Android消息机制 异步消息 & 消息队列排序

时间:2021-01-09 08:07:01

之前消息机制的原理都已经分析过了,不过对java层的消息队列的排序和异步没有详细讲过。


一、消息队列排序

一般的像我们普通调用Handler发送消息,最后都会调用MessageQueue的enqueueMessage。

    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);
}

像sendMessageAtFrontOfQueue这样只是最后的时间设置为0,自然就排在队列的前面了。

    public final boolean sendMessageAtFrontOfQueue(Message msg) {
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, 0);
}

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;//一般讲target设置为Handler
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
最后就到MessageQueue的enqueueMessage函数中去了,该函数就是根据when把这个msg插入到合适的消息队列中。

    boolean enqueueMessage(Message msg, long when) {
......

synchronized (this) {
......

msg.markInUse();
msg.when = when;
Message p = mMessages;//当前message
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) {//找到一个Message,when小于该message的when break
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;//这其实就是把该message插入合适的位置
}

// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}

最后我们再看下MessageQueue的next函数,就是消息线程循环时会不断调用MessageQueue的next来获取当前消息。

    Message next() {
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);//c层的epoll函数会阻塞

synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;// 当前msg
if (msg != null && msg.target == null) {//这里就是和异步消息有关,下节分析
// 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) {//当前消息还没到,后续可以处理空闲处理器等
// 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;.//下个消息赋值为mMessages
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;//没有消息无限阻塞
}
......



二、异步消息

在分析异步消息之前,我们先看下postSyncBarrier函数,这个函数类似enqueueMessage函数新建一个消息插入消息队列,但是postSyncBarrier插入的消息是没有target的。这点非常重要。

    private int postSyncBarrier(long when) {
// Enqueue a new sync barrier token.
// We don't need to wake the queue because the purpose of a barrier is to stall it.
synchronized (this) {
final int token = mNextBarrierToken++;
final Message msg = Message.obtain();
msg.markInUse();
msg.when = when;
msg.arg1 = token;

Message prev = null;
Message p = mMessages;
if (when != 0) {
while (p != null && p.when <= when) {// 找到一个合适的问题
prev = p;
p = p.next;
}
}
if (prev != null) { // invariant: p == prev.next
msg.next = p;
prev.next = msg;
} else {
msg.next = p;
mMessages = msg;
}
return token;
}
}
我们再来看next函数下面一段代码, 当msg的target为null时,这个时候上面的barrier(栅栏)生效了。这个时候只能执行异步消息,其它消息都跳过去。并且把这个异步消息从消息队列中去除了,但是绝不能修改mMessages。因为修改了mMessages之后,把barrier删除了,也不能回复消息队列了。
           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());//barrier生效,必须要找到一个异步的消息才返回
}
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) {//注意这个时候,prevMsg不为null
prevMsg.next = msg.next;//这里等于把这个异步消息在队列中去掉了(这个消息要被执行了)
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;//返回异步消息
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
下面我们子啊来看下isAsynchronous函数,是判断是否是异步的
    public boolean isAsynchronous() {
return (flags & FLAG_ASYNCHRONOUS) != 0;
}
setAsynchronous是设置异步和取消异步。
    public void setAsynchronous(boolean async) {
if (async) {
flags |= FLAG_ASYNCHRONOUS;
} else {
flags &= ~FLAG_ASYNCHRONOUS;
}
}
最后再来看看去除barrier,就是找到barrier的那个消息,然后从消息队列中移除。
    public void removeSyncBarrier(int token) {        // Remove a sync barrier token from the queue.        // If the queue is no longer stalled by a barrier then wake it.        synchronized (this) {            Message prev = null;            Message p = mMessages;            while (p != null && (p.target != null || p.arg1 != token)) {//找到barrier的那个消息                prev = p;                p = p.next;            }            if (p == null) {                throw new IllegalStateException("The specified message queue synchronization "                        + " barrier token has not been posted or has already been removed.");            }            final boolean needWake;            if (prev != null) {                prev.next = p.next;//把barrier从消息队列中移除                needWake = false;            } else {                mMessages = p.next;                needWake = mMessages == null || mMessages.target != null;            }            p.recycleUnchecked();            // If the loop is quitting then it is already awake.            // We can assume mPtr != 0 when mQuitting is false.            if (needWake && !mQuitting) {                nativeWake(mPtr);            }        }    }