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