iOS多线程之GCD

简单的使用：

    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"异步进行操作");
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"回到主线程，一般可用来刷新ui等操作");
        });
    });

给异步线程设置优先级：

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
    });

创建自定义串行线程：

    dispatch_queue_t queue = dispatch_queue_create("com.test.gcd.queue", NULL);
    
    dispatch_async(queue, ^{
        NSLog(@"start task 1");
        [NSThread sleepForTimeInterval:2];
        NSLog(@"stop task 1");
    });
    
    dispatch_async(queue, ^{
        NSLog(@"start task 2");
        [NSThread sleepForTimeInterval:2];
        NSLog(@"stop task 2");
    });

创建自定义并行线程:

    dispatch_queue_t queue = dispatch_queue_create("com.test.gcd.queue", DISPATCH_QUEUE_CONCURRENT);
    
    dispatch_async(queue, ^{
        NSLog(@"start task 1");
        [NSThread sleepForTimeInterval:2];
        NSLog(@"stop task 1");
    });
    
    dispatch_async(queue, ^{
        NSLog(@"start task 2");
        [NSThread sleepForTimeInterval:2];
        NSLog(@"stop task 2");
    });

如果需要当所有的异步线程结束之后调用同一个函数，可以使用GCD Group:

    dispatch_queue_t queue = dispatch_queue_create("com.test.gcd.group", DISPATCH_QUEUE_CONCURRENT);
    
    dispatch_group_t group = dispatch_group_create();
    
    dispatch_group_async(group, queue, ^{
        NSLog(@"start task 1");
        [NSThread sleepForTimeInterval:2];
        NSLog(@"stop task 1");
    });
    
    dispatch_group_async(group, queue, ^{
        NSLog(@"start task 2");
        [NSThread sleepForTimeInterval:2];
        NSLog(@"stop task 2");
    });
    
    dispatch_group_notify(group, queue, ^{
        NSLog(@"All task over");
    })

将异步线程加入到GCD group当中（注意：enter和leave函数必须成对出现，且leave函数应该放在异步线程的结束位置）：

    dispatch_group_enter(group);
    [self sendRequest1:^{
        NSLog(@"request1 done");
        dispatch_group_leave(group);
    }];
    
    dispatch_group_enter(group);
    [self sendRequest2:^{
        NSLog(@"request2 done");
        dispatch_group_leave(group);
    }];

    dispatch_group_notify(group, queue, ^{
        NSLog(@"All task over");
    });

    - (void)sendRequest1:(void(^)())block {
        dispatch_async(dispatch_get_global_queue(0, 0), ^{
            NSLog(@"start task 1");
            [NSThread sleepForTimeInterval:2];
            NSLog(@"stop task 1");
            dispatch_async(dispatch_get_main_queue(), ^{
                if (block) {
                    block();
                }
            });
        });
    }

    - (void)sendRequest2:(void(^)())block {
        dispatch_async(dispatch_get_global_queue(0, 0), ^{
            NSLog(@"start task 2");
            [NSThread sleepForTimeInterval:2];
            NSLog(@"stop task 2”);
            dispatch_async(dispatch_get_main_queue(), ^{
                if (block) {
                    block();
                }
            });
        });
    }

单例(只会执行一次)：

    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSLog(@"excute only once");
    });

延迟操作（开始了就不能取消）：

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"delay 2 sec");
    });