GCD

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/**
 *  async -- 并发队列（最常用）
 *  会不会创建线程：会，一般同时开多条
 *  任务的执行方式：并发执行
 */
- (void)asyncGlobalQueue
{
    // 获得全局的并发队列
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    // 将 任务 添加 全局队列 中去 异步 执行
    dispatch_async(queue, ^{
        NSLog(@"-----下载图片1---%@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"-----下载图片2---%@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"-----下载图片3---%@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"-----下载图片4---%@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"-----下载图片5---%@", [NSThread currentThread]);
    });
}

/**
 *  async -- 串行队列（有时候用）
 *  会不会创建线程：会，一般只开1条线程
 *  任务的执行方式：串行执行（一个任务执行完毕后再执行下一个任务）
 */
- (void)asyncSerialQueue
{
    // 1.创建一个串行队列
    dispatch_queue_t queue = dispatch_queue_create("唯一的队列名称", NULL);

    // 2.将任务添加到串行队列中 异步 执行
    dispatch_async(queue, ^{
        NSLog(@"-----下载图片1---%@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"-----下载图片2---%@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"-----下载图片3---%@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"-----下载图片4---%@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"-----下载图片5---%@", [NSThread currentThread]);
    });

    // 3.非ARC，需要释放创建的队列
//    dispatch_release(queue);
}

/**
 *  async -- 主队列(很常用)
 */
- (void)asyncMainQueue
{
    // 1.主队列(添加到主队列中的任务，都会自动放到主线程中去执行)
    dispatch_queue_t queue = dispatch_get_main_queue();

    // 2.添加 任务 到主队列中 异步 执行
    dispatch_async(queue, ^{
        NSLog(@"-----下载图片1---%@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"-----下载图片2---%@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"-----下载图片3---%@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"-----下载图片4---%@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"-----下载图片5---%@", [NSThread currentThread]);
    });
}

/*
 凡是函数名中带有create\copy\new\retain等字眼, 都应该在不需要使用这个数据的时候进行release
 GCD的数据类型在ARC环境下不需要再做release
 CF(Core Foundation)的数据类型在ARC\MRC环境下都需要再做release
 */

OC中对象的属性不能以new开头

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线程间通信

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"donwload---%@", [NSThread currentThread]);
        // 1.子线程下载图片
        NSURL *url = [NSURL URLWithString:@"http://d.hiphotos.baidu.com/image/pic/item/37d3d539b6003af3290eaf5d362ac65c1038b652.jpg"];
        NSData *data = [NSData dataWithContentsOfURL:url];
        UIImage *image = [UIImage imageWithData:data];

        // 2.回到主线程设置图片
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"setting---%@ %@", [NSThread currentThread], image);
            [self.button setImage:image forState:UIControlStateNormal];
        });
    });
}

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延时执行

- (void)download:(NSString *)url
{
    NSLog(@"download------%@---%@", url, [NSThread currentThread]);
}

- (void)delay3
{
    // 3秒后回到主线程执行block中的代码
//    dispatch_queue_t queue = dispatch_get_main_queue();
//    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), queue, ^{
//        NSLog(@"------task------%@", [NSThread currentThread]);
//    });

    // 3秒后自动开启新线程 执行block中的代码
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), queue, ^{
        NSLog(@"------task------%@", [NSThread currentThread]);
    });
}

- (void)delay2
{
    // 一旦定制好延迟任务后，不会卡主当前线程(如果当前线程是主线程，则3秒后在主线程执行)
    [self performSelector:@selector(download:) withObject:@"http://555.jpg" afterDelay:3];
}

- (void)delay1
{
    // 延迟执行不要用sleep，坏处：卡住当前线程
    [NSThread sleepForTimeInterval:3];
    NSLog(@"-----下载图片-----");
}

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一次性代码

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    NSLog(@"----touchesBegan");
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSLog(@"----once");
        HMImageDownloader *downloader = [[HMImageDownloader alloc] init];
        [downloader download];
    });
}

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队列组

会阻塞的情况

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    // 1.队列组
    dispatch_group_t group = dispatch_group_create();
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    // 2.下载图片1
    __block UIImage *image1 = nil;
    dispatch_group_async(group, queue, ^{
        NSLog(@"下载图片1开始");
        NSURL *url1 = [NSURL URLWithString:@"http://7xk9dj.com1.z0.glb.clouddn.com/refreshlayout%2Fimages%2Fstaggered24.png"];
        NSData *data1 = [NSData dataWithContentsOfURL:url1];
        image1 = [UIImage imageWithData:data1];
        NSLog(@"下载图片1结束");
    });

    // 3.下载图片2
    __block UIImage *image2 = nil;
    dispatch_group_async(group, queue, ^{
        NSLog(@"下载图片2开始");
        NSURL *url2 = [NSURL URLWithString:@"http://7xk9dj.com1.z0.glb.clouddn.com/refreshlayout%2Fimages%2Fstaggered35.png"];
        NSData *data2 = [NSData dataWithContentsOfURL:url2];
        image2 = [UIImage imageWithData:data2];
        NSLog(@"下载图片2结束");
    });

    // 4.合并图片 (保证执行完组里面的所有任务之后，再执行notify函数里面的block)
    dispatch_group_notify(group, queue, ^{
        NSLog(@"开始合并图片");
        // 开启一个位图上下文
        UIGraphicsBeginImageContextWithOptions(image1.size, NO, 0.0);

        // 绘制第1张图片
        CGFloat image1W = image1.size.width;
        CGFloat image1H = image1.size.height;
        [image1 drawInRect:CGRectMake(0, 0, image1W, image1H)];

        // 绘制第2张图片
        CGFloat image2W = image2.size.width * 0.5;
        CGFloat image2H = image2.size.height * 0.5;
        CGFloat image2Y = image1H - image2H;
        [image2 drawInRect:CGRectMake(0, image2Y, image2W, image2H)];

        // 得到上下文中的图片
        UIImage *fullImage = UIGraphicsGetImageFromCurrentImageContext();

        // 结束上下文
        UIGraphicsEndImageContext();
        NSLog(@"图片合并结束");
        // 5.回到主线程显示图片
        dispatch_async(dispatch_get_main_queue(), ^{
            self.imageView.image = fullImage;
        });
    });
}

不会阻塞通过block返回的情况

-(void)fetchConfigurationWithCompletion:(void (^)(NSError* error))completion
{
    // Define errors to be processed when everything is complete.
    // One error per service; in this example we'll have two 
    __block NSError *configError = nil;
    __block NSError *preferenceError = nil;

    // Create the dispatch group
    dispatch_group_t serviceGroup = dispatch_group_create();

    // Start the first service
    dispatch_group_enter(serviceGroup);
    [self.configService startWithCompletion:^(ConfigResponse *results, NSError* error){
        // Do something with the results
        configError = error;
        dispatch_group_leave(serviceGroup);
    }];

    // Start the second service
    dispatch_group_enter(serviceGroup);
    [self.preferenceService startWithCompletion:^(PreferenceResponse *results, NSError* error){
        // Do something with the results
        preferenceError = error;
        dispatch_group_leave(serviceGroup);
    }];

    dispatch_group_notify(serviceGroup,dispatch_get_main_queue(),^{
        // Assess any errors
        NSError *overallError = nil;
        if (configError || preferenceError)
        {
            // Either make a new error or assign one of them to the overall error
            overallError = configError ?: preferenceError;
        }
        // Now call the final completion block
        completion(overallError);
    });
}

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Swift延时执行

let delayTime = dispatch_time(DISPATCH_TIME_NOW, Int64(1 * Double(NSEC_PER_SEC)))
dispatch_after(delayTime, dispatch_get_main_queue()) {
    print("test")
}