深入理解RunLoop ❤️

IOS底层学习-DAY-17 ❤️

官方文档：Run Loops

Runloop知识树

深入理解Runloop，看我一篇就够了

runloop中的source0和source1分别处理什么时间

When performing a selector on another thread, the target thread must have an active run loop. For threads you create, this means waiting until your code explicitly starts the run loop. Because the main thread starts its own run loop, however, you can begin issuing calls on that thread as soon as the application calls the applicationDidFinishLaunching: method of the application delegate. The run loop processes all queued perform selector calls each time through the loop, rather than processing one during each loop iteration.

Table 3-2 lists the methods defined on NSObject that can be used to perform selectors on other threads. Because these methods are declared on NSObject, you can use them from any threads where you have access to Objective-C objects, including POSIX threads. These methods do not actually create a new thread to perform the selector.

The Run Loop Sequence of Events

Each time you run it, your thread’s run loop processes pending events and generates notifications for any attached observers. The order in which it does this is very specific and is as follows:

1. Notify observers that the run loop has been entered.

2. Notify observers that any ready timers are about to fire.

3. Notify observers that any input sources that are not port based are about to fire.

4. Fire any non-port-based input sources that are ready to fire.

5. If a port-based input source is ready and waiting to fire, process the event immediately. Go to step 9.

6. Notify observers that the thread is about to sleep.

7. Put the thread to sleep until one of the following events occurs:

   • An event arrives for a port-based input source.

   • A timer fires.

   • The timeout value set for the run loop expires.

   • The run loop is explicitly woken up.

8. Notify observers that the thread just woke up.

9. Process the pending event.

   • If a user-defined timer fired, process the timer event and restart the loop. Go to step 2.

   • If an input source fired, deliver the event.

   • If the run loop was explicitly woken up but has not yet timed out, restart the loop. Go to step 2.

10. Notify observers that the run loop has exited.

Because observer notifications for timer and input sources are delivered before those events actually occur, there may be a gap between the time of the notifications and the time of the actual events. If the timing between these events is critical, you can use the sleep and awake-from-sleep notifications to help you correlate the timing between the actual events.

Because timers and other periodic events are delivered when you run the run loop, circumventing that loop disrupts the delivery of those events. The typical example of this behavior occurs whenever you implement a mouse-tracking routine by entering a loop and repeatedly requesting events from the application. Because your code is grabbing events directly, rather than letting the application dispatch those events normally, active timers would be unable to fire until after your mouse-tracking routine exited and returned control to the application.

A run loop can be explicitly woken up using the run loop object. Other events may also cause the run loop to be woken up. For example, adding another non-port-based input source wakes up the run loop so that the input source can be processed immediately, rather than waiting until some other event occurs.

深入看一下 Darwin 这个核心的架构：

CFRunLoopRef 的代码是开源的，你可以在这里 http://opensource.apple.com/tarballs/CF/CF-855.17.tar.gz 下载到整个 CoreFoundation 的源码。为了方便跟踪和查看，你可以新建一个 Xcode 工程，把这堆源码拖进去看。

Node.js 的事件处理

Windows 程序的消息循环

OSX/iOS 里的 RunLoop

NSRunLoop CFRunLoopRef

RunLoop 与线程的关系

线程和 RunLoop 之间是一一对应的，其关系是保存在一个全局的 Dictionary 里

线程刚创建时并没有 RunLoop，如果你不主动获取，那它一直都不会有。

RunLoop 的创建是发生在第一次获取时，RunLoop 的销毁是发生在线程结束时。

你只能在一个线程的内部获取其 RunLoop（主线程除外）。

一个 RunLoop 包含若干个 Mode，每个 Mode 又包含若干个 Source/Timer/Observer。

每次调用 RunLoop 的主函数时，只能指定其中一个 Mode，这个Mode被称作 CurrentMode。如果需要切换 Mode，只能退出 Loop，再重新指定一个 Mode 进入。这样做主要是为了分隔开不同组的 Source/Timer/Observer，让其互不影响。

typedef CF_OPTIONS(CFOptionFlags, CFRunLoopActivity) {
     kCFRunLoopEntry = (1UL << 0),//进入runloop
     kCFRunLoopBeforeTimers = (1UL << 1),//准备处理timer
     kCFRunLoopBeforeSources = (1UL << 2),//准备处理source
     kCFRunLoopBeforeWaiting = (1UL << 5),//准备休眠
     kCFRunLoopAfterWaiting = (1UL << 6),//休眠结束，处理事件之前
     kCFRunLoopExit = (1UL << 7),//runloop退出
     kCFRunLoopAllActivities = 0x0FFFFFFFU//所有状态
};