boolean enqueueMessage(Message msg, long when) { if (msg.target == null) { throw new IllegalArgumentException("Message must have a target."); } //TODO 开始锁，当前对象锁，所以在MessageQueue在存消息的时候是不能取消息或者qiut的 synchronized (this) { if (msg.isInUse()) { throw new IllegalStateException(msg + " This message is already in use."); }
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); } } //结束锁 return true; }
@UnsupportedAppUsage 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 // 1.如果nextPollTimeoutMillis=-1，一直阻塞不会超时。 // 2.如果nextPollTimeoutMillis=0，不会阻塞，立即返回。 // 3.如果nextPollTimeoutMillis>0，最长阻塞nextPollTimeoutMillis毫秒(超时) // 如果期间有程序唤醒会立即返回。 int nextPollTimeoutMillis = 0; for (;;) { if (nextPollTimeoutMillis != 0) { Binder.flushPendingCommands(); } //native的方法，线程阻塞的作用 nativePollOnce(ptr, nextPollTimeoutMillis); //锁开始的地方 synchronized (this) { // Try to retrieve the next message. Return if found. final long now = SystemClock.uptimeMillis(); Message prevMsg = null; Message msg = mMessages; //当前链表的头结点
// 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
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; } }
if (msg != null) {
......
} else {
// No more messages.
//没有消息，nextPollTimeoutMillis复位
nextPollTimeoutMillis = -1;
}
然后又调用了这个方法，并且return了null。
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// Process the quit message now that all pending messages have been handled. //如果消息队列正在处于退出状态返回null，调用dispose();释放该消息队列 if (mQuitting) { dispose(); return null; }
