AFNetworking 中 GCD 的使用(5)

GCD in AFNetworking 的使用

AFImageDownloader 图片下载

1. 创建队列，串行，并行

- (instancetype)initWithSessionManager:(AFHTTPSessionManager *)sessionManager
                downloadPrioritization:(AFImageDownloadPrioritization)downloadPrioritization
                maximumActiveDownloads:(NSInteger)maximumActiveDownloads
                            imageCache:(id <AFImageRequestCache>)imageCache {
    if (self = [super init]) {
        ...

        NSString *name = [NSString stringWithFormat:@"com.alamofire.imagedownloader.synchronizationqueue-%@", [[NSUUID UUID] UUIDString]];
        self.synchronizationQueue = dispatch_queue_create([name cStringUsingEncoding:NSASCIIStringEncoding], DISPATCH_QUEUE_SERIAL);

        name = [NSString stringWithFormat:@"com.alamofire.imagedownloader.responsequeue-%@", [[NSUUID UUID] UUIDString]];
        self.responseQueue = dispatch_queue_create([name cStringUsingEncoding:NSASCIIStringEncoding], DISPATCH_QUEUE_CONCURRENT);
    }

    return self;
}

2. Dispatch_once

+ (instancetype)defaultInstance {
    static AFImageDownloader *sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[self alloc] init];
    });
    return sharedInstance;
}

3. ，

- (nullable AFImageDownloadReceipt *)downloadImageForURLRequest:(NSURLRequest *)request
                                                  withReceiptID:(nonnull NSUUID *)receiptID
                                                        success:(nullable void (^)(NSURLRequest *request, NSHTTPURLResponse  * _Nullable response, UIImage *responseObject))success
                                                        failure:(nullable void (^)(NSURLRequest *request, NSHTTPURLResponse * _Nullable response, NSError *error))failure {
    __block NSURLSessionDataTask *task = nil;
    dispatch_sync(self.synchronizationQueue, ^{
        if (URLIdentifier == nil) {
          ...
          // 结构返还主队列
          dispatch_async(dispatch_get_main_queue(), ^{
              failure(request, nil, error);
          });
                    ...
            return;
        }
                
        ...
       
        createdTask = [self.sessionManager
                       dataTaskWithRequest:request
                       uploadProgress:nil
                       downloadProgress:nil
                       completionHandler:^(NSURLResponse * _Nonnull response, id  _Nullable responseObject, NSError * _Nullable error) {
                         // 异步并发操作
                           dispatch_async(self.responseQueue, ^{
                               ...
// 结果返还主队列
                                 dispatch_async(dispatch_get_main_queue(), ^{
   handler.failureBlock(request, (NSHTTPURLResponse *)response, error);
                                               });
                ...
                           
}

4. 同步操作，结果async to 主队列

- (void)cancelTaskForImageDownloadReceipt:(AFImageDownloadReceipt *)imageDownloadReceipt {
    dispatch_sync(self.synchronizationQueue, ^{
        ...
            if (handler.failureBlock) {
                dispatch_async(dispatch_get_main_queue(), ^{
                    handler.failureBlock(imageDownloadReceipt.task.originalRequest, nil, error);
                });
            }
                ...
    });
}

5. 同步操作，保证线程安全

 - (AFImageDownloaderMergedTask *)safelyRemoveMergedTaskWithURLIdentifier:(NSString *)URLIdentifier {
    __block AFImageDownloaderMergedTask *mergedTask = nil;
    dispatch_sync(self.synchronizationQueue, ^{
        mergedTask = [self removeMergedTaskWithURLIdentifier:URLIdentifier];
    });
    return mergedTask;
}

- (void)safelyDecrementActiveTaskCount {
    dispatch_sync(self.synchronizationQueue, ^{
        if (self.activeRequestCount > 0) {
            self.activeRequestCount -= 1;
        }
    });
}

- (void)safelyStartNextTaskIfNecessary {
    dispatch_sync(self.synchronizationQueue, ^{
        if ([self isActiveRequestCountBelowMaximumLimit]) {
            while (self.queuedMergedTasks.count > 0) {
                AFImageDownloaderMergedTask *mergedTask = [self dequeueMergedTask];
                if (mergedTask.task.state == NSURLSessionTaskStateSuspended) {
                    [self startMergedTask:mergedTask];
                    break;
                }
            }
        }
    });
}

- (AFImageDownloaderMergedTask *)safelyGetMergedTask:(NSString *)URLIdentifier {
    __block AFImageDownloaderMergedTask *mergedTask;
    dispatch_sync(self.synchronizationQueue, ^(){
        mergedTask = self.mergedTasks[URLIdentifier];
    });
    return mergedTask;
}

总结：

1. 可以将 串行队列，与 dispatch_sync 结合，当作串行的概念，来保证数据安全， 避免竞争
2. 返还结果，异步返还主队列
3. 结果可以并发处理，场景时，串行开始，可能异步同时返还，增加处理效率。