消息机制
Java层
1.Looper构建了消息队列MessageQueue
2.Message的成员target关联Handler
3.Handler的成员mQueue关联MessageQueue,成员mLooper关联Looper
4.MessageQueue的成员mMessages关联Message
我们使用Handler时都是先要调Looper的prepare方法，用于创建Looper和MessageQueue，主线程直接使用Handler是因为在系统已经提前给我们创建好了主线程的Looper和MessageQueue
创建App进程后会调用ActivityThread的main()方法
public static void main(String[] args) {
···
Looper.prepareMainLooper();
····
Looper.loop();
}
1，创建主线程Looper
2，为主线程Handler赋值
3，Looper.looper启动循环
Looper的prepare()
Looper.prepareMainLooper
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
prepare(false)这里的prepare传的是false，我们自己创建的线程调用Looper.prepare()传递的是true表示可以退出
sMainLooper为全局主线程Looper赋值
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) { /* 如果消费者线程已有Looper绑定了,则抛出异常 */
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed)); /* 创建Looper,并绑定到消费者线程 */
}
如果Looper.prepare只能调用一次，如果多次调用会抛异常
Looper创建后会存储在ThreadLocal中，ThreadLocal可以保证线程是唯一的
Looper的构造函数
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed); //创建消息队列
mThread = Thread.currentThread(); //记录消费者线程
}
Looper的loop()
public static void loop() {
......
for (;;) {
Message msg = queue.next(); //消息队列出队得到Message事务
if (msg == null) {
return;
}
msg.target.dispatchMessage(msg);//执行具体事务
......
}
MessageQueue.next
Message next() {
···
for (;;) {
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;
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;
}
// 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
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
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;
}
}
1，nativePollOnce 如果无消息让消费者线程进入休眠状态
2，如果设置消息屏障取出异步消息
3，检测消息时间是否到达，到达取出消息，未到达设置超时时间（目标时间 - 当前时间）
4，未到达，设置超时时间，消费者线程进入休眠
5，执行IdelHanlder
msg.target.dispatchMessage(msg)
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
1，先处理Message CallBack
2，Handler的Callback
3，处理handleMessage
发送消息
Hanlder.sendMessage()，最终会调用MessageQueue的enqueueMessage
boolean enqueueMessage(Message msg, long when) {
···
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;
}
1，如果队列为空直接插入节点
2，队列不为空，按时间排序插入
3，线程休眠，插入消息为屏障，插入消息是异步消息并且在第一位，需要唤醒线程
Native层
Looper.prepare()
sp<Looper> Looper::prepare(int opts) {
......
/* 获取当前消费者线程线程绑定的Looper对象
* 由于这里首次调用prepare,还未绑定Looper,因此返回空
*/
sp<Looper> looper = Looper::getForThread();
if (looper == NULL) {
/* 创建Looper对象(见1.2),然后绑定到当前消费者线程中 */
looper = new Looper(allowNonCallbacks);
Looper::setForThread(looper);
}
return looper;
......
}
在Looper的构造函数
1，创建了eventfd
2，创建epoll文件描述符，用于监听eventfd
Looper.pollOnce
1，消费者线程调用epoll.wait检测是否有消息就绪，如果没有休眠
2，当fd就绪，消费者线程被唤醒
MessageQueue
构造函数中会创建Looper
1.Looper(Java)启动消息循环,先处理Looper(Native)事务,然后再处理Looper(Java)事务
2.Looper(Native)和Looper(Java)均无事务处理时,消费者线程会进入超时休眠状态,等待事务就绪时唤醒
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