1、iOS中一般UI上面常用两种定时器 NSTimer和CADisplayLink，那么它们分别的精确度是如何呢？

　　CADisplayLink 是用于帧刷新定时器，也就是和界面的刷新率保持一致，理想情况下FPS为60，并尽力保持60

　　NSTimer是否可以比CADisplayLink更加精确呢，最精确能下面的间隔是多少呢？NSTimer和CADisplayLink都作为一种事件加入到NSRunloop的事件源中，

　　NSRunloop不断循环，判断是否应该触发这一类事件，如下面所示，NSRunloop包涵下面这些不同的事件：

　　

typedef CF_OPTIONS(CFOptionFlags, CFRunLoopActivity) {

    kCFRunLoopEntry         = (1UL << 0), // 即将进入Loop

    kCFRunLoopBeforeTimers  = (1UL << 1), // 即将处理 Timer

    kCFRunLoopBeforeSources = (1UL << 2), // 即将处理 Source

    kCFRunLoopBeforeWaiting = (1UL << 5), // 即将进入休眠

    kCFRunLoopAfterWaiting  = (1UL << 6), // 刚从休眠中唤醒

    kCFRunLoopExit          = (1UL << 7), // 即将退出Loop

};

　　可以看出，如果想让CADisplayLink正常工作，NSRunloop至少每秒循环60次，也就是至多17ms，那么NSRunloop每秒最多循环多少次呢？

2、NSRunloop每秒循环多少次

　　通过注册一个观察者到主线程中，观察每一种事件执行的log，来看看NSRunloop的执行情况

　　

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {

    // Override point for customization after application launch.

    CFRunLoopObserverRef observer = CFRunLoopObserverCreateWithHandler(NULL, (kCFRunLoopAllActivities), YES, 0, ^(CFRunLoopObserverRef observer, CFRunLoopActivity activity)

    {

        static unsigned long count = 0;

        if(activity & kCFRunLoopEntry)

        {

            NSLog(@"activity %lu: %@", ++count, @"kCFRunLoopEntry");

        }

        else if(activity & kCFRunLoopBeforeTimers)

        {

            NSLog(@"activity %lu: %@", ++count, @"kCFRunLoopBeforeTimers");

        }

        else if(activity & kCFRunLoopBeforeSources)

        {

            NSLog(@"activity %lu: %@", ++count, @"kCFRunLoopBeforeSources");

        }

        else if(activity & kCFRunLoopBeforeWaiting)

        {

            NSLog(@"activity %lu: %@", ++count, @"kCFRunLoopBeforeWaiting");

        }

        else if(activity & kCFRunLoopAfterWaiting)

        {

            NSLog(@"activity %lu: %@", ++count, @"kCFRunLoopAfterWaiting");

        }

        else if(activity & kCFRunLoopExit)

        {

            NSLog(@"activity %lu: %@", ++count, @"kCFRunLoopExit");

        }

    });

    CFRunLoopAddObserver(CFRunLoopGetMain(), observer, kCFRunLoopCommonModes);

    return YES;

}

　　这里打印出每一类的事件，同时需要加上两类定时器到主线程中

- (void)viewDidLoad {

    [super viewDidLoad];

    // Do any additional setup after loading the view, typically from a nib.

   NSTimer *timer = [NSTimer timerWithTimeInterval:0.001 repeats:YES block:^(NSTimer * _Nonnull timer) {

       static int timerCount = 0;

       //do nothing

       NSLog(@"timer do nothing, timerCount = %d", timerCount++);

   }];

   [[NSRunLoop currentRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];

    CADisplayLink *link = [CADisplayLink displayLinkWithTarget:self selector:@selector(update:)];

    [link addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];

}

- (void)update:(id)sender

{

    static int linkCount = 0;

    NSLog(@"link do nothing, linkCount = %d", linkCount++);

}

　　可以观察出每一类事件执行的情况，测试是在iphone5s， ios10.2的系统上

输出：

NSRunloop：

2017-03-18 13:01:54.145100 NSTimerTest[1121:267663] activity 20: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.148893 NSTimerTest[1121:267663] activity 24: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.150659 NSTimerTest[1121:267663] activity 28: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.153758 NSTimerTest[1121:267663] activity 32: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.155796 NSTimerTest[1121:267663] activity 36: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.158515 NSTimerTest[1121:267663] activity 40: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.160481 NSTimerTest[1121:267663] activity 44: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.162514 NSTimerTest[1121:267663] activity 48: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.165265 NSTimerTest[1121:267663] activity 52: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.169531 NSTimerTest[1121:267663] activity 56: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.229129 NSTimerTest[1121:267663] activity 74: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.232630 NSTimerTest[1121:267663] activity 78: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.233628 NSTimerTest[1121:267663] activity 82: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.235007 NSTimerTest[1121:267663] activity 86: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.236381 NSTimerTest[1121:267663] activity 90: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.237788 NSTimerTest[1121:267663] activity 94: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.238881 NSTimerTest[1121:267663] activity 98: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.239719 NSTimerTest[1121:267663] activity 102: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.240767 NSTimerTest[1121:267663] activity 106: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.241759 NSTimerTest[1121:267663] activity 110: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.242658 NSTimerTest[1121:267663] activity 114: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.243716 NSTimerTest[1121:267663] activity 118: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.244833 NSTimerTest[1121:267663] activity 122: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.245719 NSTimerTest[1121:267663] activity 126: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.247445 NSTimerTest[1121:267663] activity 130: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.248342 NSTimerTest[1121:267663] activity 134: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.250020 NSTimerTest[1121:267663] activity 138: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.251359 NSTimerTest[1121:267663] activity 142: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.252876 NSTimerTest[1121:267663] activity 146: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.254850 NSTimerTest[1121:267663] activity 150: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.256126 NSTimerTest[1121:267663] activity 154: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.257336 NSTimerTest[1121:267663] activity 158: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.258301 NSTimerTest[1121:267663] activity 162: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.259885 NSTimerTest[1121:267663] activity 166: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.261638 NSTimerTest[1121:267663] activity 170: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.262796 NSTimerTest[1121:267663] activity 174: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.264645 NSTimerTest[1121:267663] activity 178: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.265702 NSTimerTest[1121:267663] activity 182: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.266818 NSTimerTest[1121:267663] activity 186: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.268355 NSTimerTest[1121:267663] activity 190: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.269167 NSTimerTest[1121:267663] activity 194: kCFRunLoopBeforeWaiting

2017-03-18 13:01:54.269909 NSTimerTest[1121:267663] activity 198: kCFRunLoopBeforeWaiting

　　NSTimer

2017-03-18 13:01:54.127460 NSTimerTest[1121:267663] timer do nothing, timerCount = 0

2017-03-18 13:01:54.135893 NSTimerTest[1121:267663] timer do nothing, timerCount = 1

2017-03-18 13:01:54.140639 NSTimerTest[1121:267663] timer do nothing, timerCount = 2

2017-03-18 13:01:54.142266 NSTimerTest[1121:267663] timer do nothing, timerCount = 3

2017-03-18 13:01:54.143658 NSTimerTest[1121:267663] timer do nothing, timerCount = 4

2017-03-18 13:01:54.147695 NSTimerTest[1121:267663] timer do nothing, timerCount = 5

2017-03-18 13:01:54.151489 NSTimerTest[1121:267663] timer do nothing, timerCount = 6

2017-03-18 13:01:54.154552 NSTimerTest[1121:267663] timer do nothing, timerCount = 7

2017-03-18 13:01:54.156480 NSTimerTest[1121:267663] timer do nothing, timerCount = 8

2017-03-18 13:01:54.159303 NSTimerTest[1121:267663] timer do nothing, timerCount = 9

2017-03-18 13:01:54.161246 NSTimerTest[1121:267663] timer do nothing, timerCount = 10

2017-03-18 13:01:54.163655 NSTimerTest[1121:267663] timer do nothing, timerCount = 11

2017-03-18 13:01:54.170411 NSTimerTest[1121:267663] timer do nothing, timerCount = 12

2017-03-18 13:01:54.226209 NSTimerTest[1121:267663] timer do nothing, timerCount = 13

2017-03-18 13:01:54.228032 NSTimerTest[1121:267663] timer do nothing, timerCount = 14

2017-03-18 13:01:54.231912 NSTimerTest[1121:267663] timer do nothing, timerCount = 15

2017-03-18 13:01:54.234140 NSTimerTest[1121:267663] timer do nothing, timerCount = 16

2017-03-18 13:01:54.235779 NSTimerTest[1121:267663] timer do nothing, timerCount = 17

2017-03-18 13:01:54.236939 NSTimerTest[1121:267663] timer do nothing, timerCount = 18

2017-03-18 13:01:54.238231 NSTimerTest[1121:267663] timer do nothing, timerCount = 19

2017-03-18 13:01:54.239210 NSTimerTest[1121:267663] timer do nothing, timerCount = 20

2017-03-18 13:01:54.240251 NSTimerTest[1121:267663] timer do nothing, timerCount = 21

2017-03-18 13:01:54.241185 NSTimerTest[1121:267663] timer do nothing, timerCount = 22

2017-03-18 13:01:54.242131 NSTimerTest[1121:267663] timer do nothing, timerCount = 23

2017-03-18 13:01:54.243136 NSTimerTest[1121:267663] timer do nothing, timerCount = 24

2017-03-18 13:01:54.244139 NSTimerTest[1121:267663] timer do nothing, timerCount = 25

2017-03-18 13:01:54.245170 NSTimerTest[1121:267663] timer do nothing, timerCount = 26

2017-03-18 13:01:54.246129 NSTimerTest[1121:267663] timer do nothing, timerCount = 27

2017-03-18 13:01:54.248936 NSTimerTest[1121:267663] timer do nothing, timerCount = 28

2017-03-18 13:01:54.250729 NSTimerTest[1121:267663] timer do nothing, timerCount = 29

2017-03-18 13:01:54.252273 NSTimerTest[1121:267663] timer do nothing, timerCount = 30

2017-03-18 13:01:54.253343 NSTimerTest[1121:267663] timer do nothing, timerCount = 31

2017-03-18 13:01:54.255634 NSTimerTest[1121:267663] timer do nothing, timerCount = 32

2017-03-18 13:01:54.256494 NSTimerTest[1121:267663] timer do nothing, timerCount = 33

2017-03-18 13:01:54.257829 NSTimerTest[1121:267663] timer do nothing, timerCount = 34

2017-03-18 13:01:54.259261 NSTimerTest[1121:267663] timer do nothing, timerCount = 35

2017-03-18 13:01:54.260290 NSTimerTest[1121:267663] timer do nothing, timerCount = 36

2017-03-18 13:01:54.262144 NSTimerTest[1121:267663] timer do nothing, timerCount = 37

2017-03-18 13:01:54.263531 NSTimerTest[1121:267663] timer do nothing, timerCount = 38

2017-03-18 13:01:54.266225 NSTimerTest[1121:267663] timer do nothing, timerCount = 39

2017-03-18 13:01:54.267656 NSTimerTest[1121:267663] timer do nothing, timerCount = 40

2017-03-18 13:01:54.268629 NSTimerTest[1121:267663] timer do nothing, timerCount = 41

2017-03-18 13:01:54.269641 NSTimerTest[1121:267663] timer do nothing, timerCount = 42

2017-03-18 13:01:54.271133 NSTimerTest[1121:267663] timer do nothing, timerCount = 43

2017-03-18 13:01:54.272366 NSTimerTest[1121:267663] timer do nothing, timerCount = 44

2017-03-18 13:01:54.273213 NSTimerTest[1121:267663] timer do nothing, timerCount = 45

2017-03-18 13:01:54.274115 NSTimerTest[1121:267663] timer do nothing, timerCount = 46

2017-03-18 13:01:54.275425 NSTimerTest[1121:267663] timer do nothing, timerCount = 47

2017-03-18 13:01:54.276400 NSTimerTest[1121:267663] timer do nothing, timerCount = 48

2017-03-18 13:01:54.278951 NSTimerTest[1121:267663] timer do nothing, timerCount = 49

2017-03-18 13:01:54.281740 NSTimerTest[1121:267663] timer do nothing, timerCount = 50

2017-03-18 13:01:54.283120 NSTimerTest[1121:267663] timer do nothing, timerCount = 51

2017-03-18 13:01:54.284362 NSTimerTest[1121:267663] timer do nothing, timerCount = 52

　　CADisplayLink

2017-03-18 13:01:54.138378 NSTimerTest[1121:267663] link do nothing, linkCount = 0

2017-03-18 13:01:54.149657 NSTimerTest[1121:267663] link do nothing, linkCount = 1

2017-03-18 13:01:54.166121 NSTimerTest[1121:267663] link do nothing, linkCount = 2

2017-03-18 13:01:54.225401 NSTimerTest[1121:267663] link do nothing, linkCount = 3

2017-03-18 13:01:54.233023 NSTimerTest[1121:267663] link do nothing, linkCount = 4

2017-03-18 13:01:54.247794 NSTimerTest[1121:267663] link do nothing, linkCount = 5

2017-03-18 13:01:54.265119 NSTimerTest[1121:267663] link do nothing, linkCount = 6

2017-03-18 13:01:54.280689 NSTimerTest[1121:267663] link do nothing, linkCount = 7

2017-03-18 13:01:54.298406 NSTimerTest[1121:267663] link do nothing, linkCount = 8

2017-03-18 13:01:54.315036 NSTimerTest[1121:267663] link do nothing, linkCount = 9

2017-03-18 13:01:54.331416 NSTimerTest[1121:267663] link do nothing, linkCount = 10

2017-03-18 13:01:54.346888 NSTimerTest[1121:267663] link do nothing, linkCount = 11

2017-03-18 13:01:54.363696 NSTimerTest[1121:267663] link do nothing, linkCount = 12

2017-03-18 13:01:54.380435 NSTimerTest[1121:267663] link do nothing, linkCount = 13

2017-03-18 13:01:54.398125 NSTimerTest[1121:267663] link do nothing, linkCount = 14

2017-03-18 13:01:54.414091 NSTimerTest[1121:267663] link do nothing, linkCount = 15

2017-03-18 13:01:54.430187 NSTimerTest[1121:267663] link do nothing, linkCount = 16

2017-03-18 13:01:54.447019 NSTimerTest[1121:267663] link do nothing, linkCount = 17

2017-03-18 13:01:54.463854 NSTimerTest[1121:267663] link do nothing, linkCount = 18

2017-03-18 13:01:54.480343 NSTimerTest[1121:267663] link do nothing, linkCount = 19

2017-03-18 13:01:54.513566 NSTimerTest[1121:267663] link do nothing, linkCount = 21

2017-03-18 13:01:54.530110 NSTimerTest[1121:267663] link do nothing, linkCount = 22

2017-03-18 13:01:54.547502 NSTimerTest[1121:267663] link do nothing, linkCount = 23

2017-03-18 13:01:54.565346 NSTimerTest[1121:267663] link do nothing, linkCount = 24

2017-03-18 13:01:54.580107 NSTimerTest[1121:267663] link do nothing, linkCount = 25

2017-03-18 13:01:54.598014 NSTimerTest[1121:267663] link do nothing, linkCount = 26

2017-03-18 13:01:54.629988 NSTimerTest[1121:267663] link do nothing, linkCount = 28

2017-03-18 13:01:54.646657 NSTimerTest[1121:267663] link do nothing, linkCount = 29

2017-03-18 13:01:54.663800 NSTimerTest[1121:267663] link do nothing, linkCount = 30

2017-03-18 13:01:54.680001 NSTimerTest[1121:267663] link do nothing, linkCount = 31

2017-03-18 13:01:54.697108 NSTimerTest[1121:267663] link do nothing, linkCount = 32

2017-03-18 13:01:54.713726 NSTimerTest[1121:267663] link do nothing, linkCount = 33

2017-03-18 13:01:54.729956 NSTimerTest[1121:267663] link do nothing, linkCount = 34

2017-03-18 13:01:54.746658 NSTimerTest[1121:267663] link do nothing, linkCount = 35

2017-03-18 13:01:54.763778 NSTimerTest[1121:267663] link do nothing, linkCount = 36

2017-03-18 13:01:54.779953 NSTimerTest[1121:267663] link do nothing, linkCount = 37

2017-03-18 13:01:54.796732 NSTimerTest[1121:267663] link do nothing, linkCount = 38

2017-03-18 13:01:54.813729 NSTimerTest[1121:267663] link do nothing, linkCount = 39

2017-03-18 13:01:54.829980 NSTimerTest[1121:267663] link do nothing, linkCount = 40

2017-03-18 13:01:54.846751 NSTimerTest[1121:267663] link do nothing, linkCount = 41

2017-03-18 13:01:54.863757 NSTimerTest[1121:267663] link do nothing, linkCount = 42

2017-03-18 13:01:54.879957 NSTimerTest[1121:267663] link do nothing, linkCount = 43

2017-03-18 13:01:54.896619 NSTimerTest[1121:267663] link do nothing, linkCount = 44

2017-03-18 13:01:54.913648 NSTimerTest[1121:267663] link do nothing, linkCount = 45

2017-03-18 13:01:54.930006 NSTimerTest[1121:267663] link do nothing, linkCount = 46

2017-03-18 13:01:54.946953 NSTimerTest[1121:267663] link do nothing, linkCount = 47

2017-03-18 13:01:54.963751 NSTimerTest[1121:267663] link do nothing, linkCount = 48

2017-03-18 13:01:54.980065 NSTimerTest[1121:267663] link do nothing, linkCount = 49

　　

3、总结

　　可以看出来，CADisplayLink“尽量”保证17ms执行一次，主要看NSRunloop中有没有执行其它事件时被阻塞

　　NSTimer的触发周期最小可以和NSRunloop的循环保持一致，也就是由NSRunloop的执行周期决定，大概2～3ms

　　因此这也是NSTimer也是不精确的原因

代码：https://github.com/liqiushui/NSRunloop

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