iOS中GCD定时器

前言：CADisplayLink、NSTimer 不准时

​ CADisplayLink、NSTimer是基于RunLoop机制的，如果RunLoop的任务过于繁重，有可能会导致前两个定时器不准时。

举个例子：

​ 加入我们创建了一个NSTimer定时器，每1秒钟做任务。那么，什么时候执行NSTimer呢？
​ 是在RunLoop跑圈的过程中执行NSTimer定时器，而RunLoop跑完一圈执行的时间不固定，也就导致有可能1秒钟过去了，但是RunLoop还没有执行到定时器的任务，那么，这就造成定时器有可能不准时。

一、GCD 定时器

​ GCD是不依赖与RunLoop，是直接跟系统内核交互的。时间比较准确。

GCD 定时器简单的使用：

- (void)viewDidLoad {
    [super viewDidLoad];
    NSLog(@"begin");    
    // 队列
    dispatch_queue_t queue = 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
    uint64_t start = 2.0;
    uint64_t interval = 1.0;
    dispatch_source_set_timer(timer,
                              dispatch_time(DISPATCH_TIME_NOW, start * NSEC_PER_SEC),
                              interval * NSEC_PER_SEC, 0);
    
    // 设置回调
    dispatch_source_set_event_handler(timer, ^{
        NSLog(@"111");
    });
    // 启动定时器
    dispatch_resume(timer);
    
    self.timer = timer;
}

2022-07-05 17:42:46.674345+0800 Interview02-GCD定时器[13943:350556] begin
2022-07-05 17:42:48.675440+0800 Interview02-GCD定时器[13943:350556] 111
2022-07-05 17:42:49.675542+0800 Interview02-GCD定时器[13943:350556] 111
2022-07-05 17:42:50.675350+0800 Interview02-GCD定时器[13943:350556] 111
2022-07-05 17:42:51.674523+0800 Interview02-GCD定时器[13943:350556] 111

二、GCD 定时器的实现方案

第一步：封装

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface RHGCDTimer : NSObject

+ (NSString *)timerWithBlockTask:(void(^)(void))blockTask
                      star:(float)star
                  interval:(float)interval
                    repeat:(BOOL)repeat
                     async:(BOOL)async;

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

@end

NS_ASSUME_NONNULL_END

#import "RHGCDTimer.h"

static NSMutableDictionary *timersDict;
static dispatch_semaphore_t semaphore;

@implementation RHGCDTimer

+ (void)initialize
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        timersDict = [NSMutableDictionary dictionary];
        semaphore = dispatch_semaphore_create(1);//创建一个信号量，只允许一个线程操作
    });
}

+ (NSString *)timerWithBlockTask:(void (^)(void))blockTask star:(float)star interval:(float)interval repeat:(BOOL)repeat async:(BOOL)async
{
    
    if (!blockTask || star<0 || (repeat && interval <= 0)) return nil;
    
    //创建队列，队列决定到时候任务是在哪个线程执行
    dispatch_queue_t queue = async ? dispatch_queue_create("myQueue", DISPATCH_QUEUE_SERIAL) : dispatch_get_main_queue();

    //创建一个定时器
    dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    /**
    dispatch_source_set_timer 上面的定时器
    dispatch_time_t start 开始时间  (typedef uint64_t dispatch_time_t;)
    uint64_t interval 间隔
    uint64_t leeway 误差一般写0
    */
    dispatch_source_set_timer(timer, dispatch_time(DISPATCH_TIME_NOW, star * NSEC_PER_SEC), interval *NSEC_PER_SEC, 0);

    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);//信号量
    //定时器唯一标识
    static int i = 0;
    NSString *name = [NSString stringWithFormat:@"%d", i++];
    
    //放进字典，就会产生强引用
    timersDict[name] = timer;
    dispatch_semaphore_signal(semaphore);
    
    //设置回调
    dispatch_source_set_event_handler(timer, ^{
        blockTask();
        if (!repeat) {//如果非重复执行
            [self cancelTask:name];//取消定时器
        }
    });
    //启动定时器
    dispatch_resume(timer);
    //GCD不需要销毁
    return name;
}

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

@end

第二步：使用

#import "ViewController.h"

#import "RHGCDTimer.h"

@interface ViewController ()

@property (copy, nonatomic) NSString *task;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.task = [RHGCDTimer timerWithBlockTask:^{
        NSLog(@"执行任务---%@", [NSThread currentThread]);
    } star:2.0 interval:1.0 repeat:YES async:YES];
}

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

@end

第三步：测试验证

2022-07-05 17:31:41.375918+0800 Interview02-GCD定时器[13519:337316] 执行任务---<_NSMainThread: 0x600002448880>{number = 1, name = main}
2022-07-05 17:31:42.375935+0800 Interview02-GCD定时器[13519:337316] 执行任务---<_NSMainThread: 0x600002448880>{number = 1, name = main}
2022-07-05 17:31:43.375871+0800 Interview02-GCD定时器[13519:337316] 执行任务---<_NSMainThread: 0x600002448880>{number = 1, name = main}