[iOS源码笔记]·第三方网络下载处理框架:AFNetworking网络下载处理（调用栈与缓存策略篇）

陈满iOS

发布于 2018-09-10 11:13:26

1.4K0

发布于 2018-09-10 11:13:26

文章被收录于专栏：陈满iOS陈满iOS

本篇文章中，笔者将着重介绍AFNetworking的方法调用栈，以及网络图片下载和缓存策略。在此之前，笔者写了一个官方文档翻译版，点击可跳转。

1. 数据下载调用栈

比如，我们要下载一个PDF文件，可采用示例代码如下：

- (void)DownloadPdfAndSave{

    AFHTTPSessionManager *manager = [AFHTTPSessionManager manager];
    manager.responseSerializer = [AFHTTPResponseSerializer serializer];
    manager.responseSerializer.acceptableContentTypes = [NSSet setWithObject:@"application/pdf"];
    __weak __typeof__(self) weakSelf = self;
    
    //临时配置，需要自己根据接口地址改动！！！！！！！！！！！！！！！！！！！！
    self.urlStr = @"http://10.10.100.100/test333.pdf";
    
    [manager GET:_urlStr parameters:nil progress:^(NSProgress * _Nonnull downloadProgress) {

    } success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
        __strong __typeof__(weakSelf) strongSelf = weakSelf;
        strongSelf.isWriten = [responseObject writeToFile:[self pathOfPdf] atomically:YES];
        [strongSelf openPdfByAddingSubView];
        //[strongSelf.previewController reloadData];
        
    } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
        NSLog(@"%@",error.userInfo);
    }];
}

看到这里，或许可以看manager对象的AFHTTPSessionManager类型更深层定义。

AFHTTPSessionManager.h

@interface AFHTTPSessionManager : AFURLSessionManager <NSSecureCoding, NSCopying>

AFURLSessionManager

@interface AFURLSessionManager : NSObject <NSURLSessionDelegate, NSURLSessionTaskDelegate, NSURLSessionDataDelegate, NSURLSessionDownloadDelegate, NSSecureCoding, NSCopying>

1.1 第一层

AFNetworking/AFHTTPSessionManager.m

方法

- (NSURLSessionDataTask *)GET:(NSString *)URLString
                   parameters:(id)parameters
                     progress:(void (^)(NSProgress * _Nonnull))downloadProgress
                      success:(void (^)(NSURLSessionDataTask * _Nonnull, id _Nullable))success
                      failure:(void (^)(NSURLSessionDataTask * _Nullable, NSError * _Nonnull))failure
{
...}

源码

- (NSURLSessionDataTask *)GET:(NSString *)URLString
                   parameters:(id)parameters
                     progress:(void (^)(NSProgress * _Nonnull))downloadProgress
                      success:(void (^)(NSURLSessionDataTask * _Nonnull, id _Nullable))success
                      failure:(void (^)(NSURLSessionDataTask * _Nullable, NSError * _Nonnull))failure
{

    NSURLSessionDataTask *dataTask = [self dataTaskWithHTTPMethod:@"GET"
                                                        URLString:URLString
                                                       parameters:parameters
                                                   uploadProgress:nil
                                                 downloadProgress:downloadProgress
                                                          success:success
                                                          failure:failure];

    [dataTask resume];

    return dataTask;
}

1.2 第二层

AFNetworking/AFHTTPSessionManager.m

方法

- (NSURLSessionDataTask *)dataTaskWithHTTPMethod:(NSString *)method
                                       URLString:(NSString *)URLString
                                      parameters:(id)parameters
                                  uploadProgress:(nullable void (^)(NSProgress *uploadProgress)) uploadProgress
                                downloadProgress:(nullable void (^)(NSProgress *downloadProgress)) downloadProgress
                                         success:(void (^)(NSURLSessionDataTask *, id))success
                                         failure:(void (^)(NSURLSessionDataTask *, NSError *))failure
{
...}

源码

- (NSURLSessionDataTask *)dataTaskWithHTTPMethod:(NSString *)method
                                       URLString:(NSString *)URLString
                                      parameters:(id)parameters
                                  uploadProgress:(nullable void (^)(NSProgress *uploadProgress)) uploadProgress
                                downloadProgress:(nullable void (^)(NSProgress *downloadProgress)) downloadProgress
                                         success:(void (^)(NSURLSessionDataTask *, id))success
                                         failure:(void (^)(NSURLSessionDataTask *, NSError *))failure
{
    NSError *serializationError = nil;
    NSMutableURLRequest *request = [self.requestSerializer requestWithMethod:method URLString:[[NSURL URLWithString:URLString relativeToURL:self.baseURL] absoluteString] parameters:parameters error:&serializationError];
    if (serializationError) {
        if (failure) {
            dispatch_async(self.completionQueue ?: dispatch_get_main_queue(), ^{
                failure(nil, serializationError);
            });
        }

        return nil;
    }

    __block NSURLSessionDataTask *dataTask = nil;
    dataTask = [self dataTaskWithRequest:request
                          uploadProgress:uploadProgress
                        downloadProgress:downloadProgress
                       completionHandler:^(NSURLResponse * __unused response, id responseObject, NSError *error) {
        if (error) {
            if (failure) {
                failure(dataTask, error);
            }
        } else {
            if (success) {
                success(dataTask, responseObject);
            }
        }
    }];

    return dataTask;
}

1.3 第三层

AFNetworking/AFURLSessionManager.m

方法

- (NSURLSessionDataTask *)dataTaskWithRequest:(NSURLRequest *)request
                               uploadProgress:(nullable void (^)(NSProgress *uploadProgress)) uploadProgressBlock
                             downloadProgress:(nullable void (^)(NSProgress *downloadProgress)) downloadProgressBlock
                            completionHandler:(nullable void (^)(NSURLResponse *response, id _Nullable responseObject,  NSError * _Nullable error))completionHandler 
{
...}

源码

- (NSURLSessionDataTask *)dataTaskWithRequest:(NSURLRequest *)request
                               uploadProgress:(nullable void (^)(NSProgress *uploadProgress)) uploadProgressBlock
                             downloadProgress:(nullable void (^)(NSProgress *downloadProgress)) downloadProgressBlock
                            completionHandler:(nullable void (^)(NSURLResponse *response, id _Nullable responseObject,  NSError * _Nullable error))completionHandler {

    __block NSURLSessionDataTask *dataTask = nil;
    url_session_manager_create_task_safely(^{
        dataTask = [self.session dataTaskWithRequest:request];
    });

    [self addDelegateForDataTask:dataTask uploadProgress:uploadProgressBlock downloadProgress:downloadProgressBlock completionHandler:completionHandler];

    return dataTask;
}

1.4 第四层

AFNetworking/AFURLSessionManager.m

方法

- (void)addDelegateForDataTask:(NSURLSessionDataTask *)dataTask
                uploadProgress:(nullable void (^)(NSProgress *uploadProgress)) uploadProgressBlock
              downloadProgress:(nullable void (^)(NSProgress *downloadProgress)) downloadProgressBlock
             completionHandler:(void (^)(NSURLResponse *response, id responseObject, NSError *error))completionHandler
{
...}

源码

- (void)addDelegateForDataTask:(NSURLSessionDataTask *)dataTask
                uploadProgress:(nullable void (^)(NSProgress *uploadProgress)) uploadProgressBlock
              downloadProgress:(nullable void (^)(NSProgress *downloadProgress)) downloadProgressBlock
             completionHandler:(void (^)(NSURLResponse *response, id responseObject, NSError *error))completionHandler
{
    AFURLSessionManagerTaskDelegate *delegate = [[AFURLSessionManagerTaskDelegate alloc] initWithTask:dataTask];
    delegate.manager = self;
    delegate.completionHandler = completionHandler;

    dataTask.taskDescription = self.taskDescriptionForSessionTasks;
    [self setDelegate:delegate forTask:dataTask];

    delegate.uploadProgressBlock = uploadProgressBlock;
    delegate.downloadProgressBlock = downloadProgressBlock;
}

2. 图片缓存与下载管理

2.1 调用示例

MyTableViewCell.m
- 用法：由UIImageView对象调用如下方法

- (void)setTableCellWithModel:(cellModel *)item{
    [_iconImgView setImageWithURL:[NSURL URLWithString:[NSString stringWithFormat:@"%@%@%@",kBaseImgURL,kMiddleImgURL,item.url]] placeholderImage:[UIImage imageNamed:@"imageFailed"]];
}

2.2 源代码

2.2.1 第一层

lib/AFNetworking/UIKit+AFNetworking/UIImageView+AFNetworking.m

类名:

- UIImageView+AFNetworking.m作用:
根据url建立请求代码：

- (void)setImageWithURL:(NSURL *)url
       placeholderImage:(UIImage *)placeholderImage
{
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
    [request addValue:@"image/*" forHTTPHeaderField:@"Accept"];

    [self setImageWithURLRequest:request placeholderImage:placeholderImage success:nil failure:nil];
}

2.2.2 第二层

lib/AFNetworking/UIKit+AFNetworking/UIImageView+AFNetworking.m

类名:

- UIImageView+AFNetworking.m作用：
先查询缓存，后以此决策下载任务逻辑：

1. 判断传入的请求中URL是否为空；若为空，则取消下载，imageView显示占位图片，否则，进入下一步判断
2. 判断当前的请求是否有task处于活动状态(意思可能用户单位时间内多次发了同一个请求，而在这个请求发送时已经有下载的Task执行了，这个请求将结束)
3. 根据请求查看内存缓存中是否目前有当前请求的图片，若有则查看success有无回调，有回调则将图片回调回去，否则，直接设置本ImageView的image
4. 若本地内存缓存没有数据，则先将imageView的image设置为占位图片并生成UUID，然后将请求和UUID发送出去; 若成功，则检查UUID是否和之前生成的UUID相等，以防止数据出错，没有问题则根据success是否有回调将图片发送出去;若失败，也检查UUID是否相等，并根据有无failure回调，将错误信息输出代码：

- (void)setImageWithURLRequest:(NSURLRequest *)urlRequest
              placeholderImage:(UIImage *)placeholderImage
                       success:(void (^)(NSURLRequest *request, NSHTTPURLResponse * _Nullable response, UIImage *image))success
                       failure:(void (^)(NSURLRequest *request, NSHTTPURLResponse * _Nullable response, NSError *error))failure
{

    if ([urlRequest URL] == nil) {
        [self cancelImageDownloadTask];
        self.image = placeholderImage;
        return;
    }

    if ([self isActiveTaskURLEqualToURLRequest:urlRequest]){
        return;
    }

    [self cancelImageDownloadTask];

    AFImageDownloader *downloader = [[self class] sharedImageDownloader];
    id <AFImageRequestCache> imageCache = downloader.imageCache;

    //缓存策略
    //Use the image from the image cache if it exists
    UIImage *cachedImage = [imageCache imageforRequest:urlRequest withAdditionalIdentifier:nil];
    if (cachedImage) {
        if (success) {
            success(urlRequest, nil, cachedImage);
        } else {
            self.image = cachedImage;
        }
        [self clearActiveDownloadInformation];
    } else {
        if (placeholderImage) {
            self.image = placeholderImage;
        }

        __weak __typeof(self)weakSelf = self;
        NSUUID *downloadID = [NSUUID UUID];
        AFImageDownloadReceipt *receipt;

        //下载策略
        receipt = [downloader
                   downloadImageForURLRequest:urlRequest
                   withReceiptID:downloadID
                   success:^(NSURLRequest * _Nonnull request, NSHTTPURLResponse * _Nullable response, UIImage * _Nonnull responseObject) {
                       __strong __typeof(weakSelf)strongSelf = weakSelf;
                       if ([strongSelf.af_activeImageDownloadReceipt.receiptID isEqual:downloadID]) {
                           if (success) {
                               success(request, response, responseObject);
                           } else if(responseObject) {
                               strongSelf.image = responseObject;
                           }
                           [strongSelf clearActiveDownloadInformation];
                       }

                   }
                   failure:^(NSURLRequest * _Nonnull request, NSHTTPURLResponse * _Nullable response, NSError * _Nonnull error) {
                       __strong __typeof(weakSelf)strongSelf = weakSelf;
                        if ([strongSelf.af_activeImageDownloadReceipt.receiptID isEqual:downloadID]) {
                            if (failure) {
                                failure(request, response, error);
                            }
                            [strongSelf clearActiveDownloadInformation];
                        }
                   }];

        self.af_activeImageDownloadReceipt = receipt;
    }
}

2.2.3 第三层

（1）查询缓存

lib/AFNetworking/UIKit+AFNetworking/AFAutoPurgingImageCache.m

类名：
AFAutoPurgingImageCache.m
代码：

- (nullable UIImage *)imageforRequest:(NSURLRequest *)request withAdditionalIdentifier:(NSString *)identifier {
    return [self imageWithIdentifier:[self imageCacheKeyFromURLRequest:request withAdditionalIdentifier:identifier]];
}

（2）网络下载

lib/AFNetworking/UIKit+AFNetworking/AFImageDownloader.m

类名：
AFImageDownloader.m
逻辑：
检查请求的URL是否为空，失败则则返回信息(感觉这一句有点多余，因为调用这个方法之间已经检查了URL是否为空)
在Task没有执行时，多个请求可能会同时达到该方法，因此，需要一个字典存储该URL对应的AFImageDownloaderMergedTask,若有，则取出并创建一个AFImageDownloaderResponseHandler放入该自定义Task的一个响应数组中。并将该自定义task的NSURLSessionDataTask赋值给task;若没有，则进行下一步
由于对于磁盘缓存作者是使用NSURLCache进行的，因此，根据存储策略进行不同操作以便于下载图片后进行磁盘缓存 4)之后就是从网络下载图片，这也要注意，作者是使用AFN进行异步下载的、会生成UUID和之前一样去检验下载后UUID是否发生改变；若成功，则将先缓存图片，然后将AFImageDownloaderMergedTask中数组所有响应取出并将图片等信息通过回调返回;若失败，也一样进行回调错误信息
代码：

- (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, ^{
        NSString *URLIdentifier = request.URL.absoluteString;
        if (URLIdentifier == nil) {
            if (failure) {
                NSError *error = [NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorBadURL userInfo:nil];
                dispatch_async(dispatch_get_main_queue(), ^{
                    failure(request, nil, error);
                });
            }
            return;
        }

        // 1) Append the success and failure blocks to a pre-existing request if it already exists
        AFImageDownloaderMergedTask *existingMergedTask = self.mergedTasks[URLIdentifier];
        if (existingMergedTask != nil) {
            AFImageDownloaderResponseHandler *handler = [[AFImageDownloaderResponseHandler alloc] initWithUUID:receiptID success:success failure:failure];
            [existingMergedTask addResponseHandler:handler];
            task = existingMergedTask.task;
            return;
        }

        // 2) Attempt to load the image from the image cache if the cache policy allows it
        switch (request.cachePolicy) {
            case NSURLRequestUseProtocolCachePolicy:
            case NSURLRequestReturnCacheDataElseLoad:
            case NSURLRequestReturnCacheDataDontLoad: {
                UIImage *cachedImage = [self.imageCache imageforRequest:request withAdditionalIdentifier:nil];
                if (cachedImage != nil) {
                    if (success) {
                        dispatch_async(dispatch_get_main_queue(), ^{
                            success(request, nil, cachedImage);
                        });
                    }
                    return;
                }
                break;
            }
            default:
                break;
        }

        // 3) Create the request and set up authentication, validation and response serialization
        NSUUID *mergedTaskIdentifier = [NSUUID UUID];
        NSURLSessionDataTask *createdTask;
        __weak __typeof__(self) weakSelf = self;

        createdTask = [self.sessionManager
                       dataTaskWithRequest:request
                       uploadProgress:nil
                       downloadProgress:nil
                       completionHandler:^(NSURLResponse * _Nonnull response, id  _Nullable responseObject, NSError * _Nullable error) {
                           dispatch_async(self.responseQueue, ^{
                               __strong __typeof__(weakSelf) strongSelf = weakSelf;
                               AFImageDownloaderMergedTask *mergedTask = self.mergedTasks[URLIdentifier];
                               if ([mergedTask.identifier isEqual:mergedTaskIdentifier]) {
                                   mergedTask = [strongSelf safelyRemoveMergedTaskWithURLIdentifier:URLIdentifier];
                                   if (error) {
                                       for (AFImageDownloaderResponseHandler *handler in mergedTask.responseHandlers) {
                                           if (handler.failureBlock) {
                                               dispatch_async(dispatch_get_main_queue(), ^{
                                                   handler.failureBlock(request, (NSHTTPURLResponse*)response, error);
                                               });
                                           }
                                       }
                                   } else {
                                       [strongSelf.imageCache addImage:responseObject forRequest:request withAdditionalIdentifier:nil];

                                       for (AFImageDownloaderResponseHandler *handler in mergedTask.responseHandlers) {
                                           if (handler.successBlock) {
                                               dispatch_async(dispatch_get_main_queue(), ^{
                                                   handler.successBlock(request, (NSHTTPURLResponse*)response, responseObject);
                                               });
                                           }
                                       }
                                       
                                   }
                               }
                               [strongSelf safelyDecrementActiveTaskCount];
                               [strongSelf safelyStartNextTaskIfNecessary];
                           });
                       }];

        // 4) Store the response handler for use when the request completes
        AFImageDownloaderResponseHandler *handler = [[AFImageDownloaderResponseHandler alloc] initWithUUID:receiptID
                                                                                                   success:success
                                                                                                   failure:failure];
        AFImageDownloaderMergedTask *mergedTask = [[AFImageDownloaderMergedTask alloc]
                                                   initWithURLIdentifier:URLIdentifier
                                                   identifier:mergedTaskIdentifier
                                                   task:createdTask];
        [mergedTask addResponseHandler:handler];
        self.mergedTasks[URLIdentifier] = mergedTask;

        // 5) Either start the request or enqueue it depending on the current active request count
        if ([self isActiveRequestCountBelowMaximumLimit]) {
            [self startMergedTask:mergedTask];
        } else {
            [self enqueueMergedTask:mergedTask];
        }

        task = mergedTask.task;
    });
    if (task) {
        return [[AFImageDownloadReceipt alloc] initWithReceiptID:receiptID task:task];
    } else {
        return nil;
    }
}

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原始发表：2017.07.02 ，如有侵权请联系 cloudcommunity@tencent.com 删除

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[iOS源码笔记]·第三方网络下载处理框架:AFNetworking网络下载处理（调用栈与缓存策略篇）

[iOS源码笔记]·第三方网络下载处理框架:AFNetworking网络下载处理（调用栈与缓存策略篇）

1. 数据下载调用栈

1.1 第一层

1.2 第二层

1.3 第三层

1.4 第四层

2. 图片缓存与下载管理

2.1 调用示例

2.2 源代码

2.2.1 第一层

2.2.2 第二层

2.2.3 第三层

（1）查询缓存

（2）网络下载

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