GCD常用方法

//同步: 不会开启新线程(特点:阻塞当前线程)
//异步: 具备开辟新线程的能力,但是并不一定开辟新线程(特点:不会阻塞当前线程)

//串行: 先进先出,必须完成上一个任务,才能执行下一个任务
//并发:

//同步和异步: 针对的是线程
//串行和并发: 针对的是队列

//多线程,会消耗CPU的资源: 一条子线程占512kb的内存,主线程占1M内存
//多线程原理: CPU在不同线程之间快速切换

//任务: block代码块
//死锁: 2个任务相互等待.你等我执行完,我等你执行完

//同步并发
- (void)syncConcurrent {
    NSLog(@"begin");
    dispatch_queue_t queue = dispatch_queue_create("com.gcd", DISPATCH_QUEUE_CONCURRENT);
    dispatch_sync(queue, ^{
        NSLog(@"任务1, %@", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        NSLog(@"任务2, %@", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        NSLog(@"任务3, %@", [NSThread currentThread]);
    });
    NSLog(@"end");
}

//异步并发
- (void)asyncConcurrent {
    NSLog(@"begin");
    dispatch_queue_t queue = dispatch_queue_create("com.gcd", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        NSLog(@"任务1, %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"任务2, %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"任务3, %@", [NSThread currentThread]);
    });
    NSLog(@"end");
}

//异步串行
- (void)asyncSerial {
    NSLog(@"begin");
    dispatch_queue_t queue = dispatch_queue_create("com.gcd", DISPATCH_QUEUE_SERIAL);
    dispatch_async(queue, ^{
        NSLog(@"任务1, %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"任务2, %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"任务3, %@", [NSThread currentThread]);
    });
    NSLog(@"end");
}

//同步串行
- (void)syncSerial {
    NSLog(@"begin");
    dispatch_queue_t queue = dispatch_queue_create("com.gcd", DISPATCH_QUEUE_SERIAL);
    dispatch_sync(queue, ^{
        NSLog(@"任务1, %@", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        NSLog(@"任务2, %@", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        NSLog(@"任务3, %@", [NSThread currentThread]);
    });
    NSLog(@"end");
}

//同步并发嵌套(同步)
- (void)syncConcurrent_sync {
    NSLog(@"begin");
    dispatch_queue_t queue = dispatch_queue_create("com.gcd", DISPATCH_QUEUE_CONCURRENT);
    dispatch_sync(queue, ^{
        NSLog(@"任务1, %@", [NSThread currentThread]);
        dispatch_async(queue, ^{
            NSLog(@"任务2, %@", [NSThread currentThread]);
        });
        NSLog(@"任务3, %@", [NSThread currentThread]);
    });
    NSLog(@"end");
}

//同步并发嵌套(异步)
- (void)syncConcurrent_async {
    NSLog(@"begin");
    dispatch_queue_t queue = dispatch_queue_create("com.gcd", DISPATCH_QUEUE_CONCURRENT);
    dispatch_sync(queue, ^{
        NSLog(@"任务1, %@", [NSThread currentThread]);
        dispatch_async(queue, ^{
            NSLog(@"任务2, %@", [NSThread currentThread]);
        });
        NSLog(@"任务3, %@", [NSThread currentThread]);
    });
    NSLog(@"end");
}

//同步串行嵌套(异步)
- (void)syncSerial_async {
    NSLog(@"begin");
    dispatch_queue_t queue = dispatch_queue_create("com.gcd", DISPATCH_QUEUE_SERIAL);
    dispatch_sync(queue, ^{
        NSLog(@"任务1, %@", [NSThread currentThread]);
        dispatch_async(queue, ^{
            NSLog(@"任务2, %@", [NSThread currentThread]);
        });
        NSLog(@"任务3, %@", [NSThread currentThread]);
    });
    NSLog(@"end");
}

//异步串行嵌套(异步)
- (void)asyncSerial_async {
    NSLog(@"begin");
    dispatch_queue_t queue = dispatch_queue_create("com.gcd", DISPATCH_QUEUE_SERIAL);
    dispatch_async(queue, ^{
        NSLog(@"任务1, %@", [NSThread currentThread]);
        dispatch_async(queue, ^{
            NSLog(@"任务2, %@", [NSThread currentThread]);
        });
        NSLog(@"任务3, %@", [NSThread currentThread]);
    });
    NSLog(@"end");
}

//异步串行嵌套(同步)--->死锁
- (void)asyncSerial_sync {
    NSLog(@"begin");
    dispatch_queue_t queue = dispatch_queue_create("com.gcd", DISPATCH_QUEUE_SERIAL);
    dispatch_async(queue, ^{//block1
        NSLog(@"任务1, %@", [NSThread currentThread]);
        dispatch_sync(queue, ^{//block2
            NSLog(@"任务2, %@", [NSThread currentThread]);
        });
        NSLog(@"任务3, %@", [NSThread currentThread]);
    });
    NSLog(@"end");
    //block1和block2两个任务死锁
}

//1、创建主线程（串行）
dispatch_async(dispatch_get_main_queue(), ^{
    //刷新界面代码
});

    //2、创建异步线程（并行）
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        //比较耗时的代码放这里
    });

    //3、gcd延迟
    double delayInSeconds = 1.0;
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
    dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
        //延迟代码
    });

    //4、gcd只执行一次
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        //只执行一次代码
    });

    //5、有三个任务，需要异步并发执行前两个任务，前两个任务执行完成后再执行第三个任务。
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        
        //创建组
        dispatch_group_t group=dispatch_group_create();
        
        // 关联一个任务到group
        dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            //任务一
            NSLog(@"******执行任务一******");
        });
        
        // 关联一个任务到group
        dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            //任务二
            NSLog(@"******执行任务二******");
        });
        
        // 等待组中的任务执行完毕,回到主线程执行block回调
        dispatch_group_notify(group, dispatch_get_main_queue(), ^{
            //任务三
            NSLog(@"******等待组中的任务执行完毕,回到主线程执行block回调,执行任务三******");
        });
        
    });

 //6、dispatch_barrier_async的使用，dispatch_barrier_async是在前面的任务执行结束后它才执行，而且它后面的任务等它执行完成之后才会执行
    dispatch_queue_t queue = dispatch_queue_create("create_asy_queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        NSLog(@"dispatch_async1");
    });
    dispatch_async(queue, ^{
        NSLog(@"dispatch_async2");
    });
    dispatch_barrier_async(queue, ^{
        NSLog(@"dispatch_barrier_async");
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"刷新界面");
        });
        
    });
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:1];
        NSLog(@"dispatch_async3");
    });

    /*7、GCD的另一个用处是可以让程序在后台较长久的运行。
    在没有使用GCD时，当app被按home键退出后，app仅有最多5秒钟的时候做一些保存或清理资源的工作。但是在使用GCD后，app最多有10分钟的时间在后台长久运行。这个时间可以用来做清理本地缓存，发送统计数据等工作。
    让程序在后台长久运行的示例代码如下：
    */
    // AppDelegate.h文件
    @property (assign, nonatomic) UIBackgroundTaskIdentifier backgroundUpdateTask;
    
    // AppDelegate.m文件
    - (void)applicationDidEnterBackground:(UIApplication *)application
    {
        [self beingBackgroundUpdateTask];
        // 在这里加上你需要长久运行的代码
        [self endBackgroundUpdateTask];
    }
    
    - (void)beingBackgroundUpdateTask
    {
        self.backgroundUpdateTask = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
            [self endBackgroundUpdateTask];
        }];
    }
    
    - (void)endBackgroundUpdateTask
    {
        [[UIApplication sharedApplication] endBackgroundTask: self.backgroundUpdateTask];
        self.backgroundUpdateTask = UIBackgroundTaskInvalid;
    }