简述
关于android消息机制我也看了不少文章，但是没有自己真的翻看源码，因为要尝试read the fucking source code,所以下午抽时间看了下，只能说是浅析，好了言归正传，我们从new Handler()开始分析：
首先我们看构造函数：
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;
}
我们直接看Looper.myLooper();返回一个Looper对象，然后判断mLooper是否为null，如果为null直接抛出异常，跟着初始化mQueue以及其他一些属性。重点看Looper.myLopper():
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
/**
* Returns the value of this variable for the current thread. If an entry
* doesn't yet exist for this variable on this thread, this method will
* create an entry, populating the value with the result of
* {@link #initialValue()}.
*
* @return the current value of the variable for the calling thread.
*/
@SuppressWarnings("unchecked")
public T get() {
// Optimized for the fast path.
Thread currentThread = Thread.currentThread();
Values values = values(currentThread);
if (values != null) {
Object[] table = values.table;
int index = hash & values.mask;
if (this.reference == table[index]) {
return (T) table[index + 1];
}
} else {
values = initializeValues(currentThread);
}
return (T) values.getAfterMiss(this);
}
这里简单说，以为ThreadLocal我还没看，如果明天有时间看明天再补充，现在简单说，就是通过ThreadLocal获取一个lopper对象，这个对象在这个线程是唯一的。ok创建Handler对象先分析到这里，下面看调用：handler.sendEmptyMessage(0);通过我一级一级跟下去发现，最终会调用：
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);
}
sendMessageAtTime这个方法，这个方法很短，具体就不说了，直接跟进enqueueMessage方法：
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
这里必须强调下msg.target = this，这行代码及其重要，为什么handler把自己赋值给msg.target呢？后面会讲到，我们继续跟进：
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;
}
代码比较长，我简单说就是:将新的message按时间顺序插入到消息队列中。重点来了，消息是如何取出来的呢？？首先在主线程中是这样的：
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
当应用启动的时候就直接初始化了一个Looper而且是不容许中断的prepare(false)，而非ui线程是可以中断的：
public static void prepare() {
prepare(true);
}
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));
}
剩下的我们看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();
}
}
可以看出for(;;)循环获取message，重点代码
msg.target.dispatchMessage(msg);有没有想起msg.target？没错，这里调用的就是handler的dispathMessage方法，跟进：
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
主要判断msg.callback是否为空，如果不为空就直接调用handlecallback(msg),然后判断是否传入mCallback如果传入就调用mCallback.handleMessage否则调用handleMessage，看到handleMessage(msg)有没有眼熟？没错就是我们经常重写的方法，剩下的就不多说了，那么我分析下，msg.callback是什么吧：
private static void handleCallback(Message message) {
message.callback.run();
}
就一行代码，message.callback.run(),OK我们看message.callback到底什么时候赋值的，这里不知道分析的对不对，大家看代码：
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
这是handler.post(runnable)方法，跟进getPostMessage(r)继续：
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
看到没，m.callback = r，好了这个就搞明白了吧。以上分析结束。
