Android消息机制好多人都讲过，但是自己去翻源码的时候才能明白。
今天试着讲一下，因为目标是讲清楚整体逻辑，所以不追究细节。
Message是消息机制的核心，所以从Message讲起。
1.Message是什么？
看一个从消息池中取出一个msg的方法：
public static Message obtain(Handler h, int what, int arg1, int arg2, Object obj) { Message m = obtain(); m.target = h; m.what = what; m.arg1 = arg1; m.arg2 = arg2; m.obj = obj; return m; }
一个Message由下面几个部分构成：
arg1，arg2:用于传递简单整数类型数据时使用
obj:传递的数据对象，也就是内容
what:用户自定义的消息代码，接受者可以了解这个消息的信息，作为这个消息在MessageQueue中的唯一标示。
target：一个handler，顾名思义，这个message是谁的，是handler的，感觉handler很难理解的，可以把handler理解成一个辅助类。
注：也可以使用一个message初始化另外一个message，参数里可以加入message自定义的callback
2.Messsage在哪儿待着？
在MessageQueue中，顾名思义，这是一个Message的队列。我们通过next遍历这个队列来获得msg，next方法如下所示：
Message next() { // Return here if the message loop has already quit and been disposed. // This can happen if the application tries to restart a looper after quit // which is not supported. final long ptr = mPtr; if (ptr == 0) { return null; } int pendingIdleHandlerCount = -1; // -1 only during first iteration int nextPollTimeoutMillis = 0; for (;;) { if (nextPollTimeoutMillis != 0) { Binder.flushPendingCommands(); } nativePollOnce(ptr, nextPollTimeoutMillis); synchronized (this) { // Try to retrieve the next message. Return if found. final long now = SystemClock.uptimeMillis(); Message prevMsg = null; Message msg = mMessages; if (msg != null && msg.target == null) { // Stalled by a barrier. Find the next asynchronous message in the queue. do { prevMsg = msg; msg = msg.next; } while (msg != null && !msg.isAsynchronous()); } if (msg != null) { if (now < msg.when) { // Next message is not ready. Set a timeout to wake up when it is ready. nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE); } else { // Got a message. mBlocked = false; if (prevMsg != null) { prevMsg.next = msg.next; } else { mMessages = msg.next; } msg.next = null; if (DEBUG) Log.v(TAG, "Returning message: " + msg); msg.markInUse(); return msg; } } else { // No more messages. nextPollTimeoutMillis = -1; } // Process the quit message now that all pending messages have been handled. if (mQuitting) { dispose(); return null; } // If first time idle, then get the number of idlers to run. // Idle handles only run if the queue is empty or if the first message // in the queue (possibly a barrier) is due to be handled in the future. if (pendingIdleHandlerCount < 0 && (mMessages == null || now < mMessages.when)) { pendingIdleHandlerCount = mIdleHandlers.size(); } if (pendingIdleHandlerCount <= 0) { // No idle handlers to run. Loop and wait some more. mBlocked = true; continue; } if (mPendingIdleHandlers == null) { mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)]; } mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers); } // Run the idle handlers. // We only ever reach this code block during the first iteration. for (int i = 0; i < pendingIdleHandlerCount; i++) { final IdleHandler idler = mPendingIdleHandlers[i]; mPendingIdleHandlers[i] = null; // release the reference to the handler boolean keep = false; try { keep = idler.queueIdle(); } catch (Throwable t) { Log.wtf(TAG, "IdleHandler threw exception", t); } if (!keep) { synchronized (this) { mIdleHandlers.remove(idler); } } } // Reset the idle handler count to 0 so we do not run them again. pendingIdleHandlerCount = 0; // While calling an idle handler, a new message could have been delivered // so go back and look again for a pending message without waiting. nextPollTimeoutMillis = 0; } }
3.Message从何而来？
当我们定义了一个Message后，怎么把它放在MessageQueue里的？
这个时候我们需要一个第三方的帮手，于是handler登场了。
此处，我们需要先了解一下Hanlder的成员：
final MessageQueue mQueue; final Looper mLooper; final Callback mCallback;
可以看出，handler与一个MessageQueue和一个Looper相关联，定义一个回调用的的类。
handler的初始化函数：
public Handler(Looper looper, Callback callback, boolean async) { mLooper = looper; mQueue = looper.mQueue; mCallback = callback; mAsynchronous = async; }
就是looper和looper对应 的messagequeue非配给了handler.
在Message.java中有这样一个函数：
public void sendToTarget() { target.sendMessage(this); }
可见，一个Message是由它的target，也就是一个handler调用sendMessage方法发送到MessageQueue中的，看Handler.java的源码是，会发现有好几种sendMessage方法，但最后都是调用了sendMessageAtTime方法
public boolean sendMessageAtTime(Message msg, long uptimeMillis) { MessageQueue queue = mQueue; if (queue == null) { RuntimeException e = new RuntimeException( this + " sendMessageAtTime() called with no mQueue"); Log.w("Looper", e.getMessage(), e); return false; } return enqueueMessage(queue, msg, uptimeMillis); }
可以看出，handler与一个MessageQueue相关联，如果handler关联的MessageQueue不为空的话，则入队。
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }
在将消息入队的时候，通过msg.target = this将msg与handler关联起来。
4.Message去往何处？
这个问题很明显：Message怎么从MessageQueue里出来呀，由Looper从MessageQueue中取出来：
先看看Looper的构成：
public final class Looper { static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>(); private static Looper sMainLooper; // guarded by Looper.class final MessageQueue mQueue; final Thread mThread; //...... }
可以看到Looper对应一个Thread和一个MessageQueue。
每一个Thread都对应有一个Looper么？是的，但不是默认的，如果不在主线程中，你想使用Looper的话，必须要调用一个函数：
private static void prepare(boolean quitAllowed) { if (sThreadLocal.get() != null) { throw new RuntimeException("Only one Looper may be created per thread"); } sThreadLocal.set(new Looper(quitAllowed)); }
这个函数就是维护一个ThreadLocal变量：sThreadLocl，设置属于当前线程的Looper。
这里，prepare方法巧妙地使用了ThreadLocal变量将Thread与一个Looper关联起来。
另外，注意looper中的两个方法：
public static @Nullable Looper myLooper() { return sThreadLocal.get(); } public static Looper getMainLooper() { synchronized (Looper.class) { return sMainLooper; } }
myLooper获得当前线程绑定的looper，没有则返回null。
getMainLooper获得主线程的looper，方便与主线程通信。
此时已经获得了一个Looper,准备开始取消息，调用Looper.loop()方法
public static void loop() { final Looper me = myLooper(); if (me == null) { throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } final MessageQueue queue = me.mQueue; // Make sure the identity of this thread is that of the local process, // and keep track of what that identity token actually is. Binder.clearCallingIdentity(); final long ident = Binder.clearCallingIdentity(); for (;;) { Message msg = queue.next(); // might block if (msg == null) { // No message indicates that the message queue is quitting. return; } // This must be in a local variable, in case a UI event sets the logger Printer logging = me.mLogging; if (logging != null) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } msg.target.dispatchMessage(msg); if (logging != null) { logging.println("<<<<< Finished to " + msg.target + " " + msg.callback); } // Make sure that during the course of dispatching the // identity of the thread wasn't corrupted. final long newIdent = Binder.clearCallingIdentity(); if (ident != newIdent) { Log.wtf(TAG, "Thread identity changed from 0x" + Long.toHexString(ident) + " to 0x" + Long.toHexString(newIdent) + " while dispatching to " + msg.target.getClass().getName() + " " + msg.callback + " what=" + msg.what); } msg.recycleUnchecked(); } }
我们暂时不关注细节，之关心里面的两个函数的调用
第一个：Message msg = queue.next()，这里表示从MessageQueue中取到一条信息。
第二个：msg.tartget.dispatchMessage(msg)
就是将Messag交给了handler去使用dispatchMessage()去处理，那么我们就看一下这个方法：
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }
当msg被从MessageQueue中分发出去后，被送给了handler，这时候handler会调用一个回调方法来处理这个message
(1).如果msg本身有默认的回调方法，则使用该方法处理。
(2).如果handler定义时顶一个默认的回调方法，
(3).如果上面两者都没有，则使用我们在定义Handler时重写的handleMessage方法。
大多数情况下，我们都使用第三种方式来处理信息。
5.两个简单的例子：
import android.os.Bundle; import android.os.Handler; import android.os.Message; import android.support.v7.app.AppCompatActivity; import android.view.View; import android.widget.TextView; public class UIActivity extends AppCompatActivity { private TextView tv; private Handler handler = new Handler(){ @Override public void handleMessage(Message msg) { //因为Message Queue和Looper关系，后台其实是循环的调用handleMessage方法，所以加入swith case判断 switch (msg.what){ case 0: tv = (TextView) findViewById(R.id.tv); tv.setText((CharSequence) msg.obj); } } }; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_ui); findViewById(R.id.send_text).setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { //创建一个新的线程 new Thread( new Runnable() { @Override public void run() { Message msg = new Message(); msg.what = 0 ; msg.obj = "来自另外一个线程的内容"; handler.sendMessage(msg); } } ).start(); } }); } }
第二个：
//MainActivity.java public class MainActivity extends Activity { public static final String TAG = "Main Acticity"; Button btn = null; Button btn2 = null; Handler handler = null; MyHandlerThread mHandlerThread = null; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); btn = (Button)findViewById(R.id.button); btn2 = (Button)findViewById(R.id.button2); Log.d("MainActivity.myLooper()", Looper.myLooper().toString()); Log.d("MainActivity.MainLooper", Looper.getMainLooper().toString()); btn.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View view) { mHandlerThread = new MyHandlerThread("onStartHandlerThread"); Log.d(TAG, "创建myHandlerThread对象"); mHandlerThread.start(); Log.d(TAG, "start一个Thread"); } }); btn2.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View view) { if(mHandlerThread.mHandler != null){ Message msg = new Message(); msg.what = 1; mHandlerThread.mHandler.sendMessage(msg); } } }); } } //MyHandlerThread.java public class MyHandlerThread extends Thread { public static final String TAG = "MyHT"; public Handler mHandler = null; @Override public void run() { Log.d(TAG, "进入Thread的run"); Looper.prepare(); Looper.prepare(); mHandler = new Handler(Looper.myLooper()){ @Override public void handleMessage(Message msg){ Log.d(TAG, "获得了message"); super.handleMessage(msg); } }; Looper.loop(); } }
总结：
消息机制的核心是Message，在大多数情况下要放在MessageQueue中。
使用handler将msg发送到相应的Messagequeue中，并将二者关联。
每一个Thread中有一个Looper,Looper管理一个MessageQueue，像水泵一样不断的从MessageQueue中取出msg.
取出后调用msg相关联的handler的回调方法处理message。
这样就完成了进程间的消息机制，可以在不阻塞UI线程的情况下将耗时的操作使用Handler将message传递给子线程去处理。
本文只是大致梳理了一下消息机制的框架，总结一下自己最近看的，很多细节都没有讲，等再研究一段时间后再继续写几篇深入的博客，单独分析一下各个模块。
本文疏漏之处，还望大家指正，谢谢。
参考：
https://hit-alibaba.github.io/interview/Android/basic/Android-handler-thread-looper.html
https://android.googlesource.com/platform/frameworks/base/+/refs/heads/master/core/java/android/os/MessageQueue.java
https://android.googlesource.com/platform/frameworks/base/+/refs/heads/master/core/java/android/os/Message.java
https://android.googlesource.com/platform/frameworks/base/+/refs/heads/master/core/java/android/os/Looper.java
https://android.googlesource.com/platform/frameworks/base/+/refs/heads/master/core/java/android/os/Handler.java
http://www.cnblogs.com/plokmju/p/android_handler.html
