多线程把你头搞大了吗?

多线程有好多实现方式，我先掌握一种就行了。
GCD并非想象中的那么难，只是不敢尝试，不知道应用在何处。
今天看了慕课上‘不死鸟fj’讲解的GCD，才算了解了一些基本运用。
下面我总结一下我的学习成果。（我会使用 [NSThread sleepForTimeInterval:3]模拟耗时操作）

1.最简单的使用

- (void)onGCD:(UIButton *)btn{
    
    NSLog(@"执行GCD");
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
       
        NSLog(@"执行耗时任务");
        [NSThread sleepForTimeInterval:3];
        dispatch_async(dispatch_get_main_queue(), ^{
           
            NSLog(@"回到主线程,刷新UI");
            [btn setTitle:@"ok" forState:UIControlStateNormal];
        });
        
    });
}

2.并发（也就是多几个耗时操作一同进行，相当于你做饭的时候听着歌，两不耽误）

DISPATCH_QUEUE_PRIORITY_LOW，DISPATCH_QUEUE_PRIORITY_DEFAULT，DISPATCH_QUEUE_PRIORITY_HIGH参数代表优先级别由低—>高

//并发
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
        
        NSLog(@"start task 1");
        [NSThread sleepForTimeInterval:3];
        NSLog(@"start task 1");
        
    });
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        
        NSLog(@"start task 2");
        [NSThread sleepForTimeInterval:3];
        NSLog(@"start task 2");
        
    });
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        
        NSLog(@"start task 3");
        [NSThread sleepForTimeInterval:3];
        NSLog(@"start task 3");
        
    });

3.并行/串行

参数：
DISPATCH_QUEUE_CONCURRENT--并行
DISPATCH_QUEUE_SERIAL--串行(保证执行顺序，好比接力，1棒跑完，2棒才能跑,适用于任务2必须在任务1完成的情况下才能继续运行)

//并行
dispatch_queue_t quene = dispatch_queue_create("com.task", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(quene, ^{
        
        NSLog(@"start task 1");
        [NSThread sleepForTimeInterval:3];
        NSLog(@"start task 1");
    });
    
    dispatch_async(quene, ^{
        
        NSLog(@"start task 2");
        [NSThread sleepForTimeInterval:3];
        NSLog(@"start task 2");
    });
    
    dispatch_async(quene, ^{
        
        NSLog(@"start task 3");
        [NSThread sleepForTimeInterval:3];
        NSLog(@"start task 3");
    });

4.所有任务完成之后执行下一步(适用场景：多个网络请求完成之后刷新界面)

参数：
"com.task.gcd"：唯一标识
DISPATCH_QUEUE_CONCURRENT/DISPATCH_QUEUE_SERIAL：并行/串行

- (void)onGCD:(UIButton *)btn{
    
    dispatch_queue_t queue = dispatch_queue_create("com.task.gcd", DISPATCH_QUEUE_CONCURRENT);
    
    dispatch_group_t group = dispatch_group_create();
    
    dispatch_group_enter(group);
    [self requestInfo1:^{
        
        NSLog(@"request1 done");
        dispatch_group_leave(group);
    }];
    
    dispatch_group_enter(group);
    [self requestInfo2:^{
        
        NSLog(@"request2 done");
        dispatch_group_leave(group);
    }];
    
    dispatch_group_notify(group, queue, ^{
        
        NSLog(@"全部执行完毕");
        dispatch_async(dispatch_get_main_queue(), ^{
            
            NSLog(@"回到主线程，刷新UI");
        });
    });
}

- (void)requestInfo1:(void(^)())blok{
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
       
        NSLog(@"start task 1");
        [NSThread sleepForTimeInterval:3];
        NSLog(@"start task 1");
        dispatch_async(dispatch_get_main_queue(), ^{
            if (blok) {
                blok();
            }
        });
    });
    
}

- (void)requestInfo2:(void(^)())blok{
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        
        NSLog(@"start task 2");
        [NSThread sleepForTimeInterval:3];
        NSLog(@"start task 2");
        dispatch_async(dispatch_get_main_queue(), ^{
            if (blok) {
                blok();
            }
        });
    });
    
}

5.GCD的其他应用，让代码只运行一次

- (void)onClick:(UIButton *)btn{
    
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSLog(@"我只响应一次");
    });
}