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 timer
do {
     __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;
}

8、CFRunLoopMode