消息机制主要包含4个类
Message 消息
MessageQueue 消息队列，增加一个消息enqueueMessage()，取出一个消息next()
Handler 发送消息sendMessage()和处理消息handleMessage()
Looper 死循环从MessageQueue中取出消息并发送handler.dispatchMessage()
实例
class MyThread extends Thread {
public Handler handler;
public void run() {
Looper.prepare();
handler = new Handler() {
public void handleMessage(Message msg) {
}
};
Looper.loop();
}
}
Looper
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>()
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) { //Looper.prepare()在每个线程只允许执行一次
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();
}
ThreadLocal 线程本地存储区（Thread Local Storage），每个线程都有自己私有的本地存储区，不同线程彼此不能访问对方的ThreadLocal
在这里的体现就是：虽然 sThreadLocal 是static变量，但是不同的线程调用 sThreadLocal.get() 取出来的数据是不一样的
Looper.prepare() 新建一个Looper存入static的sThreadLocal，Looper内部维护一个MessageQueue。另外，还有一个prepareMainLooper()，该方法主要在ActivityThread类中使用
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;
...
for (;;) {
Message msg = queue.next(); // might block 没消息会阻塞
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
...
msg.target.dispatchMessage(msg);
...
msg.recycleUnchecked();
}
}
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
Looper.loop() 不断循环直到没有消息时退出循环，但其实没有消息时，queue.next()会阻塞，直到有新消息才继续执行。msg.recycleUnchecked()把Message回收到消息池，以便重复利用。（Message解析在下面）
取出消息后，调用msg.target.dispatchMessage(msg)，msg.target是Handler（Handler解析在下面）
public void quit() {
mQueue.quit(false);
}
public void quitSafely() {
mQueue.quit(true);
}
Looper.quit() 最终调用的是MessageQueue.quit()
MessageQueue
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
Message next() {
...
for (;;) {
...
//阻塞操作，当等待超时或者消息队列被唤醒，才会继续
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;
//如果msg.target==null，则一直循环msg = msg.next，直到msg.target不为null，msg.target其实是Handler
if (msg != null && msg.target == null) {
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;
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
...
}
...
}
}
MessageQueue.next() nativePollOnce()取出一个message，是阻塞操作，nextPollTimeoutMillis
是下一个消息的延迟（比如handler.postDelay(r,long)里的delay），当nextPollTimeoutMillis = -1时，说明队列中没有消息，会一直阻塞。
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");
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.
// 如果队列里没有message，next()会阻塞，这里收到新的message，则需要唤醒next()
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
...
}
if (needWake) {
nativeWake(mPtr); //唤醒next()的阻塞
}
}
return true;
}
MessageQueue.enqueueMessage() 有新消息会执行nativeWake()唤醒next()里的阻塞。
MessageQueue.removeMessages() 移除队列里所有的message，放入缓存池。
Handler
public Handler() {
this(null, false);
}
public Handler(Callback callback, boolean async) {
...
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;
}
mQueue = mLooper.mQueue 说明Handler里持有的mQueue其实是Looper的。
public final boolean sendMessage(Message msg) {
return sendMessageDelayed(msg, 0);
}
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;
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; //注意这行
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
Handler.sendMessage() 最终调用Looper.mQueue.enqueueMessage()，在调用前会设置msg.target = this
public final boolean post(Runnable r) {
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
Handler.post() 也只是把runnable封装成一个msg
public final Message obtainMessage(int what) {
return Message.obtain(this, what);
}
Handler.obtainMessage() 只是调用Message.obtain()
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();
}
Handler.dispatchMessage() msg有callback就执行msg.callback.run()，没有的话，就执行handleMessage(msg)。
其实Handler.post(r) 就相当于下面这段代码，只不过系统没有开放callback变量。
Message msg = Message.obtain();
msg.callback = new Runnable(){
@Override
public void run() {
}
};
handler.sendMessage(msg);
Handler.handleMessage() 就是我们经常重写的方法。
Message
private static Message sPool;
private static final int MAX_POOL_SIZE = 50;
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++;
}
}
}
Message.recycle() 把message的变量都置空，赋给static的sPool，sPool也是一个Message，Message是一个链表，通过变量next连接。这里控制最多只能存50个。
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();
}
Message.obtain() 如果sPool里面有值，把第一个取出来。
总结
- Handler的构造函数，会执行mLooper = Looper.myLooper()。
- Handler.sendMessage() 的时候，会设置msg.target = this 然后调用 mLooper.mQueue.enqueueMessage()。
Handler.post(runnable) 其实是msg.callback=runnable，最终也是跟sendMessage() 一样的流程。 - Looper.prepare()的时候会新建一个Looper并存入sThreadLocal。
- Looper的构造函数，会执行mQueue = new MessageQueue()。
- Looper.loop() 不断循环mQueue.next()取出msg，然后调用msg.target.dispatchMessage(msg)。
- MessageQueue 里有很多native方法，MessageQueue.next()没有消息时会阻塞，MessageQueue.enqueueMessage()收到消息时会唤醒阻塞。
- 新建Message最好使用Message.obtain()，会优先从缓存池里获取，避免创建对象。
- 我们在UI线程无需调用Looper.prepare()是因为系统已经帮我们执行过Looper.prepareMainLooper()。
- 子线程不能直接修改UI，是因为UI的很多方法（比如draw）都不是线程安全的，所以引入Handler和Looper，用队列的形式确保操作UI是同步安全的。
