iOS-OC底层-RunLoop

前言

RunLoop，又一个面试常常被问到的东西，它是什么？一个运行循环，用来处理App中的各种任务，当有任务的时候，执行任务，没任务的时候进入休眠；每一个线程对应一个Runloop,主线程的RunLoop默认是开启的，子线程的需要手动开启;每一个Runloop下对应多个Modes，每一个Mode下又有多个Items事务.........这些在面试的时候基本上都能答得上来，但我也相信大部分人都只是停留在这个概念上，并没有深入的去探索过它。今天我就带着大家来一步步探索一下。

Runloop初探

都说Runloop本质上就是一个while循环，那它跟普通的while循环有什么区别呐？下面是一个普通的while循环：

int main(int argc, char * argv[]) {
    @autoreleasepool {
        while (1) {
            
        }
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));;
    }
}

运行后观察CPU的情况：

while循环的 CPU情况

再看下CFRunLoopRun的内部实现：

void CFRunLoopRun(void) {    /* DOES CALLOUT */
    int32_t result;
    do {
        result = CFRunLoopRunSpecific(CFRunLoopGetCurrent(), kCFRunLoopDefaultMode, 1.0e10, false);
        CHECK_FOR_FORK();
    } while (kCFRunLoopRunStopped != result && kCFRunLoopRunFinished != result);
}

while内部什么都没有写，CPU就飙到97%，这实在可怕。而Runloop并没有让CPU飙升，说明它并没有持续的占用CPU资源，而是有事务的时候，执行事务，没有事务的时候，进入休眠状态。
那Runloop都在处理哪些事务呐？
block应用： __CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__
timer调用：__CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__
响应source0：CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION
响应source1：__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__
GCD主队列：__CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__
Observer:__CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__
接下来我们通过代码直观的感受一下，
1.首先来执行一个NSTimer，通过bt查看堆栈信息：

- (void)viewDidLoad {
    [super viewDidLoad];
    [self sourceDemo];
}
- (void)sourceDemo{
    [NSTimer scheduledTimerWithTimeInterval:1 repeats:YES block:^(NSTimer * _Nonnull timer) {
        NSLog(@"www.zezefamily.icu");
    }];
}

NSTimer事件

Observer+touchesBegan:

- (void)viewDidLoad {
    [super viewDidLoad];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(todoNotify:) name:@"heheh" object:nil];
}
#pragma mark - 触摸事件
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
    // __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__
    NSLog(@"touchesBegan");
    [[NSNotificationCenter defaultCenter] postNotificationName:@"heheh" object:@"zezefamily"];
}
- (void)todoNotify:(NSNotification *)noti{
    // __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__
    // NSLog(@"gotNotification = %@",noti);
}

source0+observer

这里先是处理了一个source0事件(touch),后处理observer(通知)。

block

- (void)sourceDemo{
    void (^block)(void) = ^{
        NSLog(@"zezefamily");
    };
    block();
}

block事件

queue

- (void)test{
    dispatch_async(dispatch_get_main_queue(), ^{
        NSLog(@"zezefamily");
    });
}

queue

官网文档定义
RunLoop

Runloop与线程,Mode以及Item(事务)之间的关系

这里我们使用CFRunLoop，而不是NSRunloop，要知道的是NSRunloop本质是CFRunLoop的对象化封装，所以直接从CFRunLoop层来探索底层。这里通过CFRunLoop来获取下当前的运行循环和主运行循环：

- (void)viewDidLoad {
    [super viewDidLoad];
    // 主运行循环
    CFRunLoopRef mainRunloop = CFRunLoopGetMain();
    //当前运行循环
    CFRunLoopRef currentRunloop = CFRunLoopGetCurrent();
}

通过获取返回的是一个CFRunLoopRef的类型，看一下它的内部结构：

typedef struct __CFRunLoop * CFRunLoopRef;

struct __CFRunLoop {
    CFRuntimeBase _base;
    pthread_mutex_t _lock;            /* locked for accessing mode list */
    __CFPort _wakeUpPort;            // used for CFRunLoopWakeUp
    Boolean _unused;
    volatile _per_run_data *_perRunData;              // reset for runs of the run loop
    pthread_t _pthread;         // 当前runloop 所在的线程
    uint32_t _winthread;         
    CFMutableSetRef _commonModes;         // 所有的_commonModes
    CFMutableSetRef _commonModeItems;    // 所有的事务
    CFRunLoopModeRef _currentMode;        // 当前Mode,[CFRunLoopModeRef类型]
    CFMutableSetRef _modes;                     // 所有的modes
    struct _block_item *_blocks_head;
    struct _block_item *_blocks_tail;
    CFAbsoluteTime _runTime;
    CFAbsoluteTime _sleepTime;
    CFTypeRef _counterpart;
};

typedef struct __CFRunLoopMode *CFRunLoopModeRef;
struct __CFRunLoopMode {
    CFRuntimeBase _base;
    pthread_mutex_t _lock;    /* must have the run loop locked before locking this */
    CFStringRef _name;
    Boolean _stopped;
    char _padding[3];
    CFMutableSetRef _sources0;
    CFMutableSetRef _sources1;
    CFMutableArrayRef _observers;
    CFMutableArrayRef _timers;
    CFMutableDictionaryRef _portToV1SourceMap;
    __CFPortSet _portSet;
    CFIndex _observerMask;
#if USE_DISPATCH_SOURCE_FOR_TIMERS
    dispatch_source_t _timerSource;
    dispatch_queue_t _queue;
    Boolean _timerFired; // set to true by the source when a timer has fired
    Boolean _dispatchTimerArmed;
#endif
#if USE_MK_TIMER_TOO
    mach_port_t _timerPort;
    Boolean _mkTimerArmed;
#endif
#if DEPLOYMENT_TARGET_WINDOWS
    DWORD _msgQMask;
    void (*_msgPump)(void);
#endif
    uint64_t _timerSoftDeadline; /* TSR */
    uint64_t _timerHardDeadline; /* TSR */
};

CFRunLoopRef 本质是一个指向__CFRunLoop类型的结构体指针。其中包含一个pthread_t _pthread; 的线程成员变量，同时还包含了_commonModes，_commonModeItems, _currentMode, _modes。
再看一下CFRunLoopGetMain()和CFRunLoopGetCurrent()

CFRunLoopRef CFRunLoopGetMain(void) {
    CHECK_FOR_FORK();
    static CFRunLoopRef __main = NULL; // no retain needed
    if (!__main) __main = _CFRunLoopGet0(pthread_main_thread_np()); // no CAS needed
    return __main;
}

CFRunLoopRef CFRunLoopGetCurrent(void) {
    CHECK_FOR_FORK();
    CFRunLoopRef rl = (CFRunLoopRef)_CFGetTSD(__CFTSDKeyRunLoop);
    if (rl) return rl;
    return _CFRunLoopGet0(pthread_self());
}

都执行了一个同样的函数_CFRunLoopGet0(),这里就是获取RunLoop对象的函数了：

static CFMutableDictionaryRef __CFRunLoops = NULL;
static CFLock_t loopsLock = CFLockInit;
// should only be called by Foundation
// t==0 is a synonym for "main thread" that always works
CF_EXPORT CFRunLoopRef _CFRunLoopGet0(pthread_t t) {
    
    if (pthread_equal(t, kNilPthreadT)) { // 如果t== nil, t = main_queue()
        t = pthread_main_thread_np();
    }
    __CFLock(&loopsLock);
    if (!__CFRunLoops) {   //字典不存在
        __CFUnlock(&loopsLock);
        //创建字典
        CFMutableDictionaryRef dict = CFDictionaryCreateMutable(kCFAllocatorSystemDefault, 0, NULL, &kCFTypeDictionaryValueCallBacks);
        //创建runloop
        CFRunLoopRef mainLoop = __CFRunLoopCreate(pthread_main_thread_np());
        // 进行绑定 dict[@"pthread_main_thread_np"] = mainLoop   线程 = runLoop
        CFDictionarySetValue(dict, pthreadPointer(pthread_main_thread_np()), mainLoop);
        if (!OSAtomicCompareAndSwapPtrBarrier(NULL, dict, (void * volatile *)&__CFRunLoops)) {
            CFRelease(dict);
        }
        CFRelease(mainLoop);
        __CFLock(&loopsLock);
    }
    //字典存在
    //根据线程在字典中取出对应的Runloop
    CFRunLoopRef loop = (CFRunLoopRef)CFDictionaryGetValue(__CFRunLoops, pthreadPointer(t));
    __CFUnlock(&loopsLock);
    if (!loop) { // 如果loop不存在
        //创建Runloop
        CFRunLoopRef newLoop = __CFRunLoopCreate(t);
        __CFLock(&loopsLock);
        //判断是否已经保存在字典中
        loop = (CFRunLoopRef)CFDictionaryGetValue(__CFRunLoops, pthreadPointer(t));
        if (!loop) {
            //保存
            CFDictionarySetValue(__CFRunLoops, pthreadPointer(t), newLoop);
            loop = newLoop;
        }
        // don't release run loops inside the loopsLock, because CFRunLoopDeallocate may end up taking it
        __CFUnlock(&loopsLock);
        CFRelease(newLoop);
    }
    if (pthread_equal(t, pthread_self())) {   //如果线程t == 当前线程 （这里也是一层缓存处理）
        _CFSetTSD(__CFTSDKeyRunLoop, (void *)loop, NULL);
        if (0 == _CFGetTSD(__CFTSDKeyRunLoopCntr)) {
            _CFSetTSD(__CFTSDKeyRunLoopCntr, (void *)(PTHREAD_DESTRUCTOR_ITERATIONS-1), (void (*)(void *))__CFFinalizeRunLoop);
        }
    }
    return loop;
}

static CFRunLoopRef __CFRunLoopCreate(pthread_t t) {
    CFRunLoopRef loop = NULL;
    CFRunLoopModeRef rlm;
    uint32_t size = sizeof(struct __CFRunLoop) - sizeof(CFRuntimeBase);
    loop = (CFRunLoopRef)_CFRuntimeCreateInstance(kCFAllocatorSystemDefault, CFRunLoopGetTypeID(), size, NULL);
    if (NULL == loop) {
    return NULL;
    }
    (void)__CFRunLoopPushPerRunData(loop);
    __CFRunLoopLockInit(&loop->_lock);
    loop->_wakeUpPort = __CFPortAllocate();
    if (CFPORT_NULL == loop->_wakeUpPort) HALT;
    __CFRunLoopSetIgnoreWakeUps(loop);
    loop->_commonModes = CFSetCreateMutable(kCFAllocatorSystemDefault, 0, &kCFTypeSetCallBacks);
    CFSetAddValue(loop->_commonModes, kCFRunLoopDefaultMode);
    loop->_commonModeItems = NULL;
    loop->_currentMode = NULL;
    loop->_modes = CFSetCreateMutable(kCFAllocatorSystemDefault, 0, &kCFTypeSetCallBacks);
    loop->_blocks_head = NULL;
    loop->_blocks_tail = NULL;
    loop->_counterpart = NULL;
    loop->_pthread = t;
#if DEPLOYMENT_TARGET_WINDOWS
    loop->_winthread = GetCurrentThreadId();
#else
    loop->_winthread = 0;
#endif
    rlm = __CFRunLoopFindMode(loop, kCFRunLoopDefaultMode, true);
    if (NULL != rlm) __CFRunLoopModeUnlock(rlm);
    return loop;
}

_CFRunLoopGet0(pthread_t t)大致逻辑也比较简单，首先会判断__CFRunLoops(理解为全局的一个字典)是否存在，当字典不存在，先创建字典，并通过__CFRunLoopCreate()函数创建一个mainLoop主运行循环，并通过key value 的方式保存入字典，key为线程，value为runloop;
当字典存在，首先根据pthread_t t这个key，从字典中查询对应的loop对象，如果loop = nil(不存在)，通过__CFRunLoopCreate(t)创建loop对象，并保存在全局的字典中；最后返回loop对象。
这里再解读一下__CFRunLoopCreate(pthread_t t)函数：
主要是通过_CFRuntimeCreateInstance()创建一个CFRunLoopRef类型的对象loop，并进行一系列的赋值操作，这里可以看到loop->_pthread = t;。
以上过程，充分说明：RunLoop与线程是一一对应的关系。
通过LLDB看下RunLoop的输出：

(lldb) po CFRunLoopCopyAllModes(mainRunloop)
<__NSArrayM 0x600002984ff0>(
UITrackingRunLoopMode,
GSEventReceiveRunLoopMode,
kCFRunLoopDefaultMode
)

(lldb) po CFRunLoopCopyCurrentMode(mainRunloop)
kCFRunLoopDefaultMode

通过这里的输出，并结合struct __CFRunLoop，可以看到，一个RunLoop可包含多个Mode, 即RunLoop与Mode 是一对多的关系。
通过LLDB执行po mainRunloop 来查看一下RunLoop的输出，这里主要看下modes,以下是我整理的modes:

modes = {
    type = mutable set, count = 3,
    entries =>
        0 : {name = UITrackingRunLoopMode, port set = 0x4e03, queue = 0x600001770900, source = 0x600001770a00 (not fired), timer port = 0x4c03, 
            sources0 = {type = mutable set, count = 4,
                entries =>
                    0 : {signalled = No, valid = Yes, order = -1, context = {version = 0, info = 0x0, callout = PurpleEventSignalCallback (0x7fff38b30a0e)}}
                    1 : {signalled = No, valid = Yes, order = -1, context = {version = 0, info = 0x60000027c340, callout = __handleEventQueue (0x7fff48c64d04)}}
                    2 : {signalled = No, valid = Yes, order = -2, context = {version = 0, info = 0x600003729aa0, callout = __handleHIDEventFetcherDrain (0x7fff48c64d73)}}
                    3 : {signalled = Yes, valid = Yes, order = 0, context = {version = 0, info = 0x600001d7f780, callout = FBSSerialQueueRunLoopSourceHandler (0x7fff36c80bd5)}}
            },
            sources1 = {type = mutable set, count = 1,
                entries =>
                2 : {signalled = No, valid = Yes, order = -1, context = {version = 1, info = 0x3203, callout = PurpleEventCallback (0x7fff38b30a1a)}}
            },
            observers = (
                "{valid = Yes, activities = 0x1, repeats = Yes, order = -2147483647, callout = _wrapRunLoopWithAutoreleasePoolHandler (0x7fff48bc10bc), context = {type = mutable-small, count = 1, values = (\n\t0 : <0x7f9fcf00c048>\n)}}",
                "{valid = Yes, activities = 0x20, repeats = Yes, order = 0, callout = _UIGestureRecognizerUpdateObserver (0x7fff4874f06a), context = }",
                "{valid = Yes, activities = 0xa0, repeats = Yes, order = 1999000, callout = _beforeCACommitHandler (0x7fff48bf1fb7), context = }",
                "{valid = Yes, activities = 0xa0, repeats = Yes, order = 2000000, callout = _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv (0x7fff2b43cfa2), context = }",
                "{valid = Yes, activities = 0xa0, repeats = Yes, order = 2001000, callout = _afterCACommitHandler (0x7fff48bf2020), context = }",
                "{valid = Yes, activities = 0xa0, repeats = Yes, order = 2147483647, callout = _wrapRunLoopWithAutoreleasePoolHandler (0x7fff48bc10bc), context = {type = mutable-small, count = 1, values = (\n\t0 : <0x7f9fcf00c048>\n)}}"
            ),
            timers = (null),
            currently 628410160 (3209085620118608) / soft deadline in: 1.8443535e+10 sec (@ -1) / hard deadline in: 1.8443535e+10 sec (@ -1)
        },

        1 : {name = GSEventReceiveRunLoopMode, port set = 0x4b03, queue = 0x600001770a80, source = 0x600001770b80 (not fired), timer port = 0x4a03, 
            sources0 = {
                type = mutable set, count = 1,
                entries =>
                    0 : {signalled = No, valid = Yes, order = -1, context = {version = 0, info = 0x0, callout = PurpleEventSignalCallback (0x7fff38b30a0e)}}
            },
            sources1 = {
                type = mutable set, count = 1,
                entries =>
                    2 : {signalled = No, valid = Yes, order = -1, context = {version = 1, info = 0x3203, callout = PurpleEventCallback (0x7fff38b30a1a)}}
            },
            observers = (null),
            timers = (null),
            currently 628410160 (3209085622357658) / soft deadline in: 1.8443535e+10 sec (@ -1) / hard deadline in: 1.8443535e+10 sec (@ -1)
        },

        2 : {name = kCFRunLoopDefaultMode, port set = 0x1f03, queue = 0x60000177d780, source = 0x60000177d880 (not fired), timer port = 0x2203, 
            sources0 = {
                type = mutable set, count = 4,
                entries =>
                0 : {signalled = No, valid = Yes, order = -1, context = {version = 0, info = 0x0, callout = PurpleEventSignalCallback (0x7fff38b30a0e)}}
                1 : {signalled = No, valid = Yes, order = -1, context = {version = 0, info = 0x60000027c340, callout = __handleEventQueue (0x7fff48c64d04)}}
                2 : {signalled = No, valid = Yes, order = -2, context = {version = 0, info = 0x600003729aa0, callout = __handleHIDEventFetcherDrain (0x7fff48c64d73)}}
                3 : {signalled = Yes, valid = Yes, order = 0, context = {version = 0, info = 0x600001d7f780, callout = FBSSerialQueueRunLoopSourceHandler (0x7fff36c80bd5)}}
            },
            sources1 = {
                type = mutable set, count = 1,
                entries =>
                    2 : {signalled = No, valid = Yes, order = -1, context = {version = 1, info = 0x3203, callout = PurpleEventCallback (0x7fff38b30a1a)}}
                },
                observers = (
                "{valid = Yes, activities = 0x1, repeats = Yes, order = -2147483647, callout = _wrapRunLoopWithAutoreleasePoolHandler (0x7fff48bc10bc), context = {type = mutable-small, count = 1, values = (\n\t0 : <0x7f9fcf00c048>\n)}}",
                "{valid = Yes, activities = 0x20, repeats = Yes, order = 0, callout = _UIGestureRecognizerUpdateObserver (0x7fff4874f06a), context = }",
                "{valid = Yes, activities = 0xa0, repeats = Yes, order = 1999000, callout = _beforeCACommitHandler (0x7fff48bf1fb7), context = }",
                "{valid = Yes, activities = 0xa0, repeats = Yes, order = 2000000, callout = _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv (0x7fff2b43cfa2), context = }",
                "{valid = Yes, activities = 0xa0, repeats = Yes, order = 2001000, callout = _afterCACommitHandler (0x7fff48bf2020), context = }",
                "{valid = Yes, activities = 0xa0, repeats = Yes, order = 2147483647, callout = _wrapRunLoopWithAutoreleasePoolHandler (0x7fff48bc10bc), context = {type = mutable-small, count = 1, values = (\n\t0 : <0x7f9fcf00c048>\n)}}"
            ),
            timers = {type = mutable-small, count = 1, values = (
                    0 : {valid = Yes, firing = No, interval = 0, tolerance = 0, next fire date = 628409978 (-182.466853 @ 3208903165363796), callout = (Delayed Perform) UIApplication _accessibilitySetUpQuickSpeak (0x7fff25925cc0 / 0x7fff480e99fb) (/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Library/Developer/CoreSimulator/Profiles/Runtimes/iOS.simruntime/Contents/Resources/RuntimeRoot/System/Library/PrivateFrameworks/UIKitCore.framework/UIKitCore), context = }
                )},
            currently 628410160 (3209085622413965) / soft deadline in: 1.84467439e+10 sec (@ 3208903165363796) / hard deadline in: 1.84467439e+10 sec (@ 3208903165363796)
        },

}

这里可以看到，每一个Mode内部又对应着sources0,sources1,observers,timers等多个事务(items)，最终就形成了如下的关系图表：

Runloop,mode items关系图

RunLoop处理事务的具体流程

- (viod)viewDidLoad
{
    [super viewDidLoad];
    dispatch_queue_t queue = dispatch_queue_create("test_queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        [NSTimer scheduledTimerWithTimeInterval:1 repeats:YES block:^(NSTimer * _Nonnull timer) {
            NSLog(@"zezefamily");
        }];
//        [[NSRunLoop currentRunLoop] run];
    });
}

上面代码，创建了一个并发队列，并添加了一个异步任务，内部添加了一个NSTimer定时器，这里很清楚的是，当代码执行后，NSTimer并不会执行，原因是NSTimer的回调依赖于RunLoop，而子线程的RunLoop默认是没有开启的。所以当执行了[[NSRunLoop currentRunLoop] run];之后，NSTimer就开始正常执行了。下一步就看一下，RunLoop的run是如何去触发NSTimer的回调的。
[[NSRunLoop currentRunLoop] run];的本质就是CFRunLoopRun()，看一下它的源码：

CF_EXPORT void CFRunLoopRun(void);
void CFRunLoopRun(void) {    /* DOES CALLOUT */
    int32_t result;
    do {
        result = CFRunLoopRunSpecific(CFRunLoopGetCurrent(), kCFRunLoopDefaultMode, 1.0e10, false);
        CHECK_FOR_FORK();
    } while (kCFRunLoopRunStopped != result && kCFRunLoopRunFinished != result);
}

在文章开头，其实已经看到过这段代码，上面只是简单的认识了下内部是个do while循环，现在看下循环的内部：核心是一个CFRunLoopRunSpecific()函数：

SInt32 CFRunLoopRunSpecific(CFRunLoopRef rl, CFStringRef modeName, CFTimeInterval seconds, Boolean returnAfterSourceHandled) {     /* DOES CALLOUT */
    
    CHECK_FOR_FORK();
    //判断runloop状态是否为IsDeallocating 如果是直接kCFRunLoopRunFinished
    if (__CFRunLoopIsDeallocating(rl)) return kCFRunLoopRunFinished;
    __CFRunLoopLock(rl);
    //根据modeName 获取 currentMode
    CFRunLoopModeRef currentMode = __CFRunLoopFindMode(rl, modeName, false);
    // 为空判断处理
    if (NULL == currentMode || __CFRunLoopModeIsEmpty(rl, currentMode, rl->_currentMode)) {
        Boolean did = false;
        if (currentMode) __CFRunLoopModeUnlock(currentMode);
        __CFRunLoopUnlock(rl);
        return did ? kCFRunLoopRunHandledSource : kCFRunLoopRunFinished;
    }
    
    volatile _per_run_data *previousPerRun = __CFRunLoopPushPerRunData(rl);
    //将rl->_currentMode切换到currentMode
    CFRunLoopModeRef previousMode = rl->_currentMode;
    rl->_currentMode = currentMode;
    int32_t result = kCFRunLoopRunFinished;
    //判断当前活动状态是否=kCFRunLoopEntry
    if (currentMode->_observerMask & kCFRunLoopEntry ){
        //do Oserver
        __CFRunLoopDoObservers(rl, currentMode, kCFRunLoopEntry);
        //__CFRunLoopRun
        result = __CFRunLoopRun(rl, currentMode, seconds, returnAfterSourceHandled, previousMode);
    }
    //判断当前活动状态是否=kCFRunLoopExit
    if (currentMode->_observerMask & kCFRunLoopExit ){
        __CFRunLoopDoObservers(rl, currentMode, kCFRunLoopExit);
        __CFRunLoopModeUnlock(currentMode);
        __CFRunLoopPopPerRunData(rl, previousPerRun);
    }
    //将rl->_currentMode切换到previousMode(之前的模式)
    rl->_currentMode = previousMode;
    __CFRunLoopUnlock(rl);
    
    return result;
}

上面的代码，现实进行了一些容错处理；之后执行mode切换，判断RunLoop活动状态是否为kCFRunLoopEntry，如果是进入result = __CFRunLoopRun();如果RunLoop活动状态为kCFRunLoopExit,则result = kCFRunLoopRunFinished。最后返回result。
而正在处理事务(Items)逻辑的函数就是:
__CFRunLoopRun(rl, currentMode, seconds, returnAfterSourceHandled, previousMode);
这个函数的代码又臭又长，非常不好阅读，

/* rl, rlm are locked on entrance and exit */
static int32_t __CFRunLoopRun__CFRunLoopRun(CFRunLoopRef rl, CFRunLoopModeRef rlm, CFTimeInterval seconds, Boolean stopAfterHandle, CFRunLoopModeRef previousMode) {
    uint64_t startTSR = mach_absolute_time();

    if (__CFRunLoopIsStopped(rl)) {
        __CFRunLoopUnsetStopped(rl);
    return kCFRunLoopRunStopped;
    } else if (rlm->_stopped) {
    rlm->_stopped = false;
    return kCFRunLoopRunStopped;
    }
    
    mach_port_name_t dispatchPort = MACH_PORT_NULL;
    Boolean libdispatchQSafe = pthread_main_np() && ((HANDLE_DISPATCH_ON_BASE_INVOCATION_ONLY && NULL == previousMode) || (!HANDLE_DISPATCH_ON_BASE_INVOCATION_ONLY && 0 == _CFGetTSD(__CFTSDKeyIsInGCDMainQ)));
    if (libdispatchQSafe && (CFRunLoopGetMain() == rl) && CFSetContainsValue(rl->_commonModes, rlm->_name)) dispatchPort = _dispatch_get_main_queue_port_4CF();
    
#if USE_DISPATCH_SOURCE_FOR_TIMERS
    mach_port_name_t modeQueuePort = MACH_PORT_NULL;
    if (rlm->_queue) {
        modeQueuePort = _dispatch_runloop_root_queue_get_port_4CF(rlm->_queue);
        if (!modeQueuePort) {
            CRASH("Unable to get port for run loop mode queue (%d)", -1);
        }
    }
#endif
    
    dispatch_source_t timeout_timer = NULL;
    struct __timeout_context *timeout_context = (struct __timeout_context *)malloc(sizeof(*timeout_context));
    if (seconds <= 0.0) { // instant timeout
        seconds = 0.0;
        timeout_context->termTSR = 0ULL;
    } else if (seconds <= TIMER_INTERVAL_LIMIT) {
        dispatch_queue_t queue = pthread_main_np() ? __CFDispatchQueueGetGenericMatchingMain() : __CFDispatchQueueGetGenericBackground();
        timeout_timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
        dispatch_retain(timeout_timer);
        timeout_context->ds = timeout_timer;
        timeout_context->rl = (CFRunLoopRef)CFRetain(rl);
        timeout_context->termTSR = startTSR + __CFTimeIntervalToTSR(seconds);
        dispatch_set_context(timeout_timer, timeout_context); // source gets ownership of context
        dispatch_source_set_event_handler_f(timeout_timer, __CFRunLoopTimeout);
        dispatch_source_set_cancel_handler_f(timeout_timer, __CFRunLoopTimeoutCancel);
        uint64_t ns_at = (uint64_t)((__CFTSRToTimeInterval(startTSR) + seconds) * 1000000000ULL);
        dispatch_source_set_timer(timeout_timer, dispatch_time(1, ns_at), DISPATCH_TIME_FOREVER, 1000ULL);
        dispatch_resume(timeout_timer);
    } else { // infinite timeout
        seconds = 9999999999.0;
        timeout_context->termTSR = UINT64_MAX;
    }

    Boolean didDispatchPortLastTime = true;
    
    
    
    int32_t retVal = 0;
    do {
#if DEPLOYMENT_TARGET_MACOSX || DEPLOYMENT_TARGET_EMBEDDED || DEPLOYMENT_TARGET_EMBEDDED_MINI
        voucher_mach_msg_state_t voucherState = VOUCHER_MACH_MSG_STATE_UNCHANGED;
        voucher_t voucherCopy = NULL;
#endif
        uint8_t msg_buffer[3 * 1024];
#if DEPLOYMENT_TARGET_MACOSX || DEPLOYMENT_TARGET_EMBEDDED || DEPLOYMENT_TARGET_EMBEDDED_MINI
        mach_msg_header_t *msg = NULL;
        mach_port_t livePort = MACH_PORT_NULL;
#elif DEPLOYMENT_TARGET_WINDOWS
        HANDLE livePort = NULL;
        Boolean windowsMessageReceived = false;
#endif
    __CFPortSet waitSet = rlm->_portSet;

        __CFRunLoopUnsetIgnoreWakeUps(rl);

        if (rlm->_observerMask & kCFRunLoopBeforeTimers) __CFRunLoopDoObservers(rl, rlm, kCFRunLoopBeforeTimers);
        if (rlm->_observerMask & kCFRunLoopBeforeSources) __CFRunLoopDoObservers(rl, rlm, kCFRunLoopBeforeSources);

        __CFRunLoopDoBlocks(rl, rlm);

        Boolean sourceHandledThisLoop = __CFRunLoopDoSources0(rl, rlm, stopAfterHandle);
        if (sourceHandledThisLoop) {
            __CFRunLoopDoBlocks(rl, rlm);
        }

        Boolean poll = sourceHandledThisLoop || (0ULL == timeout_context->termTSR);

        if (MACH_PORT_NULL != dispatchPort && !didDispatchPortLastTime) {
#if DEPLOYMENT_TARGET_MACOSX || DEPLOYMENT_TARGET_EMBEDDED || DEPLOYMENT_TARGET_EMBEDDED_MINI
            msg = (mach_msg_header_t *)msg_buffer;
            if (__CFRunLoopServiceMachPort(dispatchPort, &msg, sizeof(msg_buffer), &livePort, 0, &voucherState, NULL)) {
                goto handle_msg;
            }
#elif DEPLOYMENT_TARGET_WINDOWS
            if (__CFRunLoopWaitForMultipleObjects(NULL, &dispatchPort, 0, 0, &livePort, NULL)) {
                goto handle_msg;
            }
#endif
        }

        didDispatchPortLastTime = false;

    if (!poll && (rlm->_observerMask & kCFRunLoopBeforeWaiting)) __CFRunLoopDoObservers(rl, rlm, kCFRunLoopBeforeWaiting);
    __CFRunLoopSetSleeping(rl);
    // do not do any user callouts after this point (after notifying of sleeping)

        // Must push the local-to-this-activation ports in on every loop
        // iteration, as this mode could be run re-entrantly and we don't
        // want these ports to get serviced.

        __CFPortSetInsert(dispatchPort, waitSet);
        
    __CFRunLoopModeUnlock(rlm);
    __CFRunLoopUnlock(rl);

        CFAbsoluteTime sleepStart = poll ? 0.0 : CFAbsoluteTimeGetCurrent();

#if DEPLOYMENT_TARGET_MACOSX || DEPLOYMENT_TARGET_EMBEDDED || DEPLOYMENT_TARGET_EMBEDDED_MINI
#if USE_DISPATCH_SOURCE_FOR_TIMERS
        do {
            if (kCFUseCollectableAllocator) {
                // objc_clear_stack(0);
                // 
                memset(msg_buffer, 0, sizeof(msg_buffer));
            }
            msg = (mach_msg_header_t *)msg_buffer;
            
            __CFRunLoopServiceMachPort(waitSet, &msg, sizeof(msg_buffer), &livePort, poll ? 0 : TIMEOUT_INFINITY, &voucherState, &voucherCopy);
            
            if (modeQueuePort != MACH_PORT_NULL && livePort == modeQueuePort) {
                // Drain the internal queue. If one of the callout blocks sets the timerFired flag, break out and service the timer.
                while (_dispatch_runloop_root_queue_perform_4CF(rlm->_queue));
                if (rlm->_timerFired) {
                    // Leave livePort as the queue port, and service timers below
                    rlm->_timerFired = false;
                    break;
                } else {
                    if (msg && msg != (mach_msg_header_t *)msg_buffer) free(msg);
                }
            } else {
                // Go ahead and leave the inner loop.
                break;
            }
        } while (1);
#else
        if (kCFUseCollectableAllocator) {
            // objc_clear_stack(0);
            // 
            memset(msg_buffer, 0, sizeof(msg_buffer));
        }
        msg = (mach_msg_header_t *)msg_buffer;
        __CFRunLoopServiceMachPort(waitSet, &msg, sizeof(msg_buffer), &livePort, poll ? 0 : TIMEOUT_INFINITY, &voucherState, &voucherCopy);
#endif
        
        
#elif DEPLOYMENT_TARGET_WINDOWS
        // Here, use the app-supplied message queue mask. They will set this if they are interested in having this run loop receive windows messages.
        __CFRunLoopWaitForMultipleObjects(waitSet, NULL, poll ? 0 : TIMEOUT_INFINITY, rlm->_msgQMask, &livePort, &windowsMessageReceived);
#endif
        
        __CFRunLoopLock(rl);
        __CFRunLoopModeLock(rlm);

        rl->_sleepTime += (poll ? 0.0 : (CFAbsoluteTimeGetCurrent() - sleepStart));

        // Must remove the local-to-this-activation ports in on every loop
        // iteration, as this mode could be run re-entrantly and we don't
        // want these ports to get serviced. Also, we don't want them left
        // in there if this function returns.

        __CFPortSetRemove(dispatchPort, waitSet);
        
        __CFRunLoopSetIgnoreWakeUps(rl);

        // user callouts now OK again
    __CFRunLoopUnsetSleeping(rl);
    if (!poll && (rlm->_observerMask & kCFRunLoopAfterWaiting)) __CFRunLoopDoObservers(rl, rlm, kCFRunLoopAfterWaiting);

        handle_msg:;
        __CFRunLoopSetIgnoreWakeUps(rl);

#if DEPLOYMENT_TARGET_WINDOWS
        if (windowsMessageReceived) {
            // These Win32 APIs cause a callout, so make sure we're unlocked first and relocked after
            __CFRunLoopModeUnlock(rlm);
        __CFRunLoopUnlock(rl);

            if (rlm->_msgPump) {
                rlm->_msgPump();
            } else {
                MSG msg;
                if (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE | PM_NOYIELD)) {
                    TranslateMessage(&msg);
                    DispatchMessage(&msg);
                }
            }
            
            __CFRunLoopLock(rl);
        __CFRunLoopModeLock(rlm);
         sourceHandledThisLoop = true;
            
            // To prevent starvation of sources other than the message queue, we check again to see if any other sources need to be serviced
            // Use 0 for the mask so windows messages are ignored this time. Also use 0 for the timeout, because we're just checking to see if the things are signalled right now -- we will wait on them again later.
            // NOTE: Ignore the dispatch source (it's not in the wait set anymore) and also don't run the observers here since we are polling.
            __CFRunLoopSetSleeping(rl);
            __CFRunLoopModeUnlock(rlm);
            __CFRunLoopUnlock(rl);
            
            __CFRunLoopWaitForMultipleObjects(waitSet, NULL, 0, 0, &livePort, NULL);
            
            __CFRunLoopLock(rl);
            __CFRunLoopModeLock(rlm);
            __CFRunLoopUnsetSleeping(rl);
            // If we have a new live port then it will be handled below as normal
        }
#endif
        if (MACH_PORT_NULL == livePort) {
            CFRUNLOOP_WAKEUP_FOR_NOTHING();
            // handle nothing
        } else if (livePort == rl->_wakeUpPort) {
            CFRUNLOOP_WAKEUP_FOR_WAKEUP();
            // do nothing on Mac OS
#if DEPLOYMENT_TARGET_WINDOWS
            // Always reset the wake up port, or risk spinning forever
            ResetEvent(rl->_wakeUpPort);
#endif
        }
#if USE_DISPATCH_SOURCE_FOR_TIMERS
        else if (modeQueuePort != MACH_PORT_NULL && livePort == modeQueuePort) {
            CFRUNLOOP_WAKEUP_FOR_TIMER();
            if (!__CFRunLoopDoTimers(rl, rlm, mach_absolute_time())) {
                // Re-arm the next timer, because we apparently fired early
                __CFArmNextTimerInMode(rlm, rl);
            }
        }
#endif
#if USE_MK_TIMER_TOO
        else if (rlm->_timerPort != MACH_PORT_NULL && livePort == rlm->_timerPort) {
            CFRUNLOOP_WAKEUP_FOR_TIMER();
            // On Windows, we have observed an issue where the timer port is set before the time which we requested it to be set. For example, we set the fire time to be TSR 167646765860, but it is actually observed firing at TSR 167646764145, which is 1715 ticks early. The result is that, when __CFRunLoopDoTimers checks to see if any of the run loop timers should be firing, it appears to be 'too early' for the next timer, and no timers are handled.
            // In this case, the timer port has been automatically reset (since it was returned from MsgWaitForMultipleObjectsEx), and if we do not re-arm it, then no timers will ever be serviced again unless something adjusts the timer list (e.g. adding or removing timers). The fix for the issue is to reset the timer here if CFRunLoopDoTimers did not handle a timer itself. 9308754
            if (!__CFRunLoopDoTimers(rl, rlm, mach_absolute_time())) {
                // Re-arm the next timer
                __CFArmNextTimerInMode(rlm, rl);
            }
        }
#endif
        else if (livePort == dispatchPort) {
            CFRUNLOOP_WAKEUP_FOR_DISPATCH();
            __CFRunLoopModeUnlock(rlm);
            __CFRunLoopUnlock(rl);
            _CFSetTSD(__CFTSDKeyIsInGCDMainQ, (void *)6, NULL);
#if DEPLOYMENT_TARGET_WINDOWS
            void *msg = 0;
#endif
            __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__(msg);
            _CFSetTSD(__CFTSDKeyIsInGCDMainQ, (void *)0, NULL);
            __CFRunLoopLock(rl);
            __CFRunLoopModeLock(rlm);
            sourceHandledThisLoop = true;
            didDispatchPortLastTime = true;
        } else {
            CFRUNLOOP_WAKEUP_FOR_SOURCE();
            
            // If we received a voucher from this mach_msg, then put a copy of the new voucher into TSD. CFMachPortBoost will look in the TSD for the voucher. By using the value in the TSD we tie the CFMachPortBoost to this received mach_msg explicitly without a chance for anything in between the two pieces of code to set the voucher again.
            voucher_t previousVoucher = _CFSetTSD(__CFTSDKeyMachMessageHasVoucher, (void *)voucherCopy, os_release);

            // Despite the name, this works for windows handles as well
            CFRunLoopSourceRef rls = __CFRunLoopModeFindSourceForMachPort(rl, rlm, livePort);
            if (rls) {
#if DEPLOYMENT_TARGET_MACOSX || DEPLOYMENT_TARGET_EMBEDDED || DEPLOYMENT_TARGET_EMBEDDED_MINI
        mach_msg_header_t *reply = NULL;
        sourceHandledThisLoop = __CFRunLoopDoSource1(rl, rlm, rls, msg, msg->msgh_size, &reply) || sourceHandledThisLoop;
        if (NULL != reply) {
            (void)mach_msg(reply, MACH_SEND_MSG, reply->msgh_size, 0, MACH_PORT_NULL, 0, MACH_PORT_NULL);
            CFAllocatorDeallocate(kCFAllocatorSystemDefault, reply);
        }
#elif DEPLOYMENT_TARGET_WINDOWS
                sourceHandledThisLoop = __CFRunLoopDoSource1(rl, rlm, rls) || sourceHandledThisLoop;
#endif
        }
            
            // Restore the previous voucher
            _CFSetTSD(__CFTSDKeyMachMessageHasVoucher, previousVoucher, os_release);
            
        }
#if DEPLOYMENT_TARGET_MACOSX || DEPLOYMENT_TARGET_EMBEDDED || DEPLOYMENT_TARGET_EMBEDDED_MINI
        if (msg && msg != (mach_msg_header_t *)msg_buffer) free(msg);
#endif
        
    __CFRunLoopDoBlocks(rl, rlm);
        

    if (sourceHandledThisLoop && stopAfterHandle) {
        retVal = kCFRunLoopRunHandledSource;
        } else if (timeout_context->termTSR < mach_absolute_time()) {
            retVal = kCFRunLoopRunTimedOut;
    } else if (__CFRunLoopIsStopped(rl)) {
            __CFRunLoopUnsetStopped(rl);
        retVal = kCFRunLoopRunStopped;
    } else if (rlm->_stopped) {
        rlm->_stopped = false;
        retVal = kCFRunLoopRunStopped;
    } else if (__CFRunLoopModeIsEmpty(rl, rlm, previousMode)) {
        retVal = kCFRunLoopRunFinished;
    }
        
#if DEPLOYMENT_TARGET_MACOSX || DEPLOYMENT_TARGET_EMBEDDED || DEPLOYMENT_TARGET_EMBEDDED_MINI
        voucher_mach_msg_revert(voucherState);
        os_release(voucherCopy);
#endif

    } while (0 == retVal);

    if (timeout_timer) {
        dispatch_source_cancel(timeout_timer);
        dispatch_release(timeout_timer);
    } else {
        free(timeout_context);
    }

    return retVal;
}

是不是一头雾水，我把一些关键的代码整理了一下，大致就是如下的样子：

SInt32 CFRunLoopRunSpecific(CFRunLoopRef rl, CFStringRef modeName, CFTimeInterval seconds, Boolean returnAfterSourceHandled) {     /* DOES CALLOUT */
    CHECK_FOR_FORK();
    if (__CFRunLoopIsDeallocating(rl)) return kCFRunLoopRunFinished;
    __CFRunLoopLock(rl);
    
    /// 首先根据modeName找到对应mode
    CFRunLoopModeRef currentMode = __CFRunLoopFindMode(rl, modeName, false);
    /// 通知 Observers: RunLoop 即将进入 loop。
    __CFRunLoopDoObservers(rl, currentMode, kCFRunLoopEntry);
    /// 内部函数，进入loop
    result = __CFRunLoopRun(rl, currentMode, seconds, returnAfterSourceHandled, previousMode);
    /// 通知 Observers: RunLoop 即将退出。
    __CFRunLoopDoObservers(rl, currentMode, kCFRunLoopExit);

    __CFRunLoopUnlock(rl);
    return result;
}

/// 核心函数
static int32_t __CFRunLoopRun(CFRunLoopRef rl, CFRunLoopModeRef rlm, CFTimeInterval seconds, Boolean stopAfterHandle, CFRunLoopModeRef previousMode) {
   
    int32_t retVal = 0;
    do {  // itmes do
        /// 通知 Observers: 即将处理timer事件
        __CFRunLoopDoObservers(rl, rlm, kCFRunLoopBeforeTimers);
        /// 通知 Observers: 即将处理Source事件
        __CFRunLoopDoObservers(rl, rlm, kCFRunLoopBeforeSources)
        /// 处理Blocks
        __CFRunLoopDoBlocks(rl, rlm);
        /// 处理sources0
        Boolean sourceHandledThisLoop = __CFRunLoopDoSources0(rl, rlm, stopAfterHandle);
        /// 处理sources0返回为YES
        if (sourceHandledThisLoop) {
            /// 处理Blocks
            __CFRunLoopDoBlocks(rl, rlm);
        }
        /// 判断有无端口消息(Source1)
        if (__CFRunLoopServiceMachPort(dispatchPort, &msg, sizeof(msg_buffer), &livePort, 0, &voucherState, NULL)) {
            /// 处理消息
            goto handle_msg;
        }
        /// 通知 Observers: 即将进入休眠
        __CFRunLoopDoObservers(rl, rlm, kCFRunLoopBeforeWaiting);
        __CFRunLoopSetSleeping(rl);
        /// 等待被唤醒
        __CFRunLoopServiceMachPort(waitSet, &msg, sizeof(msg_buffer), &livePort, poll ? 0 : TIMEOUT_INFINITY, &voucherState, &voucherCopy);
        
        // user callouts now OK again
        __CFRunLoopUnsetSleeping(rl);
        /// 通知 Observers: 被唤醒，结束休眠
        __CFRunLoopDoObservers(rl, rlm, kCFRunLoopAfterWaiting);
    handle_msg:
        if (被Timer唤醒) {
            /// 处理Timers
            __CFRunLoopDoTimers(rl, rlm, mach_absolute_time())；
        } else if (被GCD唤醒) {
            /// 处理GCD__CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__(msg);
        } else if (被Source1唤醒) {
            /// 被Source1唤醒，处理Source1
            __CFRunLoopDoSource1(rl, rlm, rls, msg, msg->msgh_size, &reply)
        }
        /// 处理block
        __CFRunLoopDoBlocks(rl, rlm);
        if (sourceHandledThisLoop && stopAfterHandle) {
            retVal = kCFRunLoopRunHandledSource;
        } else if (timeout_context->termTSR < mach_absolute_time()) {
            retVal = kCFRunLoopRunTimedOut;
        } else if (__CFRunLoopIsStopped(rl)) {
            __CFRunLoopUnsetStopped(rl);
            retVal = kCFRunLoopRunStopped;
        } else if (rlm->_stopped) {
            rlm->_stopped = false;
            retVal = kCFRunLoopRunStopped;
        } else if (__CFRunLoopModeIsEmpty(rl, rlm, previousMode)) {
            retVal = kCFRunLoopRunFinished;
        }
    } while (0 == retVal); 
    return retVal;
}

// main  dispatch queue
__CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__
// __CFRunLoopDoObservers
__CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__
// __CFRunLoopDoBlocks
__CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__
// __CFRunLoopDoSources0
__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__
// __CFRunLoopDoSource1
__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__
// __CFRunLoopDoTimers
__CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__

Run Loop Observer Activities

/* Run Loop Observer Activities */
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
};

最终就会形成这样一张熟悉的流程图：

Runloop事务处理流程图

总结

摘出来的代码有点多了，不过都是一些核心的代码片段。遗憾的是没能断点跟流程走一下。

iOS-OC底层-RunLoop

前言

Runloop初探

Runloop与线程,Mode以及Item(事务)之间的关系

RunLoop处理事务的具体流程

总结

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