AFNetworking的緩存使用

+ (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;
}

磁盤緩存

- (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;
}

內存緩存

 

圖片緩存策略（個人理解）：

圖片設置路徑->從內存字典中查找緩存的image對象->調用網絡請求->根據NSURLRequst的策略是否只讀緩存->不是只讀緩存則開啟下載操作->如果該下載已經存在則不新建下載而是只把代理（保存了成功失敗的操作和對象信息）打包到管理代理數組，不存在同時會新建下載請求->下載請求管理對象綁定了一個磁盤與內存緩存對象NNSURLCanche也就是在下載的時候優先會去獨本地緩存，下載成功之后會默認保存到本地緩存->如果是歷史下載請求且本地存在文件則直接全部數據立刻全部返回，否則按下載進度依次返回->觸發下載結束代理->查詢代理緩存數組執行與下載相關的代理內描述的操作