Runloop有效利用空闲时间

Runloop可被监听的状态

Runloop可被监听的状态是由CFRunLoopActivity结构体提供的：

public static var entry: CFRunLoopActivity { get }

    public static var beforeTimers: CFRunLoopActivity { get }

    public static var beforeSources: CFRunLoopActivity { get }

    public static var beforeWaiting: CFRunLoopActivity { get }

    public static var afterWaiting: CFRunLoopActivity { get }

    public static var exit: CFRunLoopActivity { get }

    public static var allActivities: CFRunLoopActivity { get }
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利用Runloop空闲执行操做

let flags = CFRunLoopActivity.beforeWaiting
let runloopObserVer = CFRunLoopObserverCreateWithHandler(kCFAllocatorDefault, flags.rawValue, true, 0) { (observer, activity) in
        self.perform(#selector(self.doSomething), with: nil, afterDelay: 0.01, inModes: [RunLoop.Mode.default])
    }
CFRunLoopAddObserver(CFRunLoopGetCurrent(), runloopObserVer, CFRunLoopMode.defaultMode)
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这样在beforeWaiting时会被监听到而后执行操做。因为执行操做又会唤醒Runloop这样就会进入一个循环所以须要在适当的时候移除掉监听：

CFRunLoopRemoveObserver(CFRunLoopGetCurrent(), runloopObserVer, CFRunLoopMode.defaultMode)
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