handler 源码分析

handler
Looper 轮询器
MessageQueue 消息对象

1 主线程在一创建的时候就会调用，    public static void prepareMainLooper() {}构造方法。
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}

2 在prepareMainLooper(){} 内部调用了 prepare(false);方法，这就是在子线程中new Handler()会抱错的关键

prepare(quitAllowed) {}方法里面设置了一个Looper对象，如果已经有了 Looper 对象，会抛出异常 Only one Looper may be created per thread
所以说一个 Handler只能有一个Looper对象
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));//创建一个 Looper构造器
}

3 在 Looper 的构造器中
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);//创建了 MessageQueue对象
mRun = true;
mThread = Thread.currentThread();//线程对象
}

4 但是在 handler(){}的源码构造方法中
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();//只执行这个方法
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;
}

5 查看 Looper.myLooper();
public static Looper myLooper() {
return sThreadLocal.get();//返回的是一个 Looper对象，这里就跟 2的结果一样了
}
所以在4 中抛出异常，跟2 也一样了
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
所以敢肯定（1 2 3）的原理就是主线程 Handler的工作原理
而 （4 5）就是我们手动创建 Handler的时候的工作原理。

handler.sendMessage(msg);他做的是将消息入队操作
6 经过源码跟踪，会发现在调用enqueueMessage(){}构造方法的时候，所做的事情就是将消息就行，入栈处理
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);//注意这里，想必从字面意思理解，enqueueMessage就是入栈的意思吧
}

7 看enqueueMessage()所做的事情
final boolean enqueueMessage(Message msg, long when) {
if (msg.isInUse()) {
throw new AndroidRuntimeException(msg + " This message is already in use.");
}
if (msg.target == null) {
throw new AndroidRuntimeException("Message must have a target.");
}

boolean needWake;
synchronized (this) {
if (mQuiting) {
RuntimeException e = new RuntimeException(
msg.target + " sending message to a Handler on a dead thread");
Log.w("MessageQueue", e.getMessage(), e);
return false;
}

msg.when = when;
Message p = mMessages;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg; //将消息对象的引用赋值给 Message
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; //将消息对象的引用赋值给 Message
}
}
if (needWake) {
nativeWake(mPtr);
}
return true;
}
8 那么问题来了，消息引用都传递给Message对象了，那是如何从 Message中吧消息分发出去，并响应呢？这就得看 Looper的源码中的 public static void loop() {}方法
其实 loop就是一个轮询器，在不断的从  MessageQueue中获取消息，可以看 loop()中的 Message msg = queue.next(); 内部实现源码，next() 方法就是消息队列的出队方法。不过由于这个方法的代码稍微有点长，我就不贴出来了，它的简单逻辑就是如果当前MessageQueue中存在mMessages(即待处理消息)，就将这个消息出队，然后让下一条消息成为mMessages，否则就进入一个阻塞状态，一直等到有新的消息入队
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 (;;) {//天呐，在这里居然是 for的空循环
//queue.next() 出现了，有兴趣的可以点进去看看
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);//我们又发现了什么？对，msg.target代表的是Handler，调用了dispatchMessage方法
// 这样我相信大家就都明白了为什么handleMessage()方法中可以获取到之前发送的消息了吧！
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.recycle();
}
}