Handler在Android开发中无处不在，它的使用方式想必大家都已经很熟练了，这里主要是分析它的原理。
我们从ActivityThread#main方法开始，一步步理解Handler的机制。相关代码如下：
/frameworks/base/core/java/android/app/ActivityThread.java
public static void main(String[] args) {
...
Looper.prepareMainLooper();
...
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
在ActivityThread初始化时就通过Looper建立了消息循环机制，先看下初始化部分，相关代码如下：
/frameworks/base/core/java/android/os/Looper.java
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
// 确保主线程仅有一个Looper实例
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
private static void prepare(boolean quitAllowed) {
// 确保线程仅对应一个Looper
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
这里通过ThreadLocal确保线程安全，且保证任何线程只能对应一个Looper实例。Looper实例化时，就确定了它所在的线程，同时创建了一个MessageQueue，代码如下：
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
MessageQueue稍后再研究，先看下Looper#loop的实现，代码如下：
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 (;;) {
// 等待消息，如果没有消息，可能会阻塞
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
...
try {
// 分发消息
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
...
msg.recycleUnchecked();
}
}
这里建立了一个无限循环，不断地从MessageQueue中获取消息，然后进行分发，而我们使用的Handler并没有直接绑定在Looper中，而是绑定在msg.target变量里，这样做的好处是可以创建多个Handler。下面我们转到MessageQueue中，了解下MessageQueue#next方法，代码如下：
/frameworks/base/core/java/android/os/MessageQueue.java
Message next() {
...
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; // 当前消息
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
// 这个消息是从其他线程发送过来的，不进行处理
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.
// 没到msg分发时间
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;
}
...
}
...
}
}
这里通过for循环不断的获取消息，然后等待消息分发时间到之后将消息分发出去，这样消息循环就建立好了，接下来就应该通过Handler来发送消息和处理消息。Handler发送消息的方法有很多种，我们主要使用的有以下几种：
handler.sendMessage(msg)handler.sendMessageDelayed(msg, delay)handler.post(runnable)handler.postDelayed(runnable, delay)
除了以上几个，还有几个类似的方法，甚至还有一个Handler#sendMessageAtFrontOfQueue方法，可以在消息队列的最前面插入消息，不过这些方法的原理都是一致的，我们着重分析Handler#sendMessage和Handler#post这两个方法。代码如下：
/frameworks/base/core/java/android/os/Handler.java
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
可以看到，它们最终都是调用了Handler#sendMessageDelayed方法，只是通过post方式最终将Runnable转成了Message对象，具体的做法如下：
private static Message getPostMessage(Runnable r) {
// 从一个全局的对象池里获取Message对象，可以重复使用，这样就节省了new对象的开支
Message m = Message.obtain();
m.callback = r;
return m;
}
也就是说这个Runnable实例就是作为Message的callback的。继续看Handler#sendMessageDelayed方法，代码如下：
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
...
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);
}
最终，所有的方法都会调用到这个Handler#enqueueMessage方法，将Handler本身作为Message的target参数，然后将消息入队，最后在Looper中分发。接下来看下入队的操作，代码如下：
/frameworks/base/core/java/android/os/MessageQueue.java
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) {
// 新消息需要更早分发
// 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();
// 根据时间顺序，将消息插入到合适的位置
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;
}
通过以上代码我们发现，消息在入队时，就已经按照时间顺序排列好了，最后到了分发阶段，分发是通过Handler#dispatchMessage完成的，代码如下：
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}
原来，如果在发送Message时设置了callback，就会由我们设置的Runnable来处理，否则，就通过Handler初始化时指定的Callback处理，如果都没有设置，就通过Handler#handleMessage方法来处理。这个mCallback是在构造函数中实例化的，我们看几个构造方法就明白了：
public Handler() {
this(null, false);
}
public Handler(Callback callback) {
this(callback, false);
}
public Handler(Looper looper) {
this(looper, null, false);
}
public Handler(Looper looper, Callback callback) {
this(looper, callback, false);
}
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
至此，我们对Handler机制就有了清晰的认识，它并不复杂，但是功能十分强大，掌握它的原理以后使用时会更加顺手。
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