ios 关联

 

+ (id <AFImageCache>)sharedImageCache {
    static AFImageCache *_af_defaultImageCache = nil;
    static dispatch_once_t oncePredicate;
    dispatch_once(&oncePredicate, ^{
        _af_defaultImageCache = [[AFImageCache alloc] init];

        [[NSNotificationCenter defaultCenter] addObserverForName:UIApplicationDidReceiveMemoryWarningNotification object:nil queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification * __unused notification) {
            [_af_defaultImageCache removeAllObjects];
        }];
    });

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wgnu"
    return objc_getAssociatedObject(self, @selector(sharedImageCache)) ?: _af_defaultImageCache;
#pragma clang diagnostic pop
}

//设置关联，降imageCache关联到sharedImageCache函数，接下来制定关联策略
+ (void)setSharedImageCache:(id<AFImageCache>)imageCache {
    objc_setAssociatedObject(self, @selector(sharedImageCache), imageCache, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

 以上代码来自AFNetwork源码，大致意思是通过AFNetWork下载图片的时候，我们可以指定缓存策略，如果不指定缓存策略，那么使用AFNetWork提供的默认的缓存策略。