调用Looper.quit()和Looper.quitsafely()的时候发生了什么？
根据官方文档：
Looper.quit()
调用后直接终止
Looper，不在处理任何Message，所有尝试把Message放进消息队列的操作都会失败，比如Handler.sendMessage()会返回 false，但是存在不安全性，因为有可能有Message还在消息队列中没来的及处理就终止Looper了。
Looper.quitsafely()
调用后会在所有消息都处理后再终止
Looper，所有尝试把Message放进消息队列的操作也都会失败。
Looper
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
...
for (;;) {
// 获取消息队列中的下一个消息，如果消息队列没有消息就阻塞
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);
}
// 最后回收这个 Message
msg.recycleUnchecked();
}
}
/**
* Quits the looper.
*
* Causes the {@link #loop} method to terminate without processing any
* more messages in the message queue.
*
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
*
* Using this method may be unsafe because some messages may not be delivered
* before the looper terminates. Consider using {@link #quitSafely} instead to ensure
* that all pending work is completed in an orderly manner.
*
*
* @see #quitSafely
*/
public void quit() {
mQueue.quit(false);
}
/**
* Quits the looper safely.
*
* Causes the {@link #loop} method to terminate as soon as all remaining messages
* in the message queue that are already due to be delivered have been handled.
* However pending delayed messages with due times in the future will not be
* delivered before the loop terminates.
*
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the {@link Handler#sendMessage(Message)} method will return false.
*
*/
public void quitSafely() {
mQueue.quit(true);
}
都调用了MessageQueue的quit(boolean)
MessageQueue
Message next() {
...
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;
}
if (mQuitting) {
dispose();
return null;
}
...
}
...
}
void quit(boolean safe) {
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true; // 先把 mQuitting 改为 true
if (safe) {
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}
private void removeAllMessagesLocked() {
Message p = mMessages; // mMessages 是消息队列单链表的表头
// 消息队列所有消息都执行 recycleUnchecked() 进行回收
while (p != null) {
Message n = p.next;
p.recycleUnchecked();
p = n;
}
// 最后把 mMessages 设置为 null Looper 的 for 循环就会结束
mMessages = null;
}
private void removeAllFutureMessagesLocked() {
final long now = SystemClock.uptimeMillis();
Message p = mMessages;
if (p != null) {
//如果表头的消息是延迟消息，那么整个消息队列都可以直接回收了
if (p.when > now) {
removeAllMessagesLocked();
} else {
Message n;
for (;;) {
n = p.next;
if (n == null) {
return;
}
// 如果当前消息是延迟消息，跳出循环剩下消息进入 do while 循环
if (n.when > now) {
break;
}
p = n;
}
p.next = null;
do {
p = n;
n = p.next;
p.recycleUnchecked();
} while (n != null);
}
}
}
在这里我们先回顾一下Message是怎么在Looper、MessageQueue和Handler中传递了，我们知道当Looper.loop()被执行后Handler机制就启动了，根据上面的代码可以看到loop()里面有一个for循环，只有当MessageQueue的next()返回null的时候才会退出循环终止Handler机制。再看看MessageQueue的next()我们也看到一个for循环，如果有消息的话就把消息返回，没有消息且mQuitting=false的时候继续循环下去，只有当没有消息然后mQuitting=true的时候返回null。
根据Looper代码所示Looper.quit()最终会调用MessageQueue.removeAllMessagesLocked()，而Looper.quitsafely()会调用MessageQueue.removeAllFutureMessagesLocked()。
- removeAllMessagesLocked() 表示直接把消息队列里面的消息清空
- removeAllFutureMessagesLocked() 表示把所有延迟消息清除
根据我在上面代码中写的注释我们可以总结Looper.quit()和Looper.quitsafely()做了什么和区别在哪里。
quit()实际上是把消息队列全部清空，然后让MessageQueue.next()返回null令Looper.loop()循环结束从而终止Handler机制，但是存在着不安全的地方是可能有些消息在消息队列没来得及处理。而quitsafely()做了优化，只清除消息队列中延迟信息，等待消息队列剩余信息处理完之后再终止Looper循环。
所有Message被回收都需要调用Message.recycleUnchecked()，那么Message的回收机制是怎样的呢，可以看看下面的代码和注释:
Message
/**
* Return a new Message instance from the global pool. Allows us to
* avoid allocating new objects in many cases.
*/
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
// 从缓存池中获取 Message
Message m = sPool;
sPool = m.next;
m.next = null;
// 在这里把 recycleUnchecked() 中回收时设置的 in-use 标志去掉
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}
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++;
}
}
}
recycleUnchecked()回收Message过程分为两步：
1 先把Message的变量初始化并标记为 in-use 令该Message无法被发送到消息队列。
2 判断Message缓存池缓存数量是否达到上限(默认50)，Message缓存池也是一个链表，没有的话就通过把Message以头部添加方式放到缓存池中。
Message提供一个obtain()方法从缓存池中获取回收的Message
官方推荐需要创建
Message对象的时候通过Message.obtain()来获取。