1. 测试触发时机

为了探究渲染的触发时机，我们自定义一个TestView并复写 drawRect: 方法。

虽然开发中不推荐这么使用 drawRect: 方法，这里是为了设置断点，探究渲染的触发时机。

@interface TestView : UIView
@end

@implementation TestView
- (void)drawRect:(CGRect)rect
{
    [super drawRect:rect];
}
@end

@implementation ViewController
- (void)loadView
{
    self.view = [[TestView alloc] initWithFrame:[UIScreen mainScreen].bounds];
}

- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor whiteColor];
    
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        self.view.backgroundColor = [UIColor blueColor];
    });
    
    UIButton *button = [UIButton buttonWithType:UIButtonTypeDetailDisclosure];
    [button addTarget:self action:@selector(buttonPressed) forControlEvents:UIControlEventTouchUpInside];
    button.center = self.view.center;
    [self.view addSubview:button];
}

- (void)buttonPressed
{
    self.view.backgroundColor = [UIColor redColor];
}
@end
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我们在 drawRect: 方法中设置一个断点，然后运行我们的测试代码。

1.1 情况1

刚运行起来就会进入断点，我们在 lldb 中使用 bt 打印一下调用栈。

(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
  * frame #0: 0x000000010bf67094 OCDemo`-[TestView drawRect:](self=0x00007fbd0a70ec00, _cmd="drawRect:", rect=(origin = (x = 0, y = 0), size = (width = 375, height = 812))) at ViewController.m:19:5
    frame #1: 0x00007fff491928ae UIKitCore`-[UIView(CALayerDelegate) drawLayer:inContext:] + 632
    frame #2: 0x000000010e0b7941 UIKit`-[UIViewAccessibility drawLayer:inContext:] + 74
    frame #3: 0x00007fff2b4c5ca4 QuartzCore`-[CALayer drawInContext:] + 286
    frame #4: 0x00007fff2b391d58 QuartzCore`CABackingStoreUpdate_ + 196
    frame #5: 0x00007fff2b4ce665 QuartzCore`___ZN2CA5Layer8display_Ev_block_invoke + 53
    frame #6: 0x00007fff2b4c5616 QuartzCore`-[CALayer _display] + 2026
    frame #7: 0x00007fff2b4d7d72 QuartzCore`CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 520
    frame #8: 0x00007fff2b420c04 QuartzCore`CA::Context::commit_transaction(CA::Transaction*, double) + 324
    frame #9: 0x00007fff2b4545ef QuartzCore`CA::Transaction::commit() + 649
    frame #10: 0x00007fff48ca3747 UIKitCore`__34-[UIApplication _firstCommitBlock]_block_invoke_2 + 81
    frame #11: 0x00007fff23da0b5c CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame #12: 0x00007fff23da0253 CoreFoundation`__CFRunLoopDoBlocks + 195
    frame #13: 0x00007fff23d9b043 CoreFoundation`__CFRunLoopRun + 995
    frame #14: 0x00007fff23d9a944 CoreFoundation`CFRunLoopRunSpecific + 404
    frame #15: 0x00007fff38ba6c1a GraphicsServices`GSEventRunModal + 139
    frame #16: 0x00007fff48c8b9ec UIKitCore`UIApplicationMain + 1605
    frame #17: 0x000000010bf677ba OCDemo`main(argc=1, argv=0x00007ffee3c97d28) at main.m:18:12
    frame #18: 0x00007fff51a231fd libdyld.dylib`start + 1
    frame #19: 0x00007fff51a231fd libdyld.dylib`start + 1
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从调用栈我们可以看到，App运行起来主线程的 RunLoop 会触发 -[UIApplication _firstCommitBlock] 进行第一次的绘制。

1.2 情况2

我们以前看其他博客的时候经常看到以下说法：

当在操作UI时，比如改变了frame、更新了UIView/CALayer的层次时，或者手动调用了UIView/CALayer的setNeedsLayout/setNeedsDisplay方法后，这个UIView/CALayer就被标记为待处理，并被提交到一个全局的容器去。

苹果注册了一个Observer监听BeforeWaiting(即将进入休眠)和Exit(即将退出Loop)事件，回调去执行一个很长的函数： _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv。

这个函数里会遍历所有待处理的UIView/CAlayer以执行实际的绘制和调整，并更新UI界面。

下面我们来验证一下。继续运行，执行完下面代码的中改变视图背景色的部分后

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    self.view.backgroundColor = [UIColor blueColor];
});
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会进入第二次的断点：

(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
  * frame #0: 0x00000001040cde64 OCDemo`-[TestView drawRect:](self=0x00007fdd2dc09ee0, _cmd="drawRect:", rect=(origin = (x = 0, y = 0), size = (width = 375, height = 812))) at ViewController.m:19:5
    frame #1: 0x00007fff491928ae UIKitCore`-[UIView(CALayerDelegate) drawLayer:inContext:] + 632
    frame #2: 0x00000001045a9941 UIKit`-[UIViewAccessibility drawLayer:inContext:] + 74
    frame #3: 0x00007fff2b4c5ca4 QuartzCore`-[CALayer drawInContext:] + 286
    frame #4: 0x00007fff2b391d58 QuartzCore`CABackingStoreUpdate_ + 196
    frame #5: 0x00007fff2b4ce665 QuartzCore`___ZN2CA5Layer8display_Ev_block_invoke + 53
    frame #6: 0x00007fff2b4c5616 QuartzCore`-[CALayer _display] + 2026
    frame #7: 0x00007fff2b4d7d72 QuartzCore`CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 520
    frame #8: 0x00007fff2b420c04 QuartzCore`CA::Context::commit_transaction(CA::Transaction*, double) + 324
    frame #9: 0x00007fff2b4545ef QuartzCore`CA::Transaction::commit() + 649
    frame #10: 0x00007fff2b454f81 QuartzCore`CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned long, void*) + 79
    frame #11: 0x00007fff23da0127 CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 23
    frame #12: 0x00007fff23d9abde CoreFoundation`__CFRunLoopDoObservers + 430
    frame #13: 0x00007fff23d9b12a CoreFoundation`__CFRunLoopRun + 1226
    frame #14: 0x00007fff23d9a944 CoreFoundation`CFRunLoopRunSpecific + 404
    frame #15: 0x00007fff38ba6c1a GraphicsServices`GSEventRunModal + 139
    frame #16: 0x00007fff48c8b9ec UIKitCore`UIApplicationMain + 1605
    frame #17: 0x00000001040ce77a OCDemo`main(argc=1, argv=0x00007ffeebb30d28) at main.m:18:12
    frame #18: 0x00007fff51a231fd libdyld.dylib`start + 1
    frame #19: 0x00007fff51a231fd libdyld.dylib`start + 1
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这一次我们发现主线程的 RunLoop 通过一个Observer触发了CA::Transaction::observer_callback。

我们知道 __CFRunLoopDoObservers 的API申明如下：

static void __CFRunLoopDoObservers(CFRunLoopRef rl, CFRunLoopModeRef rlm, CFRunLoopActivity activity)
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第三个参数为CFRunLoopActivity类型：

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
};
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我们打印一下第三个参数：

(lldb) po $arg3
32
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正好是我们的kCFRunLoopBeforeWaiting事件。

下面我们打印主线程的 RunLoop 并搜索 observer_callback 可以看到：

(lldb) po [NSRunLoop mainRunLoop]
...
// activities = 0xa0 kCFRunLoopBeforeWaiting | kCFRunLoopExit
4 : <CFRunLoopObserver 0x600000e1c3c0 [0x7fff8062d610]>{valid = Yes, activities = 0xa0, repeats = Yes, order = 2000000, callout = _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv (0x7fff2b454f32), context = <CFRunLoopObserver context 0x0>}
...
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我们可以看到主线程的 RunLoop 即将进入休眠的时候触发了这个观察者。

有些同学对于主线程的 RunLoop 在触发这个观察者之后，界面的渲染事件有疑问：

这时触发的渲染，是在当前轮次，还是下一轮RunLoop进行显示。

其实 BeforeWaiting 指的是，提交到这一轮 RunLoop 的事务都做完了，要进入休眠了。可以理解为每一轮 RunLoop 的 completion callback ，可以开始进行新一轮的事务提交了。而监听到 BeforeWaiting 的Observer会调用 _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv ，进行下一次的事务的准备和提交。

_ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv()
    QuartzCore:CA::Transaction::observer_callback:
        CA::Transaction::commit();
            CA::Context::commit_transaction();
                CA::Layer::layout_and_display_if_needed();
                    CA::Layer::layout_if_needed();
                        [CALayer layoutSublayers];
                            [UIView layoutSubviews];
                    CA::Layer::display_if_needed();
                        [CALayer display];
                            [UIView drawRect];
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即，下一轮的RunLoop才会刷新界面 。

1.3 情况3

我们点击按钮之后，也会更改视图的背景色：

- (void)buttonPressed
{
    self.view.backgroundColor = [UIColor redColor];
}
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我们知道，点击按钮会触发主线程的 RunLoop 的 source1 执行 __IOHIDEventSystemClientQueueCallback ，唤醒下一轮 RunLoop 。

(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
  * frame #0: 0x0000000108105e64 OCDemo`-[TestView drawRect:](self=0x00007fdeaa51d430, _cmd="drawRect:", rect=(origin = (x = 0, y = 0), size = (width = 375, height = 812))) at ViewController.m:19:5
    frame #1: 0x00007fff491928ae UIKitCore`-[UIView(CALayerDelegate) drawLayer:inContext:] + 632
    frame #2: 0x000000010a254941 UIKit`-[UIViewAccessibility drawLayer:inContext:] + 74
    frame #3: 0x00007fff2b4c5ca4 QuartzCore`-[CALayer drawInContext:] + 286
    frame #4: 0x00007fff2b391d58 QuartzCore`CABackingStoreUpdate_ + 196
    frame #5: 0x00007fff2b4ce665 QuartzCore`___ZN2CA5Layer8display_Ev_block_invoke + 53
    frame #6: 0x00007fff2b4c5616 QuartzCore`-[CALayer _display] + 2026
    frame #7: 0x00007fff2b4d7d72 QuartzCore`CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 520
    frame #8: 0x00007fff2b420c04 QuartzCore`CA::Context::commit_transaction(CA::Transaction*, double) + 324
    frame #9: 0x00007fff2b4545ef QuartzCore`CA::Transaction::commit() + 649
    frame #10: 0x00007fff48c84fc8 UIKitCore`_UIApplicationFlushRunLoopCATransactionIfTooLate + 104
    frame #11: 0x00007fff48d32ab0 UIKitCore`__handleEventQueueInternal + 7506
    frame #12: 0x00007fff48d28fb9 UIKitCore`__handleHIDEventFetcherDrain + 88
    frame #13: 0x00007fff23da0d31 CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
    frame #14: 0x00007fff23da0c5c CoreFoundation`__CFRunLoopDoSource0 + 76
    frame #15: 0x00007fff23da0434 CoreFoundation`__CFRunLoopDoSources0 + 180
    frame #16: 0x00007fff23d9b02e CoreFoundation`__CFRunLoopRun + 974
    frame #17: 0x00007fff23d9a944 CoreFoundation`CFRunLoopRunSpecific + 404
    frame #18: 0x00007fff38ba6c1a GraphicsServices`GSEventRunModal + 139
    frame #19: 0x00007fff48c8b9ec UIKitCore`UIApplicationMain + 1605
    frame #20: 0x000000010810677a OCDemo`main(argc=1, argv=0x00007ffee7af8d28) at main.m:18:12
    frame #21: 0x00007fff51a231fd libdyld.dylib`start + 1
    frame #22: 0x00007fff51a231fd libdyld.dylib`start + 1
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进入断点之后，我们发现唤醒 下一轮RunLoop 之后由 source0 执行 CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION 回调，进而触发 QuartzCore`CA::Transaction::commit() 方法。

Source1在处理任务的时候，有时会跟Source0一起配合，把一些任务分发给Source0去执行。例如刚刚提到的点击事件，先由Source1处理硬件事件，之后Source1将事件包装分发给Source0继续进行处理。

2. 总结

除了首次执行 commit 以外，更改视图显示效果之后，均在 下一轮RunLoop 才会生效，提交给 Render Server 进行渲染。

3.扩展阅读

1. iOS 保持界面流畅的技巧
2. iOS Core Animation Advanced Techniques
3. iOS 图像渲染原理

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