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。

[b]1.Message是什么？[/b]

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;
}
}

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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