Handler在Android多线程场景中应用广泛。
Handler可以将工作线程中需要更新UI的操作信息传递到UI线程（主线程）。多个线程同时更新UI的时候，可以保证线程安全。
要理解Handler的工作机制，需要了解几个类：Handler，MessageQueue，Message，Looper。
先看一张图：
简单来说，Handler发送消息Message到MessageQueue中，Looper不断获取MessageQueue中Message，然后再交由Handler来处理。
再来看一张类图：
类图中没有直观地反应Handler与Thread的关系，但可以看出：
- 每个Thread只对应一个Looper
- 每个Looper只对应一个MessageQueue
- 每个MessageQueue中有N个Message
- 每个Message最多指定一个Handler（target）来处理事件
所以，Thread和Handler是一对多的关系。
从Handler的入口开始分析，看一看Handler的构造方法。
/**
* Use the {@link Looper} for the current thread with the specified callback interface
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with respect to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
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;
}Handler构造函数里面只是做了初始化的工作，真正开启消息循环的方法是通过Looper的prepare()和loop().
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
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
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (slowDispatchThresholdMs > 0) {
final long time = end - start;
if (time > slowDispatchThresholdMs) {
Slog.w(TAG, "Dispatch took " + time + "ms on "
+ Thread.currentThread().getName() + ", h=" +
msg.target + " cb=" + msg.callback + " msg=" + msg.what);
}
}
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();
}
}loop()中有一个无限 for循环，不断地从MessageQueue中取出Message，然后通过msg.target.dispatchMessage(msg)来分发消息处理，其实这里的msg.target就是Handler，在后面源码分析中会看到。
发送消息是Handler的一个作用，通过post或者send来将Message压入MessageQueue。
-
post
post(Runnable r);
postAtTime(Runnable r, long uptimeMillis); -
Send
sendEmptyMessage(int what); sendMessageAtFrontOfQueue(Message msg); snedMessageAtTime(Message msg, long uptimeMillis); sendMessageDelayed(Message msg, long delayMills);
其实，post方法最终也是调用了send方法，将post中的Runnable转换成Message的callback。
public final boolean post(Runnable r) {
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
/*
Android系统会维护一个全局的Message池。当用户需要使用Message时，可以通过obtain直接获得，而不是自行创建。这样的设计可以避免不必要的资源浪费。
*/
Message m = Message.obtain();
m.callback = r;
return m;
}看一看send源码
public final boolean sendEmptyMessage(int what) {
return sendEmptyMessageDelayed(what, 0);
}
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
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);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}也就是说post或者send函数都是调用sendMessageDelayed方法，最终都会调用sendMessageAtTime方法，然后通过enqueueMessage方法将Message压入队列。这里有一行msg.target = this，将Handler赋值给msg的target。
MessageQueue，名字叫队列，MessageQueue有两个重要的方法。
- 元素入队
enqueueMessage(Message msg, long when); - 元素出队
next();
MessageQueue有一个成员变量Message mMessages, Message实际上是一个链表。
public final class Message implements Parcelable {
/*package*/ Handler target;
/*package*/ Runnable callback;
// sometimes we store linked lists of these things
/*package*/ Message next;
}再看一看MessageQueue中的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;
}从上述代码可以看出MessageQueue的中链表Message是按照when从小到大排序的，当一个新的Message要入列的话，会遍历链表，比较when直到插入到正确的位置。
再看看next()方法
Message next() {
...
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;
}
...
}
}这里next()方法有点类似迭代器的next()，返回第一个target不为空的异步msg。这个方法可能会阻塞。
消息处理是通过Handler的dispatchMessage()方法分发。我们来看下dispatchMessage的源码。
/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*/
public interface Callback {
/**
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
public boolean handleMessage(Message msg);
}
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}所以，消息处理的顺序是Message.callback-Handler.callback-Handler.handleMessage。
一句话概括：Handler发送消息Message到MessageQueue中，Looper不断获取MessageQueue中Message，然后再交由Handler来处理。