android异步消息机制，源码层面彻底解析（一）

Handler、Message、Loopler、MessageQueen

首先看一下我们平常使用Handler的一个最常见用法。

Handler handler =new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
//这里进行一些UI操作等处理
}

new Thread(new Runnable() {
@Override
public void run() {
Message message = Message.obtain();
........
handler.sendMessage(message);
}
});
};

看一下handler的构造函数的源码

public Handler() {
this(null, false);
}
//他会调用本类中的如下构造函数
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}

mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}

看到当mLooper == null时会抛一个“Can’t create handler inside thread that has not called Looper.prepare()”这个异常，所以我们在创建handler实例前首先需要调用Looper.prepare()

public static void prepare() {
prepare(true);
}
//将looper保存到ThreadLocal中，这里可以把ThreadLocal理解为一个以当前线程为键的Map，所以一个线程中只会有一个looper
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));
}
//我们看到在new Looper(quitAllowed)中，创建了一个消息队列MessageQueen
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}

接下来我们看handler.sendMessage(message)这个方法，从字面意思就是将信息发送出去。一般sendMessage累的方法最终都会调用sendMessageAtTime(Message msg, long uptimeMillis)这个方法

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);
}

我们看到最终会执行enqueueMessage(queue, msg, uptimeMillis)这个方法

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}

最终又会调用MessageQueen中的queue.enqueueMessage(msg, uptimeMillis)这个方法，这里的queue就是looper构造方法中创建的那个消息队列

//MessageQueen的enqueueMessage方法
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}

synchronized (this) {
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
msg.recycle();
return false;
}

msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// Inserted within the middle of the queue.  Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}

// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}

MessageQueen虽然名字是一个队列，但实质上他是一个单向链表，这个结构能快速进行插入和删除操作。从上面源码可以看出来，主要是按照发送消息的时间顺序将msg插入到消息队列中。接下来我们就需要从消息队列中取出msg了。这时候就需要调用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 (;;) {
//不断从消息队列中取出msg
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);
}
//将msg交由handler处理
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();
}
}

可以看到Looper.loop()方法通过在一个死循环中调用Message msg = queue.next()将消息不断的从消息队列中取出来。queue.next()方法的作用就是从消息队列中取msg，唯一跳出循环的方式是MessageQueen的next方法返回了null。现在msg已经取出来，下一步就是怎样将他传递给handler了对吧。所以在死循环中还有一个方法msg.target.dispatchMessage(msg) ，而msg.target就是handler，在上面handler的enqueueMessage()方法中传入的msg.target = this，this就是handler本身，接下来就看看handler的dispatchMessage()方法

public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}

如果我们采用无参的构造函数创建handler，msg.callback与mCallback均为空，所以我们会调用handleMessage(msg)，这样文章开头的那个实例整个流程就走完了，handleMessage(msg)会在handler实例所在的线程中执行。

//当我们通过这种方式创建handler时，dispatchMessage中的mCallback就不为null
public Handler(Callback callback) {
this(callback, false);
}
//Callback是一个接口，里面正好也有我们需要的handleMessage(Message msg)，dispatchMessage中的 if (mCallback != null) 语句内的内容，就是我们需要重写的handleMessage(Message msg)方法
public interface Callback {
public boolean handleMessage(Message msg);
}

//当我们调用handler.post()方法执行异步任务时
public final boolean post(Runnable r)
{
return  sendMessageDelayed(getPostMessage(r), 0);
}
//getPostMessage(r)这个方法中我们看到给m.callback赋值了，就是我们传入的runnable接口
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
//最后在handleCallback方法中我们执行了它的run方法,这也就解释了为什么在子线程中可以用handler.post(Runnable r)更新UI
private static void handleCallback(Message message) {
message.callback.run();
}

总结

梳理整个执行过程

1.调用Looper.prepare()方法，这是创建handler所必须的。在主线程中由于ActivityThread已经通过Looper.prepareMainLooper（）方法创建过looper，所以在主线程中创建handler以前无需创建looper，并通过Looper.loop()来开启主线程的消息循环。

2.通过调用handler.sendMessage(message)方法最终会执行enqueueMessage(queue, msg, uptimeMillis)，enqueueMessage又会调用MessageQueen的queue.enqueueMessage(msg, uptimeMillis)，这样消息就会被添加到消息队列中。

3.调用Looper.loop()方法在死循环中执行Message msg = queue.next()，不断的将msg从消息队列中取出来，同时执行msg.target.dispatchMessage(msg)，将消息传递给handler，由handler来处理，如我们调用的handleMessage就是处理消息的方式之一。

异步处理机制流程图

从子线程进行UI 操作的几种方式

Android 提供了几种途径来从其他线程访问 UI 线程。以下列出了几种有用的方法：

• Activity.runOnUiThread(Runnable)

• View.post(Runnable) 这里的view就是我们需要改变的ui控件

• View.postDelayed(Runnable, long)

• Handler.post(Runnable, long)

但是，随着操作日趋复杂，这类代码也会变得复杂且难以维护。 要通过工作线程处理更复杂的交互，可以考虑在工作线程中使用 Handler 处理来自 UI 线程的消息。当然，最好的解决方案或许是扩展 AsyncTask 类，此类简化了与 UI 进行交互所需执行的工作线程任务。