iOS 计时器

CADisplayLink

• 保证 调用频率和屏幕的刷新率一致 60FPS

NSTimer

• 都有循环引用的问题
• NSTimer依赖于RunLoop，如果RunLoop的任务过于繁重，可能会导致NSTimer不准时

解决循环引用

循环引用的本质是 timer和self之间的相互强引用, 只要打破这个引用循环就可以了

1.NSTimer使用block的方式创建 并对self 进行弱引用

__weak typeof(self) weakSelf = self;
self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
         [weakSelf timerTest];
}];

2.NSProxy

• NSProxy 不是NSObject的子类, 而是级别一样的基类
• NSProxy专门做消息转发的类, 继承于NSProxy的子类,在找不到方法的时候会直接进入消息转发, 省略了去父类查找的过程和动态方法解析的过程,效率比较高

#import <Foundation/Foundation.h>

@interface TimerProxy : NSProxy
+ (instancetype)proxyWithTarget:(id)target;
@property (weak, nonatomic) id target; // 弱引用target
@end

@implementation TimerProxy

+ (instancetype)proxyWithTarget:(id)target
{
    TimerProxy *proxy = [[TimerProxy alloc] init];
    proxy.target = target;
    return proxy;
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel
{
    return [self.target methodSignatureForSelector:sel];
}

- (void)forwardInvocation:(NSInvocation *)invocation
{
    [invocation invokeWithTarget:self.target];
}


self.link = [CADisplayLink displayLinkWithTarget:[TimerProxy proxyWithTarget:self] selector:@selector(linkTest)];

- (void)linkTest
{
    NSLog(@"%s", __func__);
}

- (void)dealloc
{
    NSLog(@"%s", __func__);
    [self.link invalidate];
}

解决NSTimer计时不准确

使用GCD计时器, GCD计时器不依靠Runloop计时, 而是依赖于系统内核
封装GCD计时器

#import <Foundation/Foundation.h>

@interface MSTimer : NSObject

+ (NSString *)execTask:(void(^)(void))task
           start:(NSTimeInterval)start
        interval:(NSTimeInterval)interval
         repeats:(BOOL)repeats
           async:(BOOL)async;

+ (NSString *)execTask:(id)target
              selector:(SEL)selector
                 start:(NSTimeInterval)start
              interval:(NSTimeInterval)interval
               repeats:(BOOL)repeats
                 async:(BOOL)async;

+ (void)cancelTask:(NSString *)name;

@end

#import "MSTimer.h"

@implementation MSTimer

static NSMutableDictionary *timers_;
dispatch_semaphore_t semaphore_;
+ (void)initialize
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        timers_ = [NSMutableDictionary dictionary];
        semaphore_ = dispatch_semaphore_create(1);
    });
}

+ (NSString *)execTask:(void (^)(void))task start:(NSTimeInterval)start interval:(NSTimeInterval)interval repeats:(BOOL)repeats async:(BOOL)async
{
    if (!task || start < 0 || (interval <= 0 && repeats)) return nil;
    
    // 队列
    dispatch_queue_t queue = async ? dispatch_get_global_queue(0, 0) : dispatch_get_main_queue();
    
    // 创建定时器
    dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    
    // 设置时间
    dispatch_source_set_timer(timer,
                              dispatch_time(DISPATCH_TIME_NOW, start * NSEC_PER_SEC),
                              interval * NSEC_PER_SEC, 0);
    
    
    dispatch_semaphore_wait(semaphore_, DISPATCH_TIME_FOREVER);
    // 定时器的唯一标识
    NSString *name = [NSString stringWithFormat:@"%zd", timers_.count];
    // 存放到字典中
    timers_[name] = timer;
    dispatch_semaphore_signal(semaphore_);
    
    // 设置回调
    dispatch_source_set_event_handler(timer, ^{
        task();
        
        if (!repeats) { // 不重复的任务
            [self cancelTask:name];
        }
    });
    
    // 启动定时器
    dispatch_resume(timer);
    
    return name;
}

+ (NSString *)execTask:(id)target selector:(SEL)selector start:(NSTimeInterval)start interval:(NSTimeInterval)interval repeats:(BOOL)repeats async:(BOOL)async
{
    if (!target || !selector) return nil;
    
    return [self execTask:^{
        if ([target respondsToSelector:selector]) {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
            [target performSelector:selector];
#pragma clang diagnostic pop
        }
    } start:start interval:interval repeats:repeats async:async];
}

+ (void)cancelTask:(NSString *)name
{
    if (name.length == 0) return;
    
    dispatch_semaphore_wait(semaphore_, DISPATCH_TIME_FOREVER);
    
    dispatch_source_t timer = timers_[name];
    if (timer) {
        dispatch_source_cancel(timer);
        [timers_ removeObjectForKey:name];
    }

    dispatch_semaphore_signal(semaphore_);
}

@end