面试题引发的思考：

Q: 使用CADisplayLink、NSTimer是否准时？

• CADisplayLink、NSTimer依赖于RunLoop，而事件响应、手势识别、界面刷新等也是依赖于RunLoop实现的。
• 如果RunLoop的任务过于繁重，可能会导致CADisplayLink、NSTimer不准时。

Q: 如何解决CADisplayLink、NSTimer不准时的问题呢？

• 使用GCD的定时器会更加准时。

1. GCD定时器使用：

// TODO: -----------------  ViewController类  -----------------
@interface ViewController ()
// 强引用保留dispatch_source_t对象
@property (nonatomic, strong) dispatch_source_t timer;
@end

@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];

    // 创建队列 - 主队类：timer在主线程执行
    dispatch_queue_t queue = dispatch_get_main_queue();
    // 创建队列 - 非主队列：timer在子线程执行
    //dispatch_queue_t queue = dispatch_queue_create("timer", DISPATCH_QUEUE_SERIAL);

    // 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_NOW, 1.0 * NSEC_PER_SEC, 0 * NSEC_PER_SEC);

    // 设置回调
    dispatch_source_set_event_handler(timer, ^{
        NSLog(@"timer - %@", [NSThread currentThread]);
    });

    // 启动定时器
    dispatch_resume(timer);

    self.timer = timer;
}
@end

2. GCD定时器封装：

下面对GCD定时器进行封装，以适用大多数场景：

// TODO: -----------------  MYTimer.h  -----------------
@interface MYTimer : NSObject

/// 创建定时器
/// @param task 任务block
/// @param start 开始时间
/// @param interval 时间间隔
/// @param repeats 是否重复
/// @param async 是否异步
+ (NSString *)executeTask:(void(^)(void))task
                    start:(NSTimeInterval)start
                 interval:(NSTimeInterval)interval
                  repeats:(BOOL)repeats
                    async:(BOOL)async;

/// 创建定时器
/// @param target <#target description#>
/// @param selector <#selector description#>
/// @param start 开始时间
/// @param interval 时间间隔
/// @param repeats 是否重复
/// @param async 是否异步
+ (NSString *)executeTask:(id)target
                 selector:(SEL)selector
                    start:(NSTimeInterval)start
                 interval:(NSTimeInterval)interval
                  repeats:(BOOL)repeats
                    async:(BOOL)async;

/// 取消定时器
/// @param name 任务名
+ (void)cancelTask:(NSString *)name;

@end

// TODO: -----------------  MYTimer.m  -----------------
@implementation MYTimer

// 存储定时器标识
static NSMutableDictionary *timers_;
// GCD信号量解决线程同步问题
dispatch_semaphore_t semaphore_;

// 初始化
+ (void)initialize {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        timers_ = [NSMutableDictionary dictionary];
        // 创建信号量
        semaphore_ = dispatch_semaphore_create(1);
    });
}

+ (NSString *)executeTask:(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_NOW, interval * NSEC_PER_SEC, start * NSEC_PER_SEC);

    // 等待信号量
    dispatch_semaphore_wait(semaphore_, DISPATCH_TIME_FOREVER);

    // 定时器的唯一标识
    NSString *name = [NSString stringWithFormat:@"%zd", timers_.count];
    // 将定时器存放到字典中
    timers_[name] = timer;

    // 发送信号量
    dispatch_semaphore_signal(semaphore_);

    // 设置回调
    dispatch_source_set_event_handler(timer, ^{
        // 执行任务
        task();

        if (!repeats) {
            // 取消不重复的任务
            [self cancelTask:name];
        }
    });
    
    // 启动定时器
    dispatch_resume(timer);
    
    return name;
}

+ (NSString *)executeTask:(id)target
                 selector:(SEL)selector
                    start:(NSTimeInterval)start
                 interval:(NSTimeInterval)interval
                  repeats:(BOOL)repeats
                    async:(BOOL)async
{
    if (!target || !selector) return nil;
    
    return [self executeTask:^{
        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;

    // 等待信号量
    dispatch_semaphore_wait(semaphore_, DISPATCH_TIME_FOREVER);
    
    dispatch_source_t timer = timers_[name];
    if (timer) {
        dispatch_source_cancel(timer);
        [timers_ removeObjectForKey:name];
    }
    
    // 发送信号量
    dispatch_semaphore_signal(semaphore_);
}
@end

那么MYTimer调用方法如下：

// TODO: -----------------  ViewController类  -----------------
@interface ViewController ()
@property (nonatomic, copy) NSString *task;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    self.task = [MYTimer executeTask:^{
        NSLog(@"timer - %@", [NSThread currentThread]);
    } start:2.0 interval:1.0 repeats:YES async:YES];
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    [MYTimer cancelTask:self.task];
}
@end