AFNetworking的缓存使用

+ (NSURLCache *)defaultURLCache {
    // It's been discovered that a crash will occur on certain versions
    // of iOS if you customize the cache.
    //
    // More info can be found here: https://devforums.apple.com/message/1102182#1102182
    //
    // When iOS 7 support is dropped, this should be modified to use
    // NSProcessInfo methods instead.
    if ([[[UIDevice currentDevice] systemVersion] compare:@"8.2" options:NSNumericSearch] == NSOrderedAscending) {
        return [NSURLCache sharedURLCache];
    }
    return [[NSURLCache alloc] initWithMemoryCapacity:20 * 1024 * 1024
                                         diskCapacity:150 * 1024 * 1024
                                             diskPath:@"com.alamofire.imagedownloader"];
}

+ (NSURLSessionConfiguration *)defaultURLSessionConfiguration {
    NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];

    //TODO set the default HTTP headers

    configuration.HTTPShouldSetCookies = YES;
    configuration.HTTPShouldUsePipelining = NO;

    configuration.requestCachePolicy = NSURLRequestUseProtocolCachePolicy;
    configuration.allowsCellularAccess = YES;
    configuration.timeoutIntervalForRequest = 60.0;
    configuration.URLCache = [AFImageDownloader defaultURLCache];

    return configuration;
}

磁盘缓存

- (instancetype)initWithMemoryCapacity:(UInt64)memoryCapacity preferredMemoryCapacity:(UInt64)preferredMemoryCapacity {
    if (self = [super init]) {
        self.memoryCapacity = memoryCapacity;
        self.preferredMemoryUsageAfterPurge = preferredMemoryCapacity;
        self.cachedImages = [[NSMutableDictionary alloc] init];

        NSString *queueName = [NSString stringWithFormat:@"com.alamofire.autopurgingimagecache-%@", [[NSUUID UUID] UUIDString]];
        self.synchronizationQueue = dispatch_queue_create([queueName cStringUsingEncoding:NSASCIIStringEncoding], DISPATCH_QUEUE_CONCURRENT);

        [[NSNotificationCenter defaultCenter]
         addObserver:self
         selector:@selector(removeAllImages)
         name:UIApplicationDidReceiveMemoryWarningNotification
         object:nil];

    }
    return self;
}

内存缓存

图片缓存策略（个人理解）：

图片设置路径->从内存字典中查找缓存的image对象->调用网络请求->根据NSURLRequst的策略是否只读缓存->不是只读缓存则开启下载操作->如果该下载已经存在则不新建下载而是只把代理（保存了成功失败的操作和对象信息）打包到管理代理数组，不存在同时会新建下载请求->下载请求管理对象绑定了一个磁盘与内存缓存对象NNSURLCanche也就是在下载的时候优先会去独本地缓存，下载成功之后会默认保存到本地缓存->如果是历史下载请求且本地存在文件则直接全部数据立刻全部返回，否则按下载进度依次返回->触发下载结束代理->查询代理缓存数组执行与下载相关的代理内描述的操作