原文：https://github.com/ming1016/study/wiki/CFRunLoop

简单的说run loop是事件驱动的一个大循环，如下代码所示

int main(int argc, char * argv[]) {     //程序一直运行状态     while (AppIsRunning) {          //睡眠状态，等待唤醒事件          id whoWakesMe = SleepForWakingUp();          //得到唤醒事件          id event = GetEvent(whoWakesMe);          //开始处理事件          HandleEvent(event);     }     return 0;}

Cocoa会涉及到Run Loops的

系统级：GCD，mach kernel，block，pthread
应用层：NSTimer，UIEvent，Autorelease，NSObject(NSDelayedPerforming)，NSObject(NSThreadPerformAddition)，CADisplayLink，CATransition，CAAnimation，dispatch_get_main_queue()（GCD中dispatch到main queue的block会被dispatch到main RunLoop执行），NSPort，NSURLConnection，AFNetworking(这个第三方网络请求框架使用在开启新线程中添加自己的run loop监听事件)

在Main thread堆栈中所处位置

堆栈最底层是start(dyld)，往上依次是main，UIApplication(main.m) -> GSEventRunModal(Graphic Services) -> RunLoop(包含CFRunLoopRunSpecific，CFRunLoopRun，__CFRunLoopDoSouces0，__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION) -> Handle Touch Event

RunLoop原理

CFRunLoop开源代码：http://opensource.apple.com/source/CF/CF-855.17/

执行顺序的伪代码

SetupThisRunLoopRunTimeoutTimer(); // by GCD timerdo {     __CFRunLoopDoObservers(kCFRunLoopBeforeTimers);     __CFRunLoopDoObservers(kCFRunLoopBeforeSources);     __CFRunLoopDoBlocks();     __CFRunLoopDoSource0();     CheckIfExistMessagesInMainDispatchQueue(); // GCD     __CFRunLoopDoObservers(kCFRunLoopBeforeWaiting);     var wakeUpPort = SleepAndWaitForWakingUpPorts();     // mach_msg_trap     // Zzz...     // Received mach_msg, wake up     __CFRunLoopDoObservers(kCFRunLoopAfterWaiting);     // Handle msgs     if (wakeUpPort == timerPort) {          __CFRunLoopDoTimers();     } else if (wakeUpPort == mainDispatchQueuePort) {          // GCD          __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__()     } else {          __CFRunLoopDoSource1();     }     __CFRunLoopDoBlocks();} while (!stop && !timeout);

构成

Thread包含一个CFRunLoop，一个CFRunLoop包含一种CFRunLoopMode，mode包含CFRunLoopSource，CFRunLoopTimer和CFRunLoopObserver。

CFRunLoopMode

RunLoop只能运行在一种mode下，如果要换mode当前的loop也需要停下重启成新的。利用这个机制，ScrollView过程中NSDefaultRunLoopMode的mode会切换UITrackingRunLoopMode来保证ScrollView的流畅滑动不受只能在NSDefaultRunLoopMode时处理的事件影响滑动。同时mode还是可定制的。

NSDefaultRunLoopMode：默认，空闲状态
UITrackingRunLoopMode：ScrollView滑动时
UIInitializationRunLoopMode：启动时
NSRunLoopCommonModes：Mode集合 Timer计时会被scrollView的滑动影响的问题可以通过将timer添加到NSRunLoopCommonModes来解决

//将timer添加到NSDefaultRunLoopMode中[NSTimer scheduledTimerWithTimeInterval:1.0     target:self     selector:@selector(timerTick:)     userInfo:nil     repeats:YES];//然后再添加到NSRunLoopCommonModes里NSTimer *timer = [NSTimer timerWithTimeInterval:1.0     target:self     selector:@selector(timerTick:)     userInfo:nil     repeats:YES];[[NSRunLoop currentRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];

CFRunLoopTimer

NSTimer是对RunLoopTimer的封装

+ (NSTimer *)timerWithTimeInterval:(NSTimeInterval)ti invocation:(NSInvocation *)invocation repeats:(BOOL)yesOrNo;+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)ti invocation:(NSInvocation *)invocation repeats:(BOOL)yesOrNo;- (void)performSelector:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay inModes:(NSArray *)modes;+ (CADisplayLink *)displayLinkWithTarget:(id)target selector:(SEL)sel;- (void)addToRunLoop:(NSRunLoop *)runloop forMode:(NSString *)mode;

CFRunLoopSource

source0：处理如UIEvent，CFSocket这样的事件
source1：Mach port驱动，CFMachport，CFMessagePort

CFRunLoopObserver

Cocoa框架中很多机制比如CAAnimation等都是由RunLoopObserver触发的。observer到当前状态的变化进行通知。

typedef CF_OPTIONS(CFOptionFlags, CFRunLoopActivity) {     kCFRunLoopEntry = (1UL << 0),     kCFRunLoopBeforeTimers = (1UL << 1),     kCFRunLoopBeforeSources = (1UL << 2),     kCFRunLoopBeforeWaiting = (1UL << 5),     kCFRunLoopAfterWaiting = (1UL << 6),     kCFRunLoopExit = (1UL << 7),     kCFRunLoopAllActivities = 0x0FFFFFFFU};

使用RunLoop的案例

AFNetworking

使用NSOperation+NSURLConnection并发模型都会面临NSURLConnection下载完成前线程退出导致NSOperation对象接收不到回调的问题。AFNetWorking解决这个问题的方法是按照官方的guidhttps://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSURLConnection_Class/Reference/Reference.html#//apple_ref/occ/instm/NSURLConnection/initWithRequest:delegate:startImmediately:上写的NSURLConnection的delegate方法需要在connection发起的线程runloop中调用，于是AFNetWorking直接借鉴了Apple自己的一个Demohttps://developer.apple.com/LIBRARY/IOS/samplecode/MVCNetworking/Introduction/Intro.html的实现方法单独起一个global thread，内置一个runloop，所有的connection都由这个runloop发起，回调也是它接收，不占用主线程，也不耗CPU资源。

+ (void)networkRequestThreadEntryPoint:(id)__unused object {     @autoreleasepool {          [[NSThread currentThread] setName:@"AFNetworking"];          NSRunLoop *runLoop = [NSRunLoop currentRunLoop];          [runLoop addPort:[NSMachPort port] forMode:NSDefaultRunLoopMode];          [runLoop run];     }}+ (NSThread *)networkRequestThread {     static NSThread *_networkRequestThread = nil;     static dispatch_once_t oncePredicate;     dispatch_once(&oncePredicate, ^{          _networkRequestThread =          [[NSThread alloc] initWithTarget:self               selector:@selector(networkRequestThreadEntryPoint:)               object:nil];          [_networkRequestThread start];     });     return _networkRequestThread;}

类似的可以用这个方法创建一个常驻服务的线程。

TableView中实现平滑滚动延迟加载图片

利用CFRunLoopMode的特性，可以将图片的加载放到NSDefaultRunLoopMode的mode里，这样在滚动UITrackingRunLoopMode这个mode时不会被加载而影响到。

UIImage *downloadedImage = ...;[self.avatarImageView performSelector:@selector(setImage:)     withObject:downloadedImage     afterDelay:0     inModes:@[NSDefaultRunLoopMode]];

接到程序崩溃时的信号进行自主处理例如弹出提示等

CFRunLoopRef runLoop = CFRunLoopGetCurrent();NSArray *allModes = CFBridgingRelease(CFRunLoopCopyAllModes(runLoop));while (1) {     for (NSString *mode in allModes) {          CFRunLoopRunInMode((CFStringRef)mode, 0.001, false);     }}

异步测试

- (BOOL)runUntilBlock:(BOOL(^)())block timeout:(NSTimeInterval)timeout{     __block Boolean fulfilled = NO;     void (^beforeWaiting) (CFRunLoopObserverRef observer, CFRunLoopActivity activity) =     ^(CFRunLoopObserverRef observer, CFRunLoopActivity activity) {          fulfilled = block();          if (fulfilled) {               CFRunLoopStop(CFRunLoopGetCurrent());          }     };     CFRunLoopObserverRef observer = CFRunLoopObserverCreateWithHandler(NULL, kCFRunLoopBeforeWaiting, true, 0, beforeWaiting);     CFRunLoopAddObserver(CFRunLoopGetCurrent(), observer, kCFRunLoopDefaultMode);     // Run!     CFRunLoopRunInMode(kCFRunLoopDefaultMode, timeout, false);     CFRunLoopRemoveObserver(CFRunLoopGetCurrent(), observer, kCFRunLoopDefaultMode);     CFRelease(observer);     return fulfilled;}