Handler源码中如何实现Delay

雨点打透心脏的1/2处 2022-10-01 07:43 9阅读 0赞

简要记录下调用链：

handler.postDelay-->handler.sendMessageDelay-->sendMessageAtTime-->

handler.enqueueMessage(MessageQueue,Message,long uptimeMillis)-->

messageQueue.enqueueMessage（Message msg,long when)

最后逻辑在messageQueue.enqueueMessage（Message msg,long when)方法中，贴一下这个方法代码

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; } 复制代码

大意是when还没到，阻塞，新的消息来了加入头部先去执行，等when到了再唤醒。

转载于:https://juejin.im/post/5c35408af265da61117a7bb1