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RunLoop

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简介

简单的说run loop是事件驱动的一个大循环，如下代码所示

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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

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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

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//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

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+ (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。

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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guid
https://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Demo[https://developer.apple.com/LIBRARY/IOS/samplecode/MVCNetworking/Introduction/Intro.html](https://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。

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+ (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。

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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

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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

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- (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;

}