Swift90Days – GCD 1

最近比较忙啦，就贴一下自己学习时候的关键代码，以后有机会再补上讲述的内容。

dispatch_async

看代码说话：

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
    UIImage *overlayImage = [self faceOverlayImageFromImage:_image];
    dispatch_async(dispatch_get_main_queue(), ^{
        [self fadeInNewImage:overlayImage];
    });
});

上面的代码可以发送一个自定义的异步请求，然后通过 dispatch_get_main_queue 在主线程里刷新UI。

dispatch_after

比如我们想在1秒之后弹出提示框，可以这样：

NSUInteger count = [[PhotoManager sharedManager] photos].count;
double delayInSeconds = 1.0;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC)); // 1 
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){ // 2 
    if (!count) {
        [self.navigationItem setPrompt:@"Add photos with faces to Googlyify them!"];
    } else {
        [self.navigationItem setPrompt:nil];
    }
});

dispatch_once

我们可以通过 dispatch_once 实现单例模式：

+ (instancetype)sharedManager
{
    static PhotoManager *sharedPhotoManager = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedPhotoManager = [[PhotoManager alloc] init];
        sharedPhotoManager->_photosArray = [NSMutableArray array];
    });
    return sharedPhotoManager;
}

References

• Grand Central Dispatch In-Depth: Part 1/2