Android之消息机制(三)源码解析
Android之消息机制(三)源码解析
干货，非常干。。。
目录
一、概述
二、源码
2.1 Message源码
2.1.1 实现序列化
2.1.2 obtain()方法，消息池中取消息，以链表实现队列；
2.1.3 Message的recycle(),用于回收消息，时机是在Looper.loop()方法中，执行完Handler处理后。
2.2 MessageQueue源码
2.2.1 next()方法通过when时间来取元素
2.3 Looper源码
2.3.1 prepare(),其实是采用ThreadLocal来解决线程同步的问题（TheadLocal是Looper的成员变量）；使用ThreadLocal连接了某线程和Looper对象；
2.3.2 Loop(),循环处理消息
2.3.3 quit(),会调用MessageQueue中的quit()将消息队列中消息置空，然后Looper.loop()时
2.4 Handler源码
2.4.1 sendMessage()，发送消息
2.4.2 dispatchMessage()方法，处理消息事件
三、小结
一、概述
Looper不停从MessageQueue中取Message，MessageQueue是在Looper中的（MessageQueue是Looper的一个成员变量）；
Message实现了Parceable（以便序列化，因为有时会需要进程间通信而去传递Message）
消息池，Message的obtain方法，用链表的方式实现池，首先取第一个元素，而让首指针指向下一个元素即可。
Looper中loop方法
for(;;){Message msg = queue.next();msg.target.dispatchMessage(msg);}
msg.target即为handler，每一个Message都持有一个发送者Handler的引用。
Looper.prepare()方法
调用ThreadLocal.set(Looper),ThreadLocal保证了线程安全，不让其他线程去获取主线程的Looper；
ThreadLocal(Values类似HashMap),通过ThreadLocal将Thread与Looper进行绑定；
ThreadLocal.put(this,values)
this为Looper对象，values可以用来拿当前Thread，保持唯一Thread
Handler声明在哪里，拿到的Looper就是该线程的Looper
Handler中会有mlooper=Looper.myLooper(),myLooper()就是会ThreadLocal中取values。
应用的运行，ActivityThread的looper.loop()后，如何改变主线程的执行？
通过新增Message去改变。
二、源码
2.1 Message源码
2.1.1 实现序列化
public final class Message implements Parcelable
2.1.2 obtain()方法，消息池中取消息，以链表实现队列；
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();
}
消息池中有消息就去取（以链表的方式取头指针的Message），没有消息就new出来.
2.1.3 Message的recycle(),用于回收消息，时机是在Looper.loop()方法中，执行完Handler处理后。
public void recycle() {
if (isInUse()) {
if (gCheckRecycle) {
throw new IllegalStateException("This message cannot be recycled because it "
+ "is still in use.");
}
return;
}
recycleUnchecked();
}
void recycleUnchecked() {
// Mark the message as in use while it remains in the recycled object pool.
// Clear out all other details.
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = -1;
when = 0;
target = null;
callback = null;
data = null;
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
其实看得出也是把该Message的属性进行重置，然后把该Message放到消息池的头指针上。
2.2 MessageQueue源码
2.2.1 next()方法通过when时间来取元素
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 (false) Log.v("MessageQueue", "Returning message: " + msg);
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("MessageQueue", "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;
}
}
核心代码主要是
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 (false) Log.v("MessageQueue", "Returning message: " + msg);
return msg;
}
找出when比当前时间小的Message，通过断链的方式将其取出来。
2.3 Looper源码
2.3.1 prepare(),其实是采用ThreadLocal来解决线程同步的问题（TheadLocal是Looper的成员变量）；使用ThreadLocal连接了某线程和Looper对象；
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));
}
2.3.2 Loop(),循环处理消息
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();
}
}
关键点是Message msg = queue.next(); // might block取出消息，然后，msg.target.dispatchMessage(msg);其中msg.target就是handler，这里就是进行handler处理消息，
最后处理完msg后，msg.recycleUnchecked();对消息进行回收。
2.3.3 quit(),会调用MessageQueue中的quit()将消息队列中消息置空，然后Looper.loop()时
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
则会跳出Looper.loop()的循环；
public void quit() {
mQueue.quit(false);
}
MessageQuue中代码
void quit(boolean safe) {
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true;
if (safe) {
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}
然后会执行 removeAllMessagesLocked();
private void removeAllMessagesLocked() {
Message p = mMessages;
while (p != null) {
Message n = p.next;
p.recycleUnchecked();
p = n;
}
mMessages = null;
}
将所有的Message全部进行消息置空且回收。
2.4 Handler源码
2.4.1 sendMessage()，发送消息
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);
}
最终会调到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);
}
则会去调用MessageQueue的enqueueMessage(msg, uptimeMillis)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("MessageQueue", 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;
}
该方法就是将该Message插入到队列的尾部。
2.4.2 dispatchMessage()方法，处理消息事件
Looper.loop()会去调用Handler的dispatchMessage()方法，
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
通常会采用handleMessage(msg)的方式，当然也可以采用Handler的post(Runnable r)的方式，那么其实调用的就是handleCallback(msg);
三、小结
以上源码如果看的云里雾里的话，可以考虑重点观察这条主线Looper.loop()的方法。
Looper.loop()就是从MessageQueue中得到下一个Message(MessageQueue的next()方法)，取出的message通过调用msg.target(也就是handler)去执行dispatchMessage()方法，执行完毕后调用了message的recycle()方法进行回收以便从消息池去取出。