Android消息机制：Looper,MessageQueue,Message与handler

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