认是没有 MessageQueue的不能接收Message,如果需要接收Message则需要通过prepare函数创建一个MessageQueue。具体操作请见示例代码。
prepareMainLooper函数只给主线程调用(系统处理,程序员不用处理),它会调用prepare建立Looper对象和MessageQueue。
1.4.Handler
Handler的构造函数表示Handler会有成员变量指向Looper和MessageQueue,后面我们会看到没什么需要这些引用;至于callback是实现了Callback接口的对象,后面会看到这个对象的作用。
Loop函数从MessageQueue中从前往后取出Message,然后通过Handler的dispatchMessage函数进行消息的处理(可见消息的处理是Handler负责的),消息处理完了以后通过Message对象的recycle函数放到Message Pool中,以便下次使用,通过Pool的处理提供了一定的内存管理从而加速消息对象的获取。至于需要定时处理的消息如何做到定时处理,请见 MessageQueue的next函数,它在取Message来进行处理时通过判断MessageQueue里面的Message是否符合时间要求来决定是否需要把Message取出来做处理,通过这种方式做到消息的定时处理。
Handler类的常用方法
方法签名
描 述
public void handleMessage (Messagemsg)
子类对象通过该方法接收信息
public final boolean sendEmptyMessage (int what)
发送一个只含有 what 值的消息
public final boolean sendMessage (Messagemsg)
发送消息到 Handler ,
通过 handleMessage 方法接收
public final boolean hasMessages (int what)
监测消息队列中是否还
有 what 值的消息
public final boolean post (Runnabler)
将一个线程添加到消息队列
获取消息:直接通过Message的obtain方法获取一个Message对象。
- public final Message obtainMessage(int what, int arg1, int arg2, Object obj){
- return Message.obtain(this, what, arg1, arg2, obj);
- }
public final Message obtainMessage(int what, int arg1, int arg2, Object obj){ return Message.obtain(this, what, arg1, arg2, obj);}
发送消息:通过MessageQueue的enqueueMessage把Message对象放到MessageQueue的接收消息队列中
- public boolean sendMessageAtTime(Message msg, long uptimeMillis){
- boolean sent = false;
- MessageQueue queue = mQueue;
- if (queue != null) {
- msg.target = this;
- sent = queue.enqueueMessage(msg, uptimeMillis);
- } else {
- RuntimeException e = new RuntimeException(this + " sendMessageAtTime() called with no mQueue");
- Log.w("Looper", e.getMessage(), e);
- }
- return sent;
- }
public boolean sendMessageAtTime(Message msg, long uptimeMillis){ boolean sent = false; MessageQueue queue = mQueue; if (queue != null) { msg.target = this; sent = queue.enqueueMessage(msg, uptimeMillis); } else { RuntimeException e = new RuntimeException(this + " sendMessageAtTime() called with no mQueue"); Log.w("Looper", e.getMessage(), e); } return sent;}
线程如何处理MessageQueue中接收的消息:在Looper的loop函数中循环取出MessageQueue的接收消息队列中的消息,然后调用 Hander的dispatchMessage函数对消息进行处理,至于如何处理(相应消息)则由用户指定(三个方法,优先级从高到低:Message里面的Callback,一个实现了Runnable接口的对象,其中run函数做处理工作;Handler里面的mCallback指向的一个实现了 Callback接口的对象,里面的handleMessage进行处理;处理消息Handler对象对应的类继承并实现了其中 handleMessage函数,通过这个实现的handleMessage函数处理消息)。
- public void dispatchMessage(Message msg) {
- if (msg.callback != null) {
- handleCallback(msg);
- } else {
- if (mCallback != null) {
- if (mCallback.handleMessage(msg)) {
- return;
- }
- }
- handleMessage(msg);
- }
- }
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); }}
Runnable说明:Runnable只是一个接口,实现了这个接口的类对应的对象也只是个普通的对象,并不是一个Java中的Thread。Thread类经常使用Runnable,很多人有误解,所以这里澄清一下。
从上可知以下关系图:
其中清理Message是Looper里面的loop函数指把处理过的Message放到Message的Pool里面去,如果里面已经超过最大值10个,则丢弃这个Message对象。
调用Handler是指Looper里面的loop函数从MessageQueue的接收消息队列里面取出消息,然后根据消息指向的Handler对象调用其对应的处理方法。
在Android 中Handler和Message、Thread有着很密切的关系。Handler 主要是负责Message的分发和处理。但是这个Message从哪里来的呢?Message 由一个消息队列进行管理,而消息队列却由一个Looper进行管理。Android系统中Looper负责管理线程的消息队列和消息循环,具体实现请参考Looper的源码。 可以通过Loop.myLooper()得到当前线程的Looper对象,通过Loop.getMainLooper()可以获得当前进程的主线程的 Looper对象。 Android系统的消息队列和消息循环都是针对具体线程的,一个线程可以存在(当然也可以不存在)一个消息队列和一个消息循环(Looper),特定线程的消息只能分发给本线程,不能进行跨线程,跨进程通讯。但是创建的工作线程默认是没有消息循环和消息队列的,如果想让该线程具有消息队列和消息循环,需要在线程中首先调用Looper.prepare()来创建消息队列,然后调用Looper.loop()进入消息循环。
虽说 特定线程的消息只能分发给本线程,不能进行跨线程通讯,但是由于可以通过获得线程的Looper对象来进行曲线的实现不同线程间消息的传递,代码如下:
package com.mytest.handlertest; 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.util.Log; import android.view.View; import android.view.View.OnClickListener; import android.view.ViewGroup.LayoutParams; import android.widget.Button; import android.widget.LinearLayout; import android.widget.TextView; public class HandlerTest extends Activity implements OnClickListener{ private String TAG = "HandlerTest"; private boolean bpostRunnable = false; private NoLooperThread noLooperThread = null; private OwnLooperThread ownLooperThread = null; private ReceiveMessageThread receiveMessageThread =null; private Handler mOtherThreadHandler=null; private EventHandler mHandler = null; private Button btn1 = null; private Button btn2 = null; private Button btn3 = null; private Button btn4 = null; private Button btn5 = null; private Button btn6 = null; private TextView tv = null; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); LinearLayout layout = new LinearLayout(this); LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(250, 50); layout.setOrientation(LinearLayout.VERTICAL); btn1 = new Button(this); btn1.setId(101); btn1.setText("message from main thread self"); btn1.setOnClickListener(this); layout.addView(btn1, params); btn2 = new Button(this); btn2.setId(102); btn2.setText("message from other thread to main thread"); btn2.setOnClickListener(this); layout.addView(btn2,params); btn3 = new Button(this); btn3.setId(103); btn3.setText("message to other thread from itself"); btn3.setOnClickListener(this); layout.addView(btn3, params); btn4 = new Button(this); btn4.setId(104); btn4.setText("message with Runnable as callback from other thread to main thread"); btn4.setOnClickListener(this); layout.addView(btn4, params); btn5 = new Button(this); btn5.setId(105); btn5.setText("main thread's message to other thread"); btn5.setOnClickListener(this); layout.addView(btn5, params); btn6 = new Button(this); btn6.setId(106); btn6.setText("exit"); btn6.setOnClickListener(this); layout.addView(btn6, params); tv = new TextView(this); tv.setTextColor(Color.WHITE); tv.setText(""); params = new LinearLayout.LayoutParams(LayoutParams.FILL_PARENT,LayoutParams.WRAP_CONTENT); params.topMargin=10; layout.addView(tv, params); setContentView(layout); receiveMessageThread = new ReceiveMessageThread(); receiveMessageThread.start(); } class EventHandler extends Handler{ public EventHandler(Looper looper){ super(looper); } public EventHandler(){ super(); } @Override public void handleMessage(Message msg) { super.handleMessage(msg); Log.e(TAG, "CurrentThread id:----------+>" + Thread.currentThread().getId()); switch(msg.what){ case 1: tv.setText((String)msg.obj); break; case 2: tv.setText((String)msg.obj); noLooperThread.stop(); break; case 3: Log.e(TAG,(String)msg.obj); ownLooperThread.stop(); break; default: Log.e(TAG,(String)msg.obj); break; } } } class ReceiveMessageThread extends Thread { @Override public void run(){ Looper.prepare(); mOtherThreadHandler= new Handler(){ @Override public void handleMessage(Message msg) { super.handleMessage(msg); Log.e(TAG,"-------+>"+(String)msg.obj); Log.e(TAG, "CurrentThread id:----------+>" + Thread.currentThread().getId()); } }; Log.e(TAG, "ReceiveMessageThread id:--------+>" + this.getId()); Looper.loop(); } } class NoLooperThread extends Thread { private EventHandler mNoLooperThreadHandler; @Override public void run() { Looper myLooper = Looper.myLooper(); Looper mainLooper= Looper.getMainLooper(); String msgobj; if(null == myLooper){ mNoLooperThreadHandler = new EventHandler(mainLooper); msgobj = "NoLooperThread has no looper and handleMessage function executed in main thread!"; } else{ mNoLooperThreadHandler = new EventHandler(myLooper); msgobj = "This is from NoLooperThread self and handleMessage function executed in NoLooperThread!"; } mNoLooperThreadHandler.removeMessages(0); if(bpostRunnable == false){ Message msg = mNoLooperThreadHandler.obtainMessage(2, 1, 1, msgobj); mNoLooperThreadHandler.sendMessage(msg); Log.e(TAG, "NoLooperThread id:--------+>" + this.getId()); }else{ mNoLooperThreadHandler.post(new Runnable(){ public void run() { tv.setText("update UI through handler post runnalbe mechanism!"); Log.e(TAG, "update UI id:--------+>" + Thread.currentThread().getId()); noLooperThread.stop(); } }); } } } class OwnLooperThread extends Thread{ private EventHandler mOwnLooperThreadHandler = null; @Override public void run() { Looper.prepare(); Looper myLooper = Looper.myLooper(); Looper mainLooper= Looper.getMainLooper(); String msgobj; if(null == myLooper){ mOwnLooperThreadHandler = new EventHandler(mainLooper); msgobj = "OwnLooperThread has no looper and handleMessage function executed in main thread!"; }else{ mOwnLooperThreadHandler = new EventHandler(myLooper); msgobj = "This is from OwnLooperThread self and handleMessage function executed in NoLooperThread!"; } mOwnLooperThreadHandler.removeMessages(0); Message msg = mOwnLooperThreadHandler.obtainMessage(3,1,1,msgobj); mOwnLooperThreadHandler.sendMessage(msg); Looper.loop(); } } public void onClick(View v) { switch(v.getId()){ case 101: Looper looper = Looper.myLooper(); mHandler = new EventHandler(looper); mHandler.removeMessages(0); String obj = "This main thread's message and received by itself!"; Message msg = mHandler.obtainMessage(1,1,1,obj); mHandler.sendMessage(msg); break; case 102: bpostRunnable = false; noLooperThread = new NoLooperThread(); noLooperThread.start(); break; case 103: tv.setText("please look at the error level log for other thread received message"); ownLooperThread = new OwnLooperThread(); ownLooperThread.start(); break; case 104: bpostRunnable = true; noLooperThread = new NoLooperThread(); noLooperThread.start(); break; case 105: if(null!=mOtherThreadHandler){ tv.setText("please look at the error level log for other thread received message from main thread"); String msgObj = "message from mainThread"; Message mainThreadMsg = mOtherThreadHandler.obtainMessage(1, 1, 1, msgObj); mOtherThreadHandler.sendMessage(mainThreadMsg); } break; case 106: finish(); break; } } }