本文主要从Handle、Looper、Message、MessageQueue这四个方面进行讲述。
一、首先看一下Message源码
1、Message的成员变量有：
public int what;
public int arg1;
public int arg2;
public Object obj;
public Messenger replyTo;
public int sendingUid = -1;
/*package*/ static final int FLAG_IN_USE = 1 << 0;
/*package*/ static final int FLAG_ASYNCHRONOUS = 1 << 1;
/*package*/ static final int FLAGS_TO_CLEAR_ON_COPY_FROM = FLAG_IN_USE;
/*package*/ int flags;
/*package*/ long when;
/*package*/ Bundle data;
/*package*/ Handler target;
/*package*/ Runnable callback;
/*package*/ Message next;
private static final Object sPoolSync = new Object();
private static Message sPool;
private static int sPoolSize = 0;
private static final int MAX_POOL_SIZE = 50;
private static boolean gCheckRecycle = true;从上述源码可以看出，外部无法调用Message的callback方法。在看下Handle中是如何get message的：
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
private static Message getPostMessage(Runnable r, Object token) {
Message m = Message.obtain();
m.obj = token;
m.callback = r;
return m;
}这里两个方法都是私有的。这里引入一个问题，如果你想获取当前runnable的Message该如何操作？
里面的target和callback，后面将对此进行关联。其中arg1和arg2是用来存放整型数据的，what用来保存消息标识，obj是Object类型的任意对象，replyTo是消息管理器，会关联到一个handler。通常Message对象不是直接new出来，只要调用handler中的obtainMessage方法来直接获得Message对象。这也是Android推荐的做法。
下面看一下obtainMessage的源码：
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}如果池中没有才会new一个Message。
二、MessageQueue
MessageQueue是一个final class，用来存放消息的消息队列，它具有队列的常规操作，包括：
1、构造方法新建队列
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}nativeInit()会在本地创建一个NativeMessageQueue对象，然后赋给MessageQueue中的成员变量，这一系列通过内存指针进行。
static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) {
NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();
if (!nativeMessageQueue) {
jniThrowRuntimeException(env, "Unable to allocate native queue");
return 0;
}
nativeMessageQueue->incStrong(env);
return reinterpret_cast<jlong>(nativeMessageQueue);
}2、Message元素入队
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;
}这里传入了两个参数，Message当然是当前线程的消息，那么when有什么作用用呢？
按个人目前理解when是决定事件在MessageQueue中的位置，即执行的先后顺序。我们可以看下handle下的postAtFrontOfQueue方法，它的作用是将该Message放在MessageQueue的最前面，看下它的实现。
public final boolean postAtFrontOfQueue(Runnable r)
{
return sendMessageAtFrontOfQueue(getPostMessage(r));
} public final boolean sendMessageAtFrontOfQueue(Message msg) {
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, 0);
} private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}大家可以看到它的when为0。而Handle中普通的post的方法的when为SystemClock.uptimeMillis() + delayMillis。
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}3、执行消息
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
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;
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;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
}从其中这段代码：可以验证之前的when作用的假设
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);
} 4、其他像removeMessages，dispose这里就不展开叙述了。
三、Handle
1、Handle中的成员变量
final Looper mLooper;
final MessageQueue mQueue;
final Callback mCallback;
final boolean mAsynchronous;
IMessenger mMessenger;可以看到Looper和MessageQueue都是它的成员变量。
2、Handle的构造方法
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
} 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;
} public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}从这里可以看出每个Handle只有一个MessageQueue，都为Looper.mQueue。
2、Handle加入Message
Handle加入的消息可调用的方法有：post、postDelayed、postAtTime、sendMessage、sendEmptyMessage、sendMessageAtTime等。
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
} public final boolean postDelayed(Runnable r, long delayMillis)
{
return sendMessageDelayed(getPostMessage(r), delayMillis);
} public final boolean postAtTime(Runnable r, long uptimeMillis)
{
return sendMessageAtTime(getPostMessage(r), uptimeMillis);
} public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
} public final boolean sendEmptyMessage(int what)
{
return sendEmptyMessageDelayed(what, 0);
} 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);
}post系列的方法会调用相应的sendEmptyMessage、sendEmptyMessageDelayed等方法，最终进入sendMessageAtTime中，然后调用enqueueMessage，把Message压入队列中。
由于post方法的参数是Runnable对象，所以Hander内部提供了getPostMessage方法把Runnable对象转化为Message。
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}3、Handle处理Message
3.1 下面看一下dispatchMessage和handleMessage两个方法。
如果Message中的callback不为空，通过callback来处理。由我们的Message决定。
如果Handler的mCallback不为空，通过mCallback来处理。由Handle初始化决定。
如果上面两个都为空，才调用handleMessage来处理。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}再看一下我们在何时调用dispatchMessage(msg)方法。
在Looper源码中，通过msg.target.dispatchMessage(msg)调用的。那么Looper()方法又是什么时候调用的，看第四部分。
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);
}
}
........
msg.recycleUnchecked();
}
}四、Looper
Looper也是一个final class，并且持有一个MessageQueue，MessageQueue作为线程的消息存储仓库，配合Handler, Looper一起完成一系列操作。值得注意的是，还有一个final Thread和一个final ThreadLocal<Looper>的成员变量，其中ThreadLocal负责创建一个只针对当前线程的Looper及其它相关数据对象，其它线程无法访问。
Looper调用规范：
* class LooperThread extends Thread {
* public Handler mHandler;
*
* public void run() {
* Looper.prepare();
*
* mHandler = new Handler() {
* public void handleMessage(Message msg) {
* // process incoming messages here
* }
* };
*
* Looper.loop();
* }其实就是三个步骤：
Looper.prepare()准备工作
创建消息处理的handler
调用Looper.loop()进入消息循环
看下Looper.prepare
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));
} private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}从这里可以看出一个线程只有一个Looper；一个Looper只有一个MessageQueue；Handle在构建方法中获取这个Queue。
简单梳理一下：
1、一个线程只有一个Looper；一个Looper只有一个MessageQueue；Handle在构建方法中获取这个Queue。
2、Looper.prepare -----------------> 创建一个Looper
Looper.looper -------------------> 创建一个MessageQueue
Handle.enqueueMessage -------> 将Message压入MessageQueue
Looper.loop --------------------->调用MessageQueue的next方法，获取当前需要处理的Message
Looper.loop ---------------------->调用Handle的dispatchMessage
handle.dispatchMessage ---------> 调用Handle的handleMessage
