谈 UIKit 和 CoreAnimation 在 iOS 渲染中的角色（下）

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小试牛刀之 CADisplayLink

POP 库基于 CADisplayLink 注册 VSync 信号，如何注册的呢？VSync 信号到底是个啥呢？首先，我们打印一下 CADisplayLink 的触发堆栈： 我们发现这是一个从 Port 转发过来处理的 source1，我们打印一下 Runloop，发现比没有 CADisplayLink 的 Runloop 多了一个 source1：

2 : <CFRunLoopSource 0x282dc8000 [0x1cadcf728]>{
    signalled = No, valid = Yes, order = -1, 
    context = <CFMachPort 0x282fd8160 [0x1cadcf728]>{
        valid = Yes, port = 440b, source = 0x282dc8000, 
        callout = _ZL22display_timer_callbackP12__CFMachPortPvlS1_ (0x187592b2c), 
        context = <CFMachPort context 0x2823d0000>
    }
}

有这样一个 source1 被加入了 Runloop。CADisplaylink 也是基于这个 port完成了 VSync 信号的注册工作。产生 VSync 信号的进程，每 16.7ms 进行一次到这个 port 的 mach msg 发送工作，从而不断的激活本 App 的 Runloop ，触发一个 item，完成本 App 对 VSync 的感知。这里说点题外的，看到item，很自然的联想到这里是不是一个链表之类的数据结构。实际上，我们可以多次添加不同的 CADisplayLink instance 到 Runloop 中，就像这样：

CADispalyLink *link = [CADisplayLink displayLinkWithTarget:self selector:@selector(drawSomething)];
[link addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];
CADispalyLink *link2 = [CADisplayLink displayLinkWithTarget:self selector:@selector(drawSomething2)];
[link2 addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];

但是如果这样写：

CADispalyLink *link = [CADisplayLink displayLinkWithTarget:self selector:@selector(drawSomething)];
[link addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];
CADispalyLink *link2 = [CADisplayLink displayLinkWithTarget:self selector:@selector(drawSomething)];
[link2 addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];

绿色高亮部分的差异会造成什么呢？drawSomeThing 在一个 VSync message 里被触发了两次。原因是：[CADisplayLink displayLinkWithTarget:self selector:@selector(drawSomeThing)] 这个方法内部的 [CADisplayLink displayLinkWithDisplay:target:selector:] 调用新生成了一个 DisplayLinkItem （下图汇编代码中的 v8）。

调用 [CADisplayLink addToRunLoop:forMode:]时，内部：

这里的逻辑只检测了：

下面我们总结一下基于CoreAnimation的Animation和基于POP的Animation的区别，可以用以下流程图解答： 总结一下：

但是显而易见，有以下不足：

总结可得到以下两个问题的答案：

如何进入 waiting？

调用 mach_msg，设置好 timeout 参数和 wait_port 就可以进入内核态等待 mach_msg 的来临

如何 wakeup？

说了这么一圈，我们了解了Runloop本质是在处理mach msg。那么我们什么时候做渲染呢？viewdidload、layoutSubviews 等等中代码，到底是在 Runloop 哪里被触发的呢？

Runloop 如何触发渲染？

由于 Runloop 比较多的 mode ，每个 mode 都是在组合常见的几个 item ，关于 mode 和 item 不清楚的同学可以自行查阅相关资料，这里打印出一个 Runloop 的 debugDescription 来说明：

<CFRunLoop 0x6000001d8100 [0x1c04a6510]>{wakeup port = 0x2303, stopped = false, ignoreWakeUps = false, current mode = kCFRunLoopDefaultMode,//首先，目前Runloop中存在两个mode，一个是UITracking，也就是滑动用的，一个是defaultcommon modes = <CFBasicHash 0x600003390ed0 [0x1c04a6510]>{type = mutable set, count = 2,entries => 0 : <CFString 0x1c0b2e5f0 [0x1c04a6510]>{contents = "UITrackingRunLoopMode"} 2 : <CFString 0x1c067c148 [0x1c04a6510]>{contents = "kCFRunLoopDefaultMode"}}

第一个mode——UITrackingRunLoopMode：

0 : <CFRunLoopMode 0x6000006d4410 [0x1c04a6510]>{    name = UITrackingRunLoopMode,   port set = 0x4803,   queue = 0x6000013d4e00,   source = 0x6000013d4f00 (not fired),   timer port = 0x4703,  sources0 : { 0 : <CFRunLoopSource  >{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x0, callout = PurpleEventSignalCallback (0x18aebe530)}} 1 : <CFRunLoopSource  >{signalled = No, valid = Yes, order = 0, context = <CFRunLoopSource context>{version = 0, info = 0x6000019c4a20, callout = FBSSerialQueueRunLoopSourceHandler (0x185729edc)}} 2 : <CFRunLoopSource  >{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x6000006c0000, callout = __eventQueueSourceCallback (0x1844477dc)}} 5 : <CFRunLoopSource  >{signalled = No, valid = Yes, order = -2, context = <CFRunLoopSource context>{version = 0, info = 0x600003381140, callout = __eventFetcherSourceCallback (0x184447850)}}  }, sources1 : { 0 : <CFRunLoopSource >{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 1, info = 0x3603, callout = PurpleEventCallback (0x18aebe538)}} }, observers = (    "<CFRunLoopObserver repeats = Yes, order = -2147483647, callout = _runLoopObserverCallout (0x183ee5508), context = (<_UIWeakReference: 0x600003fd4250>)}",    "<CFRunLoopObserver repeats = Yes, order = 0, callout = _UIGestureRecognizerUpdateObserver (0x183f32638), context = <CFRunLoopObserver context 0x6000016c0000>}",    "<CFRunLoopObserver repeats = Yes, order = 1999000, callout = _beforeCACommitHandler (0x1843dbf00), context = <CFRunLoopObserver context 0x125705a10>}",    "<CFRunLoopObserver repeats = Yes, order = 2000000, callout = _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv (0x187561d54), context = <CFRunLoopObserver context 0x0>}",    "<CFRunLoopObserver repeats = Yes, order = 2001000, callout = _afterCACommitHandler (0x1843dbf74), context = <CFRunLoopObserver context 0x125705a10>}",    "<CFRunLoopObserver repeats = Yes, order = 2147483647, callout = _runLoopObserverCallout (0x183ee5508), context = (\n    \"<_UIWeakReference: 0x600003fd4250>\"\n)}"    ), timers = (null),},

比较有价值的，就是这个 _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv 函数了他是由 CoreAnimation 注册到 Runloop 的一个 observer 的回调。在这个 mode 下只有一个 source1，说明现在（仅代表测试程序环境）只能被来一个端口 msg 中断，也就是 PurpleEventCallback对应的端口。

第二个 mode -GSEventReceiveRunLoopMode：没有暴露的 mode，也是跟事件下发相关，不做探究

1 : <CFRunLoopMode 0x6000006d44e0 [0x1c04a6510]>{    name = GSEventReceiveRunLoopMode, port set = 0x4603, queue = 0x6000013d4f80, source = 0x6000013d5080 (not fired), timer port = 0x4403,  sources0 = <CFBasicHash 0x600003385380 [0x1c04a6510]>{type = mutable set, count = 1,    entries => 0 : <CFRunLoopSource 0x6000008d40c0 [0x1c04a6510]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x0, callout = PurpleEventSignalCallback (0x18aebe530)}}    }, sources1 = <CFBasicHash 0x6000033853b0 [0x1c04a6510]>{type = mutable set, count = 1,    entries => 0 : <CFRunLoopSource 0x6000008d4480 [0x1c04a6510]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 1, info = 0x3603, callout = PurpleEventCallback (0x18aebe538)}}    }, observers = (null), timers = (null), currently 628607093 (779024795576) / soft deadline in: 7.68614304e+11 sec (@ -1) / hard deadline in: 7.68614304e+11 sec (@ -1)},

第三个 mode - kCFRunLoopDefaultMode。

2 : <CFRunLoopMode 0x6000006dc000 [0x1c04a6510]>{    name = kCFRunLoopDefaultMode, port set = 0x5403, queue = 0x6000013dc700, source = 0x6000013dc800 (not fired), timer port = 0x2c03,  sources0 = <CFBasicHash 0x600003385290 [0x1c04a6510]>{type = mutable set, count = 4,    entries => 0 : <CFRunLoopSource 0x6000008d40c0 [0x1c04a6510]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x0, callout = PurpleEventSignalCallback (0x18aebe530)}} 1 : <CFRunLoopSource 0x6000008dc000 [0x1c04a6510]>{signalled = No, valid = Yes, order = 0, context = <CFRunLoopSource context>{version = 0, info = 0x6000019c4a20, callout = FBSSerialQueueRunLoopSourceHandler (0x185729edc)}} 2 : <CFRunLoopSource 0x6000008d8180 [0x1c04a6510]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 0, info = 0x6000006c0000, callout = __eventQueueSourceCallback (0x1844477dc)}} 5 : <CFRunLoopSource 0x6000008d8240 [0x1c04a6510]>{signalled = No, valid = Yes, order = -2, context = <CFRunLoopSource context>{version = 0, info = 0x600003381140, callout = __eventFetcherSourceCallback (0x184447850)}}    }, sources1 = <CFBasicHash 0x6000033852f0 [0x1c04a6510]>{type = mutable set, count = 1,    entries => 0 : <CFRunLoopSource 0x6000008d43c0 [0x1c04a6510]>{signalled = No, valid = Yes, order = -1, context = <CFRunLoopSource context>{version = 1, info = 0x3603, callout = PurpleEventCallback (0x18aebe538)}}    }, observers = (    "<CFRunLoopObserver 0x600000cdc460 [0x1c04a6510]>{valid = Yes, activities = 0x1, repeats = Yes, order = -2147483647, callout = _runLoopObserverCallout (0x183ee5508), context = (\n    \"<_UIWeakReference: 0x600003fd4250>\"\n)}",    "<CFRunLoopObserver 0x600000cc0000 [0x1c04a6510]>{valid = Yes, activities = 0x20, repeats = Yes, order = 0, callout = _UIGestureRecognizerUpdateObserver (0x183f32638), context = <CFRunLoopObserver context 0x6000016c0000>}",    "<CFRunLoopObserver 0x600000cdc780 [0x1c04a6510]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 1999000, callout = _beforeCACommitHandler (0x1843dbf00), context = <CFRunLoopObserver context 0x125705a10>}",    "<CFRunLoopObserver 0x600000cd08c0 [0x1c04a6510]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 2000000, callout = _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv (0x187561d54), context = <CFRunLoopObserver context 0x0>}",    "<CFRunLoopObserver 0x600000cdc500 [0x1c04a6510]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 2001000, callout = _afterCACommitHandler (0x1843dbf74), context = <CFRunLoopObserver context 0x125705a10>}",    "<CFRunLoopObserver 0x600000cdc820 [0x1c04a6510]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 2147483647, callout = _runLoopObserverCallout (0x183ee5508), context = (\n    \"<_UIWeakReference: 0x600003fd4250>\"\n)}"    ), timers = <CFArray 0x6000019c82a0 [0x1c04a6510]>{type = mutable-small, count = 1, values = ( 0 : <CFRunLoopTimer 0x6000008d0180 [0x1c04a6510]>{valid = Yes, firing = No, interval = 0, tolerance = 0, next fire date = 628607095 (1.47744203 @ 779060301174), callout = (Delayed Perform) UIApplication _accessibilitySetUpQuickSpeak (0x1808078f4 / 0x183895594) (/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 = <CFRunLoopTimer context 0x600002898080>}    )}, currently 628607093 (779024795962) / soft deadline in: 1.47938383 sec (@ 779060301174) / hard deadline in: 1.47938379 sec (@ 779060301174)},

综上，一个普通运行起来的程序，Runloop 中大概有以下几个 source/observer：

AutoReleasePool 是在 Runloop 启动时就生成了一个基本的 pool 来使用，而 transaction 则是有需要才建立的，这一步是怎么处理的呢？前面我们看到， CATransaction 注册了 before waiting 的事件，在 before waiting 的时候，调用CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned long, void*)，这个 callback 里主要调用了CA::Transaction::commit()。

接下来我们研究一下 CATransaction 机制，并讲解CA::Transaction::commit()做了什么。

CATransaction

CATransaction 是整个 CoreAnimation 的工作流程核心，它不是对 OpenGLES 的封装的，也没有做动画插值。它是一个载体，大部分业务需求的WorkFlow都是基于CATransaction完成的。基础的用法，begin，commit，timingfunction，complete handler，隐式事务，Runloop 中的大 Transaction 我们就不谈了，不清楚的同学可以自行查阅相关已有资料。CA::Transaction::Commit() 的被调用逻辑：CA::Transaction::Commit 主要由

CA::Transaction::Commit 内部主要逻辑如下： 可以看到，我们主要做了这几件事情：

下面看 commit_If_Needed 中的具体逻辑：

综上，当 App 的 Runloop 不管是被什么事件唤醒以后，总能在进入内核态之前完成当前所有UI 的更新任务，并通过 mach_msg 完成 commit 到 render server 的工作。最常见由 iOS 开发者控制的唤醒 Runloop 的行为有：

至此我们已经解答了以下问题：

有任何你想了解的问题，欢迎在文章下方评论。

图形学时间

计算机动画扩展：计算机动画发展的很快，不再局限于某些物体的位置变换和伸缩变换，出现了很多算法实现各种效果，总结来看，计算机动画分为以下几种：

那么 CoreAnimation 在动画方面关注的显然是 2D 动画，我们也来讲解一下关于图像变形和形状混合。图像变形图像变形有两种基本的方法：

形状混合

二维图形动画，都可以简化为为多边形处理。二维的形状混合，即在两个关键帧的多边形之间插入新的多边形。插入新的多边形，需要解决的问题就是前后两个关键帧之间，顶点的对应关系和顶点之间插值路径的问题。这两个问题在 siggraph 1992 和 1993 的论文上得到了解。

参考资料

[1]深入理解 RunLoop: https://blog.ibireme.com/2015/05/18/Runloop/

[2]Apple CFRunloop 源码: https://opensource.apple.com/source/CF/CF-635/CFRunLoop.c.auto.html

[3]界面渲染的整体流程: http://blog.handy.wang/blog/2015/10/03/uiviewyu-calayerxie-zuo-xuan-ran-jie-mian-de-guo-cheng/

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