iOS学习笔记-108.多线程07——CGD栅栏函数、延时、一次性代码

时间：2021-01-10 08:10:16

多线程07——CGD栅栏函数、延时、一次性代码

一、说明

1.1 栅栏函数说明

dispatch_barrier_async(dispatch_queue_t queue, dispatch_block_t block);

在前面的任务执行结束后它才执行，除此之外只有它本身执行完了，它后面的任才能被执行

示例代码

 dispatch_barrier_async(queue, ^{
NSLog(@"++++++++++++++++++++++++++++++");
 });

1.2 延时执行

dispatch_after(dispatch_time_t when,
    dispatch_queue_t queue,
    dispatch_block_t block);

可以使用已经内置的代码模板

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(<delayInSeconds> * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        <code to be executed after a specified delay>
    });

使用说明

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
//2秒钟后执行的代码。。。。。。
});

1.3 一次性代码

使用 dispatch_once 函数能保证某段代码在程序运行过程中只被执行1次

static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
// 只执行1次的代码(这里面默认是线程安全的)
});

二、栅栏函数

2.1 说明

我们创建四个任务。在栅栏函数前面创建两个 ，栅栏函数后面创建两个。我们得到的结果应该是：

  1. 先执行前面两个任务，执行完了到第2步
  2. 执行栅栏函数，执行完了到第3步
  3. 执行后面的两个任务

2.2 代码

/*栅栏函数*/
-(void)barrier{
dispatch_queue_t queue = dispatch_queue_create("com.wiming", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(queue, ^{
for (int i=0; i<5; i++) {
NSLog(@"download1--%zd---%@",i,[NSThread currentThread]);
        }
    });
dispatch_async(queue, ^{
for (int i=0; i<5; i++) {
NSLog(@"download2--%zd---%@",i,[NSThread currentThread]);
        }
    });
    dispatch_barrier_async(queue, ^{
NSLog(@"++++++++++++++++++++++++++++++");
    });
dispatch_async(queue, ^{
for (int i=0; i<5; i++) {
NSLog(@"download3--%zd---%@",i,[NSThread currentThread]);
        }
    });
dispatch_async(queue, ^{
for (int i=0; i<5; i++) {
NSLog(@"download4--%zd---%@",i,[NSThread currentThread]);
        }
    });
}

2.3 结果

[72846:415833] download1--0---<NSThread: 0x600000076b00>{number = 4, name = (null)}
[72846:415834] download2--0---<NSThread: 0x60800007d900>{number = 3, name = (null)}
[72846:415833] download1--1---<NSThread: 0x600000076b00>{number = 4, name = (null)}
[72846:415834] download2--1---<NSThread: 0x60800007d900>{number = 3, name = (null)}
[72846:415833] download1--2---<NSThread: 0x600000076b00>{number = 4, name = (null)}
[72846:415834] download2--2---<NSThread: 0x60800007d900>{number = 3, name = (null)}
[72846:415833] download1--3---<NSThread: 0x600000076b00>{number = 4, name = (null)}
[72846:415834] download2--3---<NSThread: 0x60800007d900>{number = 3, name = (null)}
[72846:415833] download1--4---<NSThread: 0x600000076b00>{number = 4, name = (null)}
[72846:415834] download2--4---<NSThread: 0x60800007d900>{number = 3, name = (null)}
[72846:415834] ++++++++++++++++++++++++++++++
[72846:415834] download3--0---<NSThread: 0x60800007d900>{number = 3, name = (null)}
[72846:415833] download4--0---<NSThread: 0x600000076b00>{number = 4, name = (null)}
[72846:415834] download3--1---<NSThread: 0x60800007d900>{number = 3, name = (null)}
[72846:415833] download4--1---<NSThread: 0x600000076b00>{number = 4, name = (null)}
[72846:415834] download3--2---<NSThread: 0x60800007d900>{number = 3, name = (null)}
[72846:415833] download4--2---<NSThread: 0x600000076b00>{number = 4, name = (null)}
[72846:415834] download3--3---<NSThread: 0x60800007d900>{number = 3, name = (null)}
[72846:415833] download4--3---<NSThread: 0x600000076b00>{number = 4, name = (null)}
[72846:415834] download3--4---<NSThread: 0x60800007d900>{number = 3, name = (null)}
[72846:415833] download4--4---<NSThread: 0x600000076b00>{number = 4, name = (null)}

2.4 结果分析

通过上面的结果我们可以看到

任务1和任务2（download1和download2），首先执行，它们执行完了 才执行 栅栏函数里面的代码，栅栏函数里面的代码执行完了 ，才开始执行任务3和任务4（download3和download4）

三、延时执行

3.1. 调用NSObject的方法

//第一个参数：方法选择器，就是时间到了以后要执行的方法
//第二个参数：方法的参数
//第三个参数：延时的时长
- (void)performSelector:(SEL)aSelector withObject:(nullable id)anArgument afterDelay:(NSTimeInterval)delay;

示例代码

/*延时*/
-(void)delay{
NSLog(@"start------");
//第一种方式
    [self performSelector:@selector(task) withObject:nil afterDelay:2.0];
}

-(void)task{
NSLog(@"task-------%@",[NSThread currentThread]);
}

结果

2017-09-03 01:48:42.194 03_UIview83多线程GCD[74790:425940] start------
2017-09-03 01:48:44.196 03_UIview83多线程GCD[74790:425940] task-------<NSThread: 0x608000068dc0>{number = 1, name = main}

我们可以看一下时间，确实是2s

01:48:42
01:48:44

3.2 使用NSTime

使用的方式是

+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)ti target:(id)aTarget selector:(SEL)aSelector userInfo:(nullable id)userInfo repeats:(BOOL)yesOrNo;

示例代码

/*延时*/
-(void)delay{
NSLog(@"start------");    
//第二种方式
    [NSTimer scheduledTimerWithTimeInterval:2.0 target:self selector:@selector(task) userInfo:nil repeats:NO];
}

-(void)task{
NSLog(@"task-------%@",[NSThread currentThread]);
}

结果

2017-09-03 03:13:55.639 03_UIview83多线程GCD[77570:439579] start------
2017-09-03 03:13:57.641 03_UIview83多线程GCD[77570:439579] task-------<NSThread: 0x600000074d00>{number = 1, name = main}

3.3 使用GCD函数

示例代码

/*延时*/
-(void)delay{
NSLog(@"start------");
//GCD
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
NSLog(@"GCD-------%@",[NSThread currentThread]);
    });
}

结果

2017-09-03 03:15:57.027 03_UIview83多线程GCD[78003:442676] start------
2017-09-03 03:15:59.218 03_UIview83多线程GCD[78003:442676] GCD-------<NSThread: 0x608000066cc0>{number = 1, name = main}

四、一次性代码

示例代码

/*一次性代码*/
-(void)once{
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
NSLog(@"------one------");
    });
}

结果 程序运行以后，无论操作多少次都只会执行一次

2017-09-03 03:19:17.991 03_UIview83多线程GCD[78649:447665] ------one------
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