handler 总结
handler架构包含那几个实体
handler：插入消息到messagequeue
looper：从messagequeue中去消息
messagequeue：存储消息，链表结构
如何保证每个线程有一个looper
使用ThreadLocal<Looper>
如何唤醒messagequeue
插入消息的时候去唤醒它，底层使用epoll，往epoll写入消息唤醒他。
空闲消息处理器和同步屏障机制听过吗？
都是优先级相关，同步屏障机制优先级最高，空闲消息处理器优先级最低。
https://blog.csdn.net/asdgbc/article/details/79148180
https://blog.csdn.net/kc58236582/article/details/52919904
分析一下去消息的next方法
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
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis); //第一次取消息会马上返回，因为nextPollTimeoutMillis=0
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) { //如果没有target，就优先处理
// 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) { //消息还没到，再等等，这个等待时间会传到epoll
// 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.
//处理空闲handler
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; //没有消息，也没有空闲的handler，block住，mBlocked为true的话，插入message就会唤醒epoll
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.
//处理空闲handler
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.
//能到这里，说明处理了空闲handler，下次nativepollonce也不需要等待，直接返回
nextPollTimeoutMillis = 0;
}
}
解析一些epoll函数
int epoll_create(int size) 创建epoll
int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event)
第一个参数是epoll_create返回值，第二个参数代表操作，第三个参数代表监听的fd，第四个参数是告诉内核需要监听什么事
int epoll_wait(int epfd, struct epoll_event * events, int maxevents, int timeout)
等待事件的产生，类似于select()调用。参数events用来从内核得到事件的集合，maxevents表示每次能处理的最大事件数，告之内核这个events有多大，这个maxevents的值不能大于创建epoll_create()时的size，参数timeout是超时时间（毫秒，0会立即返回，-1将不确定，也有说法说是永久阻塞）。该函数返回需要处理的事件数目，如返回0表示已超时。
handler里面都是怎样的
epoll_create：
mEpollFd = epoll_create(EPOLL_SIZE_HINT);
epoll_ctl：
eventItem.events = EPOLLIN;
eventItem.data.fd = mWakeEventFd;
int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem);
mWakeEventFd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
epoll_wait：
struct epoll_event eventItems[EPOLL_MAX_EVENTS];
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
native looper机制是怎样的？
通过addfd来实现的。也是通过epoll来实现等待，可以有callback。
https://blog.csdn.net/chwan_gogogo/article/details/46953563
已经有消息在处理的情况下，需要唤醒吗？
不需要！！
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) { //可以看到这里有判断，needWake才唤醒的。
nativeWake(mPtr);
}
}
return true;
}
