iOS RunLoop应用分析—原来这些都在使用RunLoop
1、事件响应
iOS设备的事件响应，是有RunLoop参与的。 提起iOS设备的事件响应，相信大家都会有一个大概的了解:
(1)用户触发事件->(2)系统将事件转交到对应APP的事件队列->(3)APP从消息队列头取出事件->(4)交由Main Window进行消息分发->(5)找到合适的Responder进行处理，如果没找到，则会沿着Responder chain返回到APP层，丢弃不响应该事件
这里涉及到两个问题，(3)到(5)步是由进程内处理的，而(1)到(2)步则涉及到设备硬件，iOS操作系统，以及目标APP之间的通信，通信的大致步骤是什么样的呢？ 当我们的APP在接收到任何事件请求之前，main RunLoop都是处于mach_msg_trap休眠状态中的，那么，又是谁唤醒它的呢？
借助lldb，执行：
po [NSRunLoop mainRunLoop]复制代码
打印信息如下：
<CFRunLoop 0x60000307c900 [0x1dc655ae0]>{wakeup port = 0x2203, stopped = false, ignoreWakeUps = false,
current mode = kCFRunLoopDefaultMode,
common modes = <CFBasicHash 0x600000252b20 [0x1dc655ae0]>{type = mutable set, count = 2,
entries =>
0 : <CFString 0x1d008c078 [0x1dc655ae0]>{contents = "UITrackingRunLoopMode"}
2 : <CFString 0x1dc88d768 [0x1dc655ae0]>{contents = "kCFRunLoopDefaultMode"}
}
,
common mode items = <CFBasicHash 0x60000023cb70 [0x1dc655ae0]>{type = mutable set, count = 9,
entries =>
0 : <CFRunLoopSource 0x600003964300 [0x1dc655ae0]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x0, callout = PurpleEventSignalCallback (0x18c03d28c)}}
2 : <CFRunLoopObserver 0x600003d7c280 [0x1dc655ae0]>{valid = Yes, activities = 0x20, repeats = Yes, order = 0, callout = _UIGestureRecognizerUpdateObserver (0x1848adbd0), context = <CFRunLoopObserver context 0x60000277c620>}
3 : <CFRunLoopObserver 0x600003d606e0 [0x1dc655ae0]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 1999000, callout = _beforeCACommitHandler (0x184dc4888), context = <CFRunLoopObserver context 0x151004ae0>}
4 : <CFRunLoopObserver 0x600003d60500 [0x1dc655ae0]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 2001000, callout = _afterCACommitHandler (0x184dc488c), context = <CFRunLoopObserver context 0x151004ae0>}
5 : <CFRunLoopSource 0x600003968000 [0x1dc655ae0]>{signalled = No, valid = Yes, order = 0, context = <CFRunLoopSource context>{version = 0, info = 0x6000019568c0, callout = FBSSerialQueueRunLoopSourceHandler (0x18619d338)}}
6 : <CFRunLoopSource 0x600003960300 [0x1dc655ae0]>{signalled = No, valid = Yes, order = -2, context = <CFRunLoopSource context>{version = 0, info = 0x600000252e80, callout = __eventFetcherSourceCallback (0x184e38034)}}
9 : <CFRunLoopObserver 0x600003d7c5a0 [0x1dc655ae0]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 2000000, callout = _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv (0x188468b28), context = <CFRunLoopObserver context 0x0>}
10 : <CFRunLoopSource 0x600003960180 [0x1dc655ae0]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x60000377c0d0, callout = __eventQueueSourceCallback (0x184e37f7c)}}
12 : <CFRunLoopSource 0x6000039606c0 [0x1dc655ae0]>{signalled = No, valid = Yes, order = 0, context = <CFRunLoopSource MIG Server> {port = 24075, subsystem = 0x1d0773ff8, context = 0x600002879140}}
}
,
modes = <CFBasicHash 0x600000252c70 [0x1dc655ae0]>{type = mutable set, count = 4,
entries =>
2 : <CFRunLoopMode 0x6000037681a0 [0x1dc655ae0]>{name = UITrackingRunLoopMode, port set = 0x4703, queue = 0x600002260400, source = 0x600002261280 (not fired), timer port = 0x4603,
sources0 = <CFBasicHash 0x60000023cc00 [0x1dc655ae0]>{type = mutable set, count = 4,
entries =>
0 : <CFRunLoopSource 0x600003964300 [0x1dc655ae0]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x0, callout = PurpleEventSignalCallback (0x18c03d28c)}}
1 : <CFRunLoopSource 0x600003968000 [0x1dc655ae0]>{signalled = No, valid = Yes, order = 0, context = <CFRunLoopSource context>{version = 0, info = 0x6000019568c0, callout = FBSSerialQueueRunLoopSourceHandler (0x18619d338)}}
2 : <CFRunLoopSource 0x600003960180 [0x1dc655ae0]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x60000377c0d0, callout = __eventQueueSourceCallback (0x184e37f7c)}}
4 : <CFRunLoopSource 0x600003960300 [0x1dc655ae0]>{signalled = No, valid = Yes, order = -2, context = <CFRunLoopSource context>{version = 0, info = 0x600000252e80, callout = __eventFetcherSourceCallback (0x184e38034)}}
}
,
sources1 = <CFBasicHash 0x60000023c6c0 [0x1dc655ae0]>{type = mutable set, count = 1,
entries =>
0 : <CFRunLoopSource 0x6000039606c0 [0x1dc655ae0]>{signalled = No, valid = Yes, order = 0, context = <CFRunLoopSource MIG Server> {port = 24075, subsystem = 0x1d0773ff8, context = 0x600002879140}}
}
,
observers = (
"<CFRunLoopObserver 0x600003d7c280 [0x1dc655ae0]>{valid = Yes, activities = 0x20, repeats = Yes, order = 0, callout = _UIGestureRecognizerUpdateObserver (0x1848adbd0), context = <CFRunLoopObserver context 0x60000277c620>}",
"<CFRunLoopObserver 0x600003d606e0 [0x1dc655ae0]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 1999000, callout = _beforeCACommitHandler (0x184dc4888), context = <CFRunLoopObserver context 0x151004ae0>}",
"<CFRunLoopObserver 0x600003d7c5a0 [0x1dc655ae0]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 2000000, callout = _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv (0x188468b28), context = <CFRunLoopObserver context 0x0>}",
"<CFRunLoopObserver 0x600003d60500 [0x1dc655ae0]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 2001000, callout = _afterCACommitHandler (0x184dc488c), context = <CFRunLoopObserver context 0x151004ae0>}"
),
timers = (null),
currently 663695489 (6974909751689) / soft deadline in: 7.68614046e+11 sec (@ -1) / hard deadline in: 7.68614046e+11 sec (@ -1)
},
3 : <CFRunLoopMode 0x600003768270 [0x1dc655ae0]>{name = GSEventReceiveRunLoopMode, port set = 0x3803, queue = 0x600002261300, source = 0x600002261400 (not fired), timer port = 0x3903,
sources0 = <CFBasicHash 0x60000023ce10 [0x1dc655ae0]>{type = mutable set, count = 1,
entries =>
0 : <CFRunLoopSource 0x600003964300 [0x1dc655ae0]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x0, callout = PurpleEventSignalCallback (0x18c03d28c)}}
}
,
sources1 = <CFBasicHash 0x60000023ca20 [0x1dc655ae0]>{type = mutable set, count = 0,
entries =>
}
,
observers = (null),
timers = (null),
currently 663695489 (6974909768440) / soft deadline in: 7.68614046e+11 sec (@ -1) / hard deadline in: 7.68614046e+11 sec (@ -1)
},
4 : <CFRunLoopMode 0x6000037680d0 [0x1dc655ae0]>{name = kCFRunLoopDefaultMode, port set = 0x3103, queue = 0x600002260e00, source = 0x600002261080 (not fired), timer port = 0x5103,
sources0 = <CFBasicHash 0x60000023cc60 [0x1dc655ae0]>{type = mutable set, count = 4,
entries =>
0 : <CFRunLoopSource 0x600003964300 [0x1dc655ae0]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x0, callout = PurpleEventSignalCallback (0x18c03d28c)}}
1 : <CFRunLoopSource 0x600003968000 [0x1dc655ae0]>{signalled = No, valid = Yes, order = 0, context = <CFRunLoopSource context>{version = 0, info = 0x6000019568c0, callout = FBSSerialQueueRunLoopSourceHandler (0x18619d338)}}
2 : <CFRunLoopSource 0x600003960180 [0x1dc655ae0]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x60000377c0d0, callout = __eventQueueSourceCallback (0x184e37f7c)}}
4 : <CFRunLoopSource 0x600003960300 [0x1dc655ae0]>{signalled = No, valid = Yes, order = -2, context = <CFRunLoopSource context>{version = 0, info = 0x600000252e80, callout = __eventFetcherSourceCallback (0x184e38034)}}
}
,
sources1 = <CFBasicHash 0x60000023cc30 [0x1dc655ae0]>{type = mutable set, count = 1,
entries =>
0 : <CFRunLoopSource 0x6000039606c0 [0x1dc655ae0]>{signalled = No, valid = Yes, order = 0, context = <CFRunLoopSource MIG Server> {port = 24075, subsystem = 0x1d0773ff8, context = 0x600002879140}}
}
,
observers = (
"<CFRunLoopObserver 0x600003d7c280 [0x1dc655ae0]>{valid = Yes, activities = 0x20, repeats = Yes, order = 0, callout = _UIGestureRecognizerUpdateObserver (0x1848adbd0), context = <CFRunLoopObserver context 0x60000277c620>}",
"<CFRunLoopObserver 0x600003d606e0 [0x1dc655ae0]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 1999000, callout = _beforeCACommitHandler (0x184dc4888), context = <CFRunLoopObserver context 0x151004ae0>}",
"<CFRunLoopObserver 0x600003d7c5a0 [0x1dc655ae0]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 2000000, callout = _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv (0x188468b28), context = <CFRunLoopObserver context 0x0>}",
"<CFRunLoopObserver 0x600003d60500 [0x1dc655ae0]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 2001000, callout = _afterCACommitHandler (0x184dc488c), context = <CFRunLoopObserver context 0x151004ae0>}"
),
timers = <CFArray 0x60000287d920 [0x1dc655ae0]>{type = mutable-small, count = 0, values = ()},
currently 663695489 (6974909768895) / soft deadline in: 7.68614046e+11 sec (@ -1) / hard deadline in: 7.68614046e+11 sec (@ -1)
},
5 : <CFRunLoopMode 0x600003768b60 [0x1dc655ae0]>{name = kCFRunLoopCommonModes, port set = 0x5c07, queue = 0x600002262b80, source = 0x600002262c80 (not fired), timer port = 0x5d03,
sources0 = (null),
sources1 = (null),
observers = (null),
timers = (null),
currently 663695489 (6974909785136) / soft deadline in: 7.68614046e+11 sec (@ -1) / hard deadline in: 7.68614046e+11 sec (@ -1)
},
}
}复制代码这里有一坨，我们从中间找出三个关键的代码段！！
a、kCFRunLoopDefaultMode
<CFRunLoopMode 0x6000037680d0 [0x1dc655ae0]>{name = kCFRunLoopDefaultMode, port set = 0x3103, queue = 0x600002260e00, source = 0x600002261080 (not fired), timer port = 0x5103,
sources0 = <CFBasicHash 0x60000023cc60 [0x1dc655ae0]>{type = mutable set, count = 4,
entries =>
0 : <CFRunLoopSource 0x600003964300 [0x1dc655ae0]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x0, callout = PurpleEventSignalCallback (0x18c03d28c)}}
1 : <CFRunLoopSource 0x600003968000 [0x1dc655ae0]>{signalled = No, valid = Yes, order = 0, context = <CFRunLoopSource context>{version = 0, info = 0x6000019568c0, callout = FBSSerialQueueRunLoopSourceHandler (0x18619d338)}}
2 : <CFRunLoopSource 0x600003960180 [0x1dc655ae0]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x60000377c0d0, callout = __eventQueueSourceCallback (0x184e37f7c)}}
4 : <CFRunLoopSource 0x600003960300 [0x1dc655ae0]>{signalled = No, valid = Yes, order = -2, context = <CFRunLoopSource context>{version = 0, info = 0x600000252e80, callout = __eventFetcherSourceCallback (0x184e38034)}}
}
,
sources1 = <CFBasicHash 0x60000023cc30 [0x1dc655ae0]>{type = mutable set, count = 1,
entries =>
0 : <CFRunLoopSource 0x6000039606c0 [0x1dc655ae0]>{signalled = No, valid = Yes, order = 0, context = <CFRunLoopSource MIG Server> {port = 24075, subsystem = 0x1d0773ff8, context = 0x600002879140}}
}
,
observers = (
"<CFRunLoopObserver 0x600003d7c280 [0x1dc655ae0]>{valid = Yes, activities = 0x20, repeats = Yes, order = 0, callout = _UIGestureRecognizerUpdateObserver (0x1848adbd0), context = <CFRunLoopObserver context 0x60000277c620>}",
"<CFRunLoopObserver 0x600003d606e0 [0x1dc655ae0]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 1999000, callout = _beforeCACommitHandler (0x184dc4888), context = <CFRunLoopObserver context 0x151004ae0>}",
"<CFRunLoopObserver 0x600003d7c5a0 [0x1dc655ae0]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 2000000, callout = _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv (0x188468b28), context = <CFRunLoopObserver context 0x0>}",
"<CFRunLoopObserver 0x600003d60500 [0x1dc655ae0]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 2001000, callout = _afterCACommitHandler (0x184dc488c), context = <CFRunLoopObserver context 0x151004ae0>}"
),
timers = <CFArray 0x60000287d920 [0x1dc655ae0]>{type = mutable-small, count = 0, values = ()},
currently 663695489 (6974909768895) / soft deadline in: 7.68614046e+11 sec (@ -1) / hard deadline in: 7.68614046e+11 sec (@ -1)
},复制代码b、UITrackingRunLoopMode
x<CFRunLoopMode 0x6000037681a0 [0x1dc655ae0]>{name = UITrackingRunLoopMode, port set = 0x4703, queue = 0x600002260400, source = 0x600002261280 (not fired), timer port = 0x4603,
sources0 = <CFBasicHash 0x60000023cc00 [0x1dc655ae0]>{type = mutable set, count = 4,
entries =>
0 : <CFRunLoopSource 0x600003964300 [0x1dc655ae0]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x0, callout = PurpleEventSignalCallback (0x18c03d28c)}}
1 : <CFRunLoopSource 0x600003968000 [0x1dc655ae0]>{signalled = No, valid = Yes, order = 0, context = <CFRunLoopSource context>{version = 0, info = 0x6000019568c0, callout = FBSSerialQueueRunLoopSourceHandler (0x18619d338)}}
2 : <CFRunLoopSource 0x600003960180 [0x1dc655ae0]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x60000377c0d0, callout = __eventQueueSourceCallback (0x184e37f7c)}}
4 : <CFRunLoopSource 0x600003960300 [0x1dc655ae0]>{signalled = No, valid = Yes, order = -2, context = <CFRunLoopSource context>{version = 0, info = 0x600000252e80, callout = __eventFetcherSourceCallback (0x184e38034)}}
}
,
sources1 = <CFBasicHash 0x60000023c6c0 [0x1dc655ae0]>{type = mutable set, count = 1,
entries =>
0 : <CFRunLoopSource 0x6000039606c0 [0x1dc655ae0]>{signalled = No, valid = Yes, order = 0, context = <CFRunLoopSource MIG Server> {port = 24075, subsystem = 0x1d0773ff8, context = 0x600002879140}}
}
,
observers = (
"<CFRunLoopObserver 0x600003d7c280 [0x1dc655ae0]>{valid = Yes, activities = 0x20, repeats = Yes, order = 0, callout = _UIGestureRecognizerUpdateObserver (0x1848adbd0), context = <CFRunLoopObserver context 0x60000277c620>}",
"<CFRunLoopObserver 0x600003d606e0 [0x1dc655ae0]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 1999000, callout = _beforeCACommitHandler (0x184dc4888), context = <CFRunLoopObserver context 0x151004ae0>}",
"<CFRunLoopObserver 0x600003d7c5a0 [0x1dc655ae0]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 2000000, callout = _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv (0x188468b28), context = <CFRunLoopObserver context 0x0>}",
"<CFRunLoopObserver 0x600003d60500 [0x1dc655ae0]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 2001000, callout = _afterCACommitHandler (0x184dc488c), context = <CFRunLoopObserver context 0x151004ae0>}"
),
timers = (null),
currently 663695489 (6974909751689) / soft deadline in: 7.68614046e+11 sec (@ -1) / hard deadline in: 7.68614046e+11 sec (@ -1)
},复制代码在kCFRunLoopDefaultMode，UITrackingRunLoopMode 下面均注册了的source0事件源：
<CFRunLoopSource 0x600003960180 [0x1dc655ae0]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x60000377c0d0, callout = __eventQueueSourceCallback (0x184e37f7c)}}复制代码他的回调函数是__eventQueueSourceCallback，没错，APP就是通过这个回调函数来处理事件队列的。
但是，我们注意到了，__eventQueueSourceCallback所对应的source类型是0，也就是说它本身不会唤醒休眠的main RunLoop, main线程自身在休眠状态中也不可能自己去唤醒自己~
所以：系统肯定还有一个子线程，用来接收事件并唤醒main thread，并将事件传递到main thread上！！
com.apple.uikit.eventfetch-thread复制代码
看线程的名字就知道，它是UIKit所创建的用于接收event的线程(以下我们简称为event fetch thread)。 我们打印出com.apple.uikit.eventfetch-thread的RunLoop 的 kCFRunLoopDefaultMode。
<CFRunLoopMode 0x604000190740 [0x107f54bb0]>{name = kCFRunLoopDefaultMode, port set = 0x1d03, queue = 0x6040001569a0, source = 0x604000190810 (not fired), timer port = 0x2b03,
sources0 = <CFBasicHash 0x60800005f8f0 [0x107f54bb0]>{type = mutable set, count = 1,
entries =>
0 : <CFRunLoopSource 0x608000167440 [0x107f54bb0]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x600000131940, callout = _UIEventFetcherTriggerHandOff (0x108d71599)}}
}
,
sources1 = <CFBasicHash 0x608000240b10 [0x107f54bb0]>{type = mutable set, count = 3,
entries =>
0 : <CFRunLoopSource 0x604000167a40 [0x107f54bb0]>{signalled = No, valid = Yes, order = 0, context = <CFMachPort 0x608000156dc0 [0x107f54bb0]>{valid = Yes, port = 300f, source = 0x604000167a40, callout = __IOHIDEventSystemClientQueueCallback (0x10b17904d), context = <CFMachPort context 0x7f8b28d02ae0>}}
1 : <CFRunLoopSource 0x6040001678c0 [0x107f54bb0]>{signalled = No, valid = Yes, order = 1, context = <CFMachPort 0x608000157080 [0x107f54bb0]>{valid = Yes, port = 4e0f, source = 0x6040001678c0, callout = __IOMIGMachPortPortCallback (0x10b185692), context = <CFMachPort context 0x6080000d3780>}}
2 : <CFRunLoopSource 0x604000167980 [0x107f54bb0]>{signalled = No, valid = Yes, order = 0, context = <CFMachPort 0x608000156f20 [0x107f54bb0]>{valid = Yes, port = 4c07, source = 0x604000167980, callout = __IOHIDEventSystemClientAvailabilityCallback (0x10b179281), context = <CFMachPort context 0x7f8b28d02ae0>}}
}
,
observers = (null),
timers = (null),
currently 547998693 (173591812879054) / soft deadline in: 1.84465705e+10 sec (@ -1) / hard deadline in: 1.84465705e+10 sec (@ -1)
},
}复制代码里面有sources1，callout: __IOHIDEventSystemClientQueueCallback
既然这个是source1类型，则是可以被系统通过mach port唤醒event fetch thread的RunLoop， 来执行__IOHIDEventSystemClientQueueCallback回调的。
到此破案了！！结论如下：
用户触发事件， IOKit.framework 生成一个 IOHIDEvent 事件并由 SpringBoard 接收，SpringBoard会利用mach port，产生source1，来唤醒目标APP的com.apple.uikit.eventfetch-thread的RunLoop。Eventfetch thread会将main runloop 中__handleEventQueue所对应的source0设置为signaled == Yes状态，同时唤醒main RunLoop。mainRunLoop则调用__eventQueueSourceCallback进行事件队列处理。
2、手势识别
iOS的手势识别也依赖于RunLoop。
当系统识别出一个手势时，会打断touch系列的回调，同时更新标记对应手势的UIGestureRecognizer状态(Needing update?)
而UIKit会向main RunLoop注册一个observer（其实是分别在tracking mode，default mode和common modes下分别注册）:
<CFRunLoopObserver 0x600003d7c280 [0x1dc655ae0]>{valid = Yes, activities = 0x20, repeats = Yes, order = 0, callout = _UIGestureRecognizerUpdateObserver (0x1848adbd0), context = <CFRunLoopObserver context 0x60000277c620>}复制代码这里查看一下它的activities属性0x20，结合CF中的位标记定义:
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
};复制代码可知该observer监听main RunLoop的kCFRunLoopBeforeWaiting事件。每当main RunLoop即将休眠时，该observer被触发，同时调用回调函数_UIGestureRecognizerUpdateObserver。_UIGestureRecognizerUpdateObserver会检测当前需要被更新状态的recognizer（创建，触发，销毁）。
如果有手势被触发，在_UIGestureRecognizerUpdateObserver回调中会借助UIKit一个内部类UIGestureEnvironment 来进行一系列处理。其中会向APP的event queue中投递一个gesture event，这个gesture event的处理流程应该和上面的事件处理类似的，内部会调用__handleEventQueueInternal处理该gesture event，并通过UIKit内部类UIGestureEnvironment 来处理这个gesture event，并最终回调到我们自己所写的gesture回调中。
手势识别 的过程可以大致总结如下：
当上面的 _UIApplicationHandleEventQueue()识别了一个手势时，其首先会调用 Cancel 将当前的 touchesBegin/Move/End 系列回调打断。随后系统将对应的 UIGestureRecognizer 标记为待处理。
苹果注册了一个 Observer 监测 BeforeWaiting (Loop即将进入休眠) 事件，这个 Observer 的回调函数是 _UIGestureRecognizerUpdateObserver()，其内部会获取所有刚被标记为待处理的 GestureRecognizer，并执行GestureRecognizer 的回调。
当有 UIGestureRecognizer 的变化(创建/销毁/状态改变)时，这个回调都会进行相应处理。
3、页面的渲染
当我们调用 [UIView setNeedsDisplay] 时，这时会调用当前 View.layer 的 [view.layer setNeedsDisplay]方法。
这等于给当前的 layer 打上了一个标记，而此时并没有直接进行绘制工作。而是会到当前的 Runloop 即将休眠，也就是 beforeWaiting 时才会进行绘制工作。
紧接着会调用 [CALayer display]，进入到真正绘制的工作。CALayer 层会判断自己的 delegate 有没有实现异步绘制的代理方法 displayer:，这个代理方法是异步绘制的入口，如果没有实现这个方法，那么会继续进行系统绘制的流程，然后绘制结束。
CALayer 内部会创建一个 Backing Store，用来获取图形上下文。接下来会判断这个 layer 是否有 delegate。
如果有的话，会调用 [layer.delegate drawLayer:inContext:]，并且会返回给我们 [UIView DrawRect:] 的回调，让我们在系统绘制的基础之上再做一些事情。
如果没有 delegate，那么会调用 [CALayer drawInContext:]。
以上两个分支，最终 CALayer 都会将位图提交到 Backing Store，最后提交给 GPU。
至此绘制的过程结束。
4、NSTimer
NSTimer 其实就是 CFRunLoopTimer，他们之间是 toll-free bridged 的 一"码"当先
typedef struct CF_BRIDGED_MUTABLE_TYPE(NSTimer) __CFRunLoopTimer * CFRunLoopTimerRef;
struct __CFRunLoopTimer {
CFRuntimeBase _base;
uint16_t _bits;
pthread_mutex_t _lock;
CFRunLoopRef _runLoop; // timer对应的runloop
CFMutableSetRef _rlModes; // timer对应的runloop modes
CFAbsoluteTime _nextFireDate;
CFTimeInterval _interval; /* immutable */
CFTimeInterval _tolerance; /* mutable */
uint64_t _fireTSR; /* timer本次需要被触发的时间， TSR units */
CFIndex _order; /* immutable */
CFRunLoopTimerCallBack _callout; /* timer 回调 immutable */
CFRunLoopTimerContext _context; /* immutable, except invalidation */
};复制代码Timer的触发流程大致是这样的：
用户添加timer到runloop的某个或几个mode下；
根据timer是否设置了tolerance，如果没有设置，则调用底层xnu内核的mk_timer注册一个mach-port事件，如果设置了tolerance，则注册一个GCD timer；
当由XNU内核或GCD管理的timer的 fire time到了，通过对应的mach port唤醒RunLoop(mk_timer对应rlm的_timerPort, GCD timer对应GCD queue port)；
RunLoop执行__CFRunLoopDoTimers，其中会调用__CFRunLoopDoTimer， DoTimer方法里面会根据当前mach time和Timer的fireTSR属性，判断fireTSR 是否< 当前的mach time，如果小于，则触发timer，同时更新下一次fire时间fireTSR。
5、RunLoop和线程应用场景
线程和RunLoop是一一对应的,其映射关系是保存在一个全局的 Dictionary 里
自己创建的线程默认是没有开启RunLoop的
怎么创建一个常驻线程？
为当前线程开启一个RunLoop（第一次调用 [NSRunLoop currentRunLoop]方法时实际是会先去创建一个RunLoop）
向当前RunLoop中添加一个Port/Source等维持RunLoop的事件循环（如果RunLoop的mode中一个item都没有，RunLoop会退出）
启动该RunLoop
@autoreleasepool {
NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
[[NSRunLoop currentRunLoop] addPort:[NSMachPort port] forMode:NSDefaultRunLoopMode];
[runLoop run];
}复制代码怎样保证子线程数据回来更新UI的时候不打断用户的滑动操作？
当我们在子请求数据的同时滑动浏览当前页面，如果数据请求成功要切回主线程更新UI，那么就会影响当前正在滑动的体验。
我们就可以将更新UI事件放在主线程的NSDefaultRunLoopMode上执行即可，这样就会等用户不再滑动页面，主线程RunLoop由UITrackingRunLoopMode切换到NSDefaultRunLoopMode时再去更新UI
[self performSelectorOnMainThread:@selector(reloadData) withObject:nil waitUntilDone:NO modes:@[NSDefaultRunLoopMode]];复制代码
易错代码分析
NSLog(@"1");
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"2");
[self performSelector:@selector(test) withObject:nil afterDelay:10];
NSLog(@"3");
});
NSLog(@"4");
- (void)test{
NSLog(@"5");
}复制代码打印结果:1423，test方法并不会执行。
原因分析:如果是带afterDelay的延时函数，会在内部创建一个 NSTimer，然后添加到当前线程的RunLoop中。由于当前线程没有开启RunLoop，所以该方法会失效。
NSLog(@"1");
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"2");
[[NSRunLoop currentRunLoop] run];
[self performSelector:@selector(test) withObject:nil afterDelay:10];
NSLog(@"3");
});
NSLog(@"4");
- (void)test{
NSLog(@"5");
}复制代码打印结果:1423，test方法并不会执行。
原因分析:如果RunLoop的mode中一个item都没有，RunLoop会退出。即在调用RunLoop的run方法后，由于其mode中没有添加任何item去维持RunLoop的时间循环，RunLoop随即还是会退出。 所以我们自己启动RunLoop，一定要在添加item。
NSLog(@"1");
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"2");
[self performSelector:@selector(test) withObject:nil afterDelay:10];
[[NSRunLoop currentRunLoop] run];
NSLog(@"3");
});
NSLog(@"4");
- (void)test{
NSLog(@"5");
}复制代码打印结果:14253，test方法会执行。
6、PerformSelector 相关
PerformSelector 的实现原理？
当调用 NSObject 的 performSelecter:afterDelay: 后，实际上其内部会创建一个 Timer 并添加到当前线程的 RunLoop 中。所以如果当前线程没有 RunLoop，则这个方法会失效。
当调用 performSelector:onThread: 时，实际上其会创建一个 Timer 加到对应的线程去，同样的，如果对应线程没有 RunLoop 该方法也会失效。
作者：小小小_小朋友
链接：https://juejin.cn/post/7056282331132198919