SDWebImage解析之SDWebImageDownloade

作者: DevHuangjb | 来源:发表于2019-01-04 23:28 被阅读0次

SDWebImageDownloaderOperation是针对图片的某个具体的下载任务。SDWebImageDownloader角色则是类似于一个下载器，可以发起执行一个或多个下载任务，并对这些任务进行管理：任务的执行顺序，停止任务，取消任务，控制任务的并发量等。

首先来看一下SDWebImageDownloader.h头文件

定义了一个下载的配置选项SDWebImageDownloaderOptions

typedef NS_OPTIONS(NSUInteger, SDWebImageDownloaderOptions) {
//下载优先级低
    SDWebImageDownloaderLowPriority = 1,
    SDWebImageDownloaderLowPriority = 2,
    //渐进式的解码下载图片
    SDWebImageDownloaderProgressiveDownload = 1 << 1,
    //配置了这个选项就会使用系统默认的缓存机制，否则使用SDImageCache缓存
    SDWebImageDownloaderUseNSURLCache = 1 << 2,
    //结合SDWebImageDownloaderUseNSURLCache选项，忽略缓存的图片
    SDWebImageDownloaderIgnoreCachedResponse = 1 << 3,
    //当app退到后台，申请后台继续下载
    SDWebImageDownloaderContinueInBackground = 1 << 4,
    //设置NSMutableURLRequest.HTTPShouldHandleCookies = YES;
    SDWebImageDownloaderHandleCookies = 1 << 5,
    //允许非法证书请求
    SDWebImageDownloaderAllowInvalidSSLCertificates = 1 << 6,
    //下载优先级高
    SDWebImageDownloaderHighPriority = 1 << 7,
    //如果下载的图片太大(大于60M)则对图片进行缩小
    SDWebImageDownloaderScaleDownLargeImages = 1 << 8,
};

�下面是任务执行的顺序

typedef NS_ENUM(NSInteger, SDWebImageDownloaderExecutionOrder) {
//先入先出
    SDWebImageDownloaderFIFOExecutionOrder,
    //先入后出
    SDWebImageDownloaderLIFOExecutionOrder
};

这里有个类SDWebImageDownloadToken 用于关联某个特定的SDWebImageDownloaderOperation，可以让外界方便的取消下载中的任务。

//这个类遵守SDWebImageOperation协议，实现了cancel方法
@interface SDWebImageDownloadToken : NSObject <SDWebImageOperation>
//图片url
@property (nonatomic, strong, nullable) NSURL *url;
//SDWebImageDownloaderOperation： - (nullable id)addHandlersForProgress:completed:后返回的字典结构，里面包含这个operation的progressBlock和completedBlock
@property (nonatomic, strong, nullable) id downloadOperationCancelToken;
//关联的SDWebImageDownloaderOperation
@property (nonatomic, weak, nullable) NSOperation<SDWebImageDownloaderOperationInterface> *downloadOperation;
@end

@implementation SDWebImageDownloadToken
- (void)cancel {
    if (self.downloadOperation) {
        SDWebImageDownloadToken *cancelToken = self.downloadOperationCancelToken;
        if (cancelToken) {
        //调用SDWebImageDownloaderOperation的 cancel:方法来移除回调函数，当所有回调函数都移除了，这个下载任务就会被取消掉
            [self.downloadOperation cancel:cancelToken];
        }
    }
}
@end

下面来看看SDWebImageDownloader向外部暴露的属性和方法：

@interface SDWebImageDownloader : NSObject
//是否要对下载的图片预绘制解码
@property (assign, nonatomic) BOOL shouldDecompressImages;
//最大并发下载数
@property (assign, nonatomic) NSInteger maxConcurrentDownloads;
//当前下载任务数
@property (readonly, nonatomic) NSUInteger currentDownloadCount;
//超市时间,默认15s
@property (assign, nonatomic) NSTimeInterval downloadTimeout;
//内部用于配置NSUrlSession的NSURLSessionConfiguration
@property (readonly, nonatomic, nonnull) NSURLSessionConfiguration *sessionConfiguration;
//任务的执行顺序
@property (assign, nonatomic) SDWebImageDownloaderExecutionOrder executionOrder;
//单例获取
+ (nonnull instancetype)sharedDownloader;
//https证书
@property (strong, nonatomic, nullable) NSURLCredential *urlCredential;
@property (strong, nonatomic, nullable) NSString *username;
@property (strong, nonatomic, nullable) NSString *password;
//用于过滤http请求头参数
@property (nonatomic, copy, nullable) SDWebImageDownloaderHeadersFilterBlock headersFilter;
//
- (nonnull instancetype)initWithSessionConfiguration:(nullable NSURLSessionConfiguration *)sessionConfiguration NS_DESIGNATED_INITIALIZER;
//设置http请求头参数
- (void)setValue:(nullable NSString *)value forHTTPHeaderField:(nullable NSString *)field;
//获取指定http请求头信息
- (nullable NSString *)valueForHTTPHeaderField:(nullable NSString *)field;
//指定自定义的下载类
- (void)setOperationClass:(nullable Class)operationClass;
//下载图片，设置下载选项，指定回调block
- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
                                                   options:(SDWebImageDownloaderOptions)options
                                                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                                 completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock;
//取消
- (void)cancel:(nullable SDWebImageDownloadToken *)token;
//暂停
- (void)setSuspended:(BOOL)suspended;
//取消所有下载任务
- (void)cancelAllDownloads;
//重新指定NSURLSessionConfiguration配置NSUrlSession
- (void)createNewSessionWithConfiguration:(nonnull NSURLSessionConfiguration *)sessionConfiguration;
//使NSUrlSession失效，cancelPendingOperations：是否取消下载中的任务
- (void)invalidateSessionAndCancel:(BOOL)cancelPendingOperations;

@end

接下来看看SDWebImageDownloader 的实现

@interface SDWebImageDownloader () <NSURLSessionTaskDelegate, NSURLSessionDataDelegate>
@property (strong, nonatomic, nonnull) NSOperationQueue *downloadQueue;
//最后添加的任务：当指定为LIFO，如果有新下载任务，则之前最后添加的任务会依赖于新任务执行完达到LIFO效果
@property (weak, nonatomic, nullable) NSOperation *lastAddedOperation;
//自定义的下载类
@property (assign, nonatomic, nullable) Class operationClass;
//管理的下载任务集合
@property (strong, nonatomic, nonnull) NSMutableDictionary<NSURL *, SDWebImageDownloaderOperation *> *URLOperations;
//request请求头
@property (strong, nonatomic, nullable) SDHTTPHeadersMutableDictionary *HTTPHeaders;
//保证任务集合在多线程数据安全所用的信号量
@property (strong, nonatomic, nonnull) dispatch_semaphore_t operationsLock; 
//保证HTTPHeaders在多线程数据安全所用的信号量
@property (strong, nonatomic, nonnull) dispatch_semaphore_t headersLock; 
@property (strong, nonatomic) NSURLSession *session;
@end

在initialize中为请求添加指示器：

+ (void)initialize {
    // 我们需要额外导入SDNetworkActivityIndicator.h
    if (NSClassFromString(@"SDNetworkActivityIndicator")) {

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
        //通过runtime来实例化一个指示器
        id activityIndicator = [NSClassFromString(@"SDNetworkActivityIndicator") performSelector:NSSelectorFromString(@"sharedActivityIndicator")];
#pragma clang diagnostic pop

        // 因为指示器是用sharedActivityIndicator获取的单例，添加监听前先移除
        [[NSNotificationCenter defaultCenter] removeObserver:activityIndicator name:SDWebImageDownloadStartNotification object:nil];
        [[NSNotificationCenter defaultCenter] removeObserver:activityIndicator name:SDWebImageDownloadStopNotification object:nil];
        [[NSNotificationCenter defaultCenter] addObserver:activityIndicator
                                                 selector:NSSelectorFromString(@"startActivity")
                                                     name:SDWebImageDownloadStartNotification object:nil];
        [[NSNotificationCenter defaultCenter] addObserver:activityIndicator
                                                 selector:NSSelectorFromString(@"stopActivity")
                                                     name:SDWebImageDownloadStopNotification object:nil];
    }
}

下面来看构造函数：

- (nonnull instancetype)initWithSessionConfiguration:(nullable NSURLSessionConfiguration *)sessionConfiguration {
    if ((self = [super init])) {
        _operationClass = [SDWebImageDownloaderOperation class];
        //默认预绘制解码下载图片
        _shouldDecompressImages = YES;
        //默认FIFO执行顺序
        _executionOrder = SDWebImageDownloaderFIFOExecutionOrder;
        _downloadQueue = [NSOperationQueue new];
        //默认并发数为6
        _downloadQueue.maxConcurrentOperationCount = 6;
        _downloadQueue.name = @"com.hackemist.SDWebImageDownloader";
        _URLOperations = [NSMutableDictionary new];
#ifdef SD_WEBP
        _HTTPHeaders = [@{@"Accept": @"image/webp,image/*;q=0.8"} mutableCopy];
#else
        _HTTPHeaders = [@{@"Accept": @"image/*;q=0.8"} mutableCopy];
#endif
        _operationsLock = dispatch_semaphore_create(1);
        _headersLock = dispatch_semaphore_create(1);
        //默认超时时间15s
        _downloadTimeout = 15.0;
        [self createNewSessionWithConfiguration:sessionConfiguration];
    }
    return self;
}

下面来看看如何发起和管理一个下载任务

- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
                                                   options:(SDWebImageDownloaderOptions)options
                                                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                                 completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock

内部调用：

- (nullable SDWebImageDownloadToken *)addProgressCallback:(SDWebImageDownloaderProgressBlock)progressBlock
                                           completedBlock:(SDWebImageDownloaderCompletedBlock)completedBlock
                                                   forURL:(nullable NSURL *)url
                                           createCallback:(SDWebImageDownloaderOperation *(^)(void))createCallback

来为一个SDWebImageDownloaderOperation 添加progressBlock和completedBlock。同时传递了一个createCallback来实例化一个SDWebImageDownloaderOperation。

我们来看看如何实例化一个SDWebImageDownloaderOperation:

       __strong __typeof (wself) sself = wself;
        NSTimeInterval timeoutInterval = sself.downloadTimeout;
        if (timeoutInterval == 0.0) {
            timeoutInterval = 15.0;
        }

        // 是否使用系统默认的缓存策略
        NSURLRequestCachePolicy cachePolicy = options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData;
        NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url
                                                                    cachePolicy:cachePolicy
                                                                timeoutInterval:timeoutInterval];
        
        request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
        request.HTTPShouldUsePipelining = YES;
        if (sself.headersFilter) {
        //如果指定过滤请求头，使用block过滤
            request.allHTTPHeaderFields = sself.headersFilter(url, [sself allHTTPHeaderFields]);
        }
        else {
            request.allHTTPHeaderFields = [sself allHTTPHeaderFields];
        }
        SDWebImageDownloaderOperation *operation = [[sself.operationClass alloc] initWithRequest:request inSession:sself.session options:options];
        operation.shouldDecompressImages = sself.shouldDecompressImages;
        
        if (sself.urlCredential) {
            operation.credential = sself.urlCredential;
        } else if (sself.username && sself.password) {
            operation.credential = [NSURLCredential credentialWithUser:sself.username password:sself.password persistence:NSURLCredentialPersistenceForSession];
        }
        //通过下载选项配置请求优先级
        if (options & SDWebImageDownloaderHighPriority) {
            operation.queuePriority = NSOperationQueuePriorityHigh;
        } else if (options & SDWebImageDownloaderLowPriority) {
            operation.queuePriority = NSOperationQueuePriorityLow;
        }
        
        if (sself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
        //指定LIFO
            [sself.lastAddedOperation addDependency:operation];
            sself.lastAddedOperation = operation;
        }

        return operation;

下面来看看如何关联回调block:

- (nullable SDWebImageDownloadToken *)addProgressCallback:(SDWebImageDownloaderProgressBlock)progressBlock
                                           completedBlock:(SDWebImageDownloaderCompletedBlock)completedBlock
                                                   forURL:(nullable NSURL *)url
                                           createCallback:(SDWebImageDownloaderOperation *(^)(void))createCallback {
    if (url == nil) {
        if (completedBlock != nil) {
            completedBlock(nil, nil, nil, NO);
        }
        return nil;
    }
    
    LOCK(self.operationsLock);
    //首先确认下这个operation是否已经在任务集合中，保证当前针对一个url只有一个下载任务，防止同一时刻反复下载一张图片
    SDWebImageDownloaderOperation *operation = [self.URLOperations objectForKey:url];
    if (!operation || operation.isFinished) {
        operation = createCallback();
        __weak typeof(self) wself = self;
        operation.completionBlock = ^{
            __strong typeof(wself) sself = wself;
            if (!sself) {
                return;
            }
            LOCK(sself.operationsLock);
            [sself.URLOperations removeObjectForKey:url];
            UNLOCK(sself.operationsLock);
        };
        [self.URLOperations setObject:operation forKey:url];
        //添加到下载队列，开始下载
        [self.downloadQueue addOperation:operation];
    }
    UNLOCK(self.operationsLock);
    
//为operation增加回调block
    id downloadOperationCancelToken = [operation addHandlersForProgress:progressBlock completed:completedBlock];
    //配置SDWebImageDownloadToken，包装了回调函数，operation,url。返回给外包调用者来管理下载任务
    SDWebImageDownloadToken *token = [SDWebImageDownloadToken new];
    token.downloadOperation = operation;
    token.url = url;
    token.downloadOperationCancelToken = downloadOperationCancelToken;

    return token;
}

关于NSURLSession代理方法：

#pragma mark NSURLSessionDataDelegate

- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
didReceiveResponse:(NSURLResponse *)response
 completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler {
    SDWebImageDownloaderOperation *dataOperation = [self operationWithTask:dataTask];
    if ([dataOperation respondsToSelector:@selector(URLSession:dataTask:didReceiveResponse:completionHandler:)]) {
        [dataOperation URLSession:session dataTask:dataTask didReceiveResponse:response completionHandler:completionHandler];
    } else {
        if (completionHandler) {
            completionHandler(NSURLSessionResponseAllow);
        }
    }
}

都是通过这种形式来把代理操作转发到SDWebImageDownloaderOperation的代理方法里面去执行。

operationWithTask通过NSURLSessionTask的taskIdentifier匹配来找出相对应的SDWebImageDownloaderOperation。

- (SDWebImageDownloaderOperation *)operationWithTask:(NSURLSessionTask *)task {
    SDWebImageDownloaderOperation *returnOperation = nil;
    for (SDWebImageDownloaderOperation *operation in self.downloadQueue.operations) {
        if (operation.dataTask.taskIdentifier == task.taskIdentifier) {
            returnOperation = operation;
            break;
        }
    }
    return returnOperation;
}

SDWebImage解析之SDWebImageDownloade

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