系统的代理方法

@interface AFURLSessionManagerTaskDelegate : NSObject <NSURLSessionTaskDelegate, NSURLSessionDataDelegate, NSURLSessionDownloadDelegate>

KVO

- (instancetype)initWithTask:(NSURLSessionTask *)task {
    self = [super init];
    if (!self) {
        return nil;
    }
    
    _mutableData = [NSMutableData data];
    _uploadProgress = [[NSProgress alloc] initWithParent:nil userInfo:nil];
    _downloadProgress = [[NSProgress alloc] initWithParent:nil userInfo:nil];
    
    __weak __typeof__(task) weakTask = task;
    for (NSProgress *progress in @[ _uploadProgress, _downloadProgress ])
    {
        progress.totalUnitCount = NSURLSessionTransferSizeUnknown;
        progress.cancellable = YES;
        progress.cancellationHandler = ^{
            [weakTask cancel];
        };
        progress.pausable = YES;
        progress.pausingHandler = ^{
            [weakTask suspend];
        };
        if ([progress respondsToSelector:@selector(setResumingHandler:)]) {
            progress.resumingHandler = ^{
                [weakTask resume];
            };
        }
        [progress addObserver:self
                   forKeyPath:NSStringFromSelector(@selector(fractionCompleted))
                      options:NSKeyValueObservingOptionNew
                      context:NULL];
...