iOS网络——SDWebImage SDImageDownloader源码解析
2017-11-01 18:11
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你要知道的NSURLSession都在这里
转载请注明出处 http://blog.csdn.net/u014205968/article/details/78416946本系列文章主要讲解iOS中网络请求类NSURLSession的使用方法进行详解,同时也会以此为扩展,讲解SDWebImage中图片下载功能的源码分析,讲解AFNetworking相关源码分析。本系列文章主要分为以下几篇进行讲解,读者可按需查阅。
iOS网络——NSURLSession详解及SDWebImage源码解析
iOS网络——SDWebImage SDImageDownloader源码解析
iOS网络——AFNetworking AFURLSessionManager源码解析
iOS网络——AFNetworking AFHttpSessionManager源码解析
SDWebImage SDWebImageDownloader源码解析
前一篇文章中讲解了SDWebImageDownloaderOperation是如何自定义
NSOperation子类以及如何使用
NSURLSession实现下载的,本文将会讲解
SDWebImageDownloader类,来探索
SDWebImage如何实现多线程下载多张图片的。
首先看一下接口声明代码:
//下载选项设置的一系列枚举 typedef NS_OPTIONS(NSUInteger, SDWebImageDownloaderOptions) { SDWebImageDownloaderLowPriority = 1 << 0, SDWebImageDownloaderProgressiveDownload = 1 << 1, SDWebImageDownloaderUseNSURLCache = 1 << 2, SDWebImageDownloaderIgnoreCachedResponse = 1 << 3, SDWebImageDownloaderContinueInBackground = 1 << 4, SDWebImageDownloaderHandleCookies = 1 << 5, SDWebImageDownloaderAllowInvalidSSLCertificates = 1 << 6, SDWebImageDownloaderHighPriority = 1 << 7, SDWebImageDownloaderScaleDownLargeImages = 1 << 8, }; //下载图片时的顺序,FIFO或者LIFO typedef NS_ENUM(NSInteger, SDWebImageDownloaderExecutionOrder) { SDWebImageDownloaderFIFOExecutionOrder, SDWebImageDownloaderLIFOExecutionOrder }; //声明通知的全局变量名 FOUNDATION_EXPORT NSString * _Nonnull const SDWebImageDownloadStartNotification; FOUNDATION_EXPORT NSString * _Nonnull const SDWebImageDownloadStopNotification; //进度回调块 typedef void(^SDWebImageDownloaderProgressBlock)(NSInteger receivedSize, NSInteger expectedSize, NSURL * _Nullable targetURL); //下载完成的回调块 typedef void(^SDWebImageDownloaderCompletedBlock)(UIImage * _Nullable image, NSData * _Nullable data, NSError * _Nullable error, BOOL finished); //http首部数据字典 typedef NSDictionary<NSString *, NSString *> SDHTTPHeadersDictionary; //http首部可变数据字典 typedef NSMutableDictionary<NSString *, NSString *> SDHTTPHeadersMutableDictionary; //http首部过滤的一个回调块 typedef SDHTTPHeadersDictionary * _Nullable (^SDWebImageDownloaderHeadersFilterBlock)(NSURL * _Nullable url, SDHTTPHeadersDictionary * _Nullable headers);
上面就是一些枚举、变量、自定义类型的声明。
/* 自定义token类,用于取消下载任务 这个token第二个属性其实就是SDWebImageDownloaderOperation中使用的token即回调块的字典 目的相同,都是为了取消特定的下载任务 */ @interface SDWebImageDownloadToken : NSObject @property (nonatomic, strong, nullable) NSURL *url; @property (nonatomic, strong, nullable) id downloadOperationCancelToken; @end //异步下载图片 @interface SDWebImageDownloader : NSObject //是否压缩图片 @property (assign, nonatomic) BOOL shouldDecompressImages; //支持的最大同时下载图片的数量,其实就是NSOperationQueue支持的最大并发数 @property (assign, nonatomic) NSInteger maxConcurrentDownloads; //当前正在下载图片的数量,其实就是NSOperationQueue的operationCount即正在执行下载任务的operation的数量 @property (readonly, nonatomic) NSUInteger currentDownloadCount; //下载时连接服务器的超时时间,默认15s @property (assign, nonatomic) NSTimeInterval downloadTimeout; //session运行模式,默认使用默认模式,即 defaultSessionConfiguration @property (readonly, nonatomic, nonnull) NSURLSessionConfiguration *sessionConfiguration; //执行下载任务的顺序,FIFO或LIFO @property (assign, nonatomic) SDWebImageDownloaderExecutionOrder executionOrder; //类方法,获取全局共享的单例对象 + (nonnull instancetype)sharedDownloader; //默认的URL credential @property (strong, nonatomic, nullable) NSURLCredential *urlCredential; //用户名,有些图片下载的地址需要做用户认证 @property (strong, nonatomic, nullable) NSString *username; //密码 @property (strong, nonatomic, nullable) NSString *password; //过滤http首部的回调块 @property (nonatomic, copy, nullable) SDWebImageDownloaderHeadersFilterBlock headersFilter; //初始化方法,不使用全局共享的downloader时创建 - (nonnull instancetype)initWithSessionConfiguration:(nullable NSURLSessionConfiguration *)sessionConfiguration NS_DESIGNATED_INITIALIZER; //为http首部设置值 - (void)setValue:(nullable NSString *)value forHTTPHeaderField:(nullable NSString *)field; //返回http首部的值 - (nullable NSString *)valueForHTTPHeaderField:(nullable NSString *)field; //设置下载类的Class,默认使用SDWebImageDownloaderOperation,开发者可以自定义只需实现相关协议 - (void)setOperationClass:(nullable Class)operationClass; /* 下载url对应的图片 设置下载配置选项、进度回调块、下载完成回调块 返回一个token,用于取消对应的下载任务 */ - (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock; //取消下载任务,需要传入上面创建下载任务时返回的token - (void)cancel:(nullable SDWebImageDownloadToken *)token; //设置下载队列NSOperationQueue挂起 - (void)setSuspended:(BOOL)suspended; //设置取消NSOperationQueue队列中的所有下载任务 - (void)cancelAllDownloads; //要求downloader使用特定运行模式创建一个NSURLSession对象 - (void)createNewSessionWithConfiguration:(nonnull NSURLSessionConfiguration *)sessionConfiguration; @end
上面定义了一系列的属性和方法,接下来看一下
.m文件的源码:
@implementation SDWebImageDownloadToken @end //遵守NSURLSessionTaskDelegate和NSURLSessionDataDelegate协议 @interface SDWebImageDownloader () <NSURLSessionTaskDelegate, NSURLSessionDataDelegate> //定义一个NSOperationQueue的下载队列 @property (strong, nonatomic, nonnull) NSOperationQueue *downloadQueue; //最近一次添加进队列的operation主要用于LIFO时设置依赖 @property (weak, nonatomic, nullable) NSOperation *lastAddedOperation; //operationClass默认是SDWebImageDownloaderOperation @property (assign, nonatomic, nullable) Class operationClass; //可变字典,key是图片的URL,value是对应的下载任务Operation @property (strong, nonatomic, nonnull) NSMutableDictionary<NSURL *, SDWebImageDownloaderOperation *> *URLOperations; //<NSString*, NSString*>类型的字典,存储http首部 @property (strong, nonatomic, nullable) SDHTTPHeadersMutableDictionary *HTTPHeaders; //一个GCD的队列 @property (SDDispatchQueueSetterSementics, nonatomic, nullable) dispatch_queue_t barrierQueue; //NSURLSession对象 @property (strong, nonatomic) NSURLSession *session; @end @implementation SDWebImageDownloader //类方法,类加载的时候执行 + (void)initialize { //如果导入了SDNetworkActivityIndicator文件,就会展示一个小菊花 if (NSClassFromString(@"SDNetworkActivityIndicator")) { #pragma clang diagnostic push #pragma clang diagnostic ignored "-Warc-performSelector-leaks" id activityIndicator = [NSClassFromString(@"SDNetworkActivityIndicator") performSelector:NSSelectorFromString(@"sharedActivityIndicator")]; #pragma clang diagnostic pop //删除通知后重新添加通知,防止重复添加出现异常 [[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]; } } //类方法,返回单例对象 + (nonnull instancetype)sharedDownloader { static dispatch_once_t once; static id instance; dispatch_once(&once, ^{ instance = [self new]; }); return instance; } //初始化方法调用下面的初始化方法,NSURLSession运行在默认模式下 - (nonnull instancetype)init { return [self initWithSessionConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]]; } //初始化方法 - (nonnull instancetype)initWithSessionConfiguration:(nullable NSURLSessionConfiguration *)sessionConfiguration { if ((self = [super init])) { //默认使用SDWebImageDownloaderOperation作为下载任务Operation _operationClass = [SDWebImageDownloaderOperation class]; _shouldDecompressImages = YES; //默认下载顺序FIFO _executionOrder = SDWebImageDownloaderFIFOExecutionOrder; //创建NSOperationQueue并设置最大并发数为6,即同时最多可以下载6张图片 _downloadQueue = [NSOperationQueue new]; _downloadQueue.maxConcurrentOperationCount = 6; //设置名称 _downloadQueue.name = @"com.hackemist.SDWebImageDownloader"; //设置下载webp格式图片的http首部 _URLOperations = [NSMutableDictionary new]; #ifdef SD_WEBP _HTTPHeaders = [@{@"Accept": @"image/webp,image/*;q=0.8"} mutableCopy]; #else _HTTPHeaders = [@{@"Accept": @"image/*;q=0.8"} mutableCopy]; #endif //创建一个GCD并发队列 _barrierQueue = dispatch_queue_create("com.hackemist.SDWebImageDownloaderBarrierQueue", DISPATCH_QUEUE_CONCURRENT); //默认超时时间15s _downloadTimeout = 15.0; //创建一个sessionConfiguration运行默认的NSURLSession对象 [self createNewSessionWithConfiguration:sessionConfiguration]; } return self; } //创建指定运行模式的NSURLSession对象,可能已经创建过了 - (void)createNewSessionWithConfiguration:(NSURLSessionConfiguration *)sessionConfiguration { //可能已经创建过,所以需要取消前一个session的下载任务并打破引用循环 [self cancelAllDownloads]; if (self.session) { [self.session invalidateAndCancel]; } sessionConfiguration.timeoutIntervalForRequest = self.downloadTimeout; self.session = [NSURLSession sessionWithConfiguration:sessionConfiguration delegate:self delegateQueue:nil]; } //在析构函数中调用session的invalidateAndCancel方法取消下载任务并打破引用循环 - (void)dealloc { [self.session invalidateAndCancel]; self.session = nil; //NSOperationQueue取消所有的下载操作 [self.downloadQueue cancelAllOperations]; //释放GCD队列 SDDispatchQueueRelease(_barrierQueue); }
上面的各种初始化、析构函数也都很简单,不再赘述了。
//为http首部设置键值对 - (void)setValue:(nullable NSString *)value forHTTPHeaderField:(nullable NSString *)field { if (value) { self.HTTPHeaders[field] = value; } else { [self.HTTPHeaders removeObjectForKey:field]; } } //获取http首部的值 - (nullable NSString *)valueForHTTPHeaderField:(nullable NSString *)field { return self.HTTPHeaders[field]; } //设置最大同时下载图片的数量,即NSOperationQueue最大并发数 - (void)setMaxConcurrentDownloads:(NSInteger)maxConcurrentDownloads { _downloadQueue.maxConcurrentOperationCount = maxConcurrentDownloads; } //当前正在下载图片数量,即NSOperationQueue中正在执行的operation数量 - (NSUInteger)currentDownloadCount { return _downloadQueue.operationCount; } //获取最大同时下载图片的数量 - (NSInteger)maxConcurrentDownloads { return _downloadQueue.maxConcurrentOperationCount; } //获取NSURLSession的运行模式配置 - (NSURLSessionConfiguration *)sessionConfiguration { return self.session.configuration; } //设置operation的Class类对象 - (void)setOperationClass:(nullable Class)operationClass { if (operationClass && [operationClass isSubclassOfClass:[NSOperation class]] && [operationClass conformsToProtocol:@protocol(SDWebImageDownloaderOperationInterface)]) { _operationClass = operationClass; } else { _operationClass = [SDWebImageDownloaderOperation class]; } }
上面的方法都是一些
setter和
getter。
//下载图片的方法 - (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url options:(SDWebImageDownloaderOptions)options progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock { __weak SDWebImageDownloader *wself = self; //直接调用另一个方法,后面大片的block代码目的就是为了创建一个SDWebImageDownloaderOperation类的对象 return [self addProgressCallback:progressBlock completedBlock:completedBlock forURL:url createCallback:^SDWebImageDownloaderOperation *{ //block中为了防止引用循环和空指针,先weak后strong __strong __typeof (wself) sself = wself; //设置超时时间 NSTimeInterval timeoutInterval = sself.downloadTimeout; if (timeoutInterval == 0.0) { timeoutInterval = 15.0; } //设置缓存策略 NSURLRequestCachePolicy cachePolicy = options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData; //创建一个可变的request镀锡 NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:cachePolicy timeoutInterval:timeoutInterval]; //设置cookie的处理策略 request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies); request.HTTPShouldUsePipelining = YES; //过滤http首部 if (sself.headersFilter) { request.allHTTPHeaderFields = sself.headersFilter(url, [sself.HTTPHeaders copy]); } else { request.allHTTPHeaderFields = sself.HTTPHeaders; } //创建一个SDWebImageDownloaderOperation类的对象 //传入request、session和下载选项配置options SDWebImageDownloaderOperation *operation = [[sself.operationClass alloc] initWithRequest:request inSession:sself.session options:options]; //设置是否压缩图片 operation.shouldDecompressImages = sself.shouldDecompressImages; //设置认证凭证和https相关 if (sself.urlCredential) { operation.credential = sself.urlCredential; } else if (sself.username && sself.password) { operation.credential = [NSURLCredential credentialWithUser:sself.username password:sself.password persistence:NSURLCredentialPersistenceForSession]; } //设置下载优先级 if (options & SDWebImageDownloaderHighPriority) { operation.queuePriority = NSOperationQueuePriorityHigh; } else if (options & SDWebImageDownloaderLowPriority) { operation.queuePriority = NSOperationQueuePriorityLow; } //向队列中添加创建的下载任务,之后这个operation就会被线程调度来执行其start方法 [sself.downloadQueue addOperation:operation]; //如果是LIFO就设置一个依赖 if (sself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) { // Emulate LIFO execution order by systematically adding new operations as last operation's dependency [sself.lastAddedOperation addDependency:operation]; sself.lastAddedOperation = operation; } //返回operation return operation; }]; } //取消一个下载任务,需要传入上一个方法返回的token,其实具体的token是由下一个方法创建的 - (void)cancel:(nullable SDWebImageDownloadToken *)token { //异步执行,阻塞队列从而串行的删除任务,可以避免竞争条件的产生 dispatch_barrier_async(self.barrierQueue, ^{ //通过token的url获取到这个Operation SDWebImageDownloaderOperation *operation = self.URLOperations[token.url]; //调用Operation自定义的cancel方法来取消任务,传入一个回调块字典的token BOOL canceled = [operation cancel:token.downloadOperationCancelToken]; //如果取消了就从字典中移除掉这个键值对 if (canceled) { [self.URLOperations removeObjectForKey:token.url]; } }); } /* 前面download方法调用的方法,返回一个token createCallback就是download方法写的,用于创建一个SDWebImageDownloaderOperation */ - (nullable SDWebImageDownloadToken *)addProgressCallback:(SDWebImageDownloaderProgressBlock)progressBlock completedBlock:(SDWebImageDownloaderCompletedBlock)completedBlock forURL:(nullable NSURL *)url createCallback:(SDWebImageDownloaderOperation *(^)(void))createCallback { //URL为nil就调用下载完成回调块,返回nil if (url == nil) { if (completedBlock != nil) { completedBlock(nil, nil, nil, NO); } return nil; } //定义一个token变量 __block SDWebImageDownloadToken *token = nil; //同步方法,阻塞当前线程也阻塞队列,防止产生竞争条件 dispatch_barrier_sync(self.barrierQueue, ^{ //通过URL获取Operation SDWebImageDownloaderOperation *operation = self.URLOperations[url]; if (!operation) { //如果URL对应的Operation不存在就调用,createCallback块创建一个 operation = createCallback(); //添加进字典中 self.URLOperations[url] = operation; __weak SDWebImageDownloaderOperation *woperation = operation; //设置Operation下载完成的回调块 operation.completionBlock = ^{ //同步方法阻塞当前线程阻塞队列 dispatch_barrier_sync(self.barrierQueue, ^{ //先weak后strong SDWebImageDownloaderOperation *soperation = woperation; if (!soperation) return; //下载完成就从字典中删除 if (self.URLOperations[url] == soperation) { [self.URLOperations removeObjectForKey:url]; }; }); }; } //取消下载任务时的token,第一个值就是url,第二个值就是回调块字典 id downloadOperationCancelToken = [operation addHandlersForProgress:progressBlock completed:completedBlock]; token = [SDWebImageDownloadToken new]; token.url = url; token.downloadOperationCancelToken = downloadOperationCancelToken; }); return token; }
上面的方法也比较容易懂,不再赘述了。
//设置是否挂起下载队列 - (void)setSuspended:(BOOL)suspended { self.downloadQueue.suspended = suspended; } //取消所有的下载任务 - (void)cancelAllDownloads { [self.downloadQueue cancelAllOperations]; } #pragma mark Helper methods //通过NSURLSessionTask找到NSOPerationQueue里的任务 - (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; } #pragma mark NSURLSessionDataDelegate //下面这些代理方法,都是先通过NSURLSessionTask找到对应的SDWebImageDownloaderOperation //然后调用Operation的代理方法, - (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveResponse:(NSURLResponse *)response completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler { // Identify the operation that runs this task and pass it the delegate method SDWebImageDownloaderOperation *dataOperation = [self operationWithTask:dataTask]; [dataOperation URLSession:session dataTask:dataTask didReceiveResponse:response completionHandler:completionHandler]; } - (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data { // Identify the operation that runs this task and pass it the delegate method SDWebImageDownloaderOperation *dataOperation = [self operationWithTask:dataTask]; [dataOperation URLSession:session dataTask:dataTask didReceiveData:data]; } - (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask willCacheResponse:(NSCachedURLResponse *)proposedResponse completionHandler:(void (^)(NSCachedURLResponse *cachedResponse))completionHandler { // Identify the operation that runs this task and pass it the delegate method SDWebImageDownloaderOperation *dataOperation = [self operationWithTask:dataTask]; [dataOperation URLSession:session dataTask:dataTask willCacheResponse:proposedResponse completionHandler:completionHandler]; } #pragma mark NSURLSessionTaskDelegate - (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error { // Identify the operation that runs this task and pass it the delegate method SDWebImageDownloaderOperation *dataOperation = [self operationWithTask:task]; [dataOperation URLSession:session task:task didCompleteWithError:error]; } - (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task willPerformHTTPRedirection:(NSHTTPURLResponse *)response newRequest:(NSURLRequest *)request completionHandler:(void (^)(NSURLRequest * _Nullable))completionHandler { completionHandler(request); } - (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler { // Identify the operation that runs this task and pass it the delegate method SDWebImageDownloaderOperation *dataOperation = [self operationWithTask:task]; [dataOperation URLSession:session task:task didReceiveChallenge:challenge completionHandler:completionHandler]; } @end
这里的代理方法运用比较值得学习吧,
SDWebImageDownloaderOperation需要传入一个
NSURLSession对象,但是这个对象不一定可用,如果不可用
SDWebImageDownloaderOperation就会自己创建一个
NSURLSession对象,但如果它可用,那
SDWebImageDownloaderOperation就不能接收回调方法,所以,本类在回调方法中直接调用
SDWebImageDownloaderOperation的回调方法。
经过两篇文章的源码讲解,
SDWebImage关于图片下载的部分也就全部讲解完了,主要使用了自定义
NSOperation子类,并在这个自定义
NSOperation子类中通过一个可用的
NSURLSession来创建一个执行服务器交互数据的
NSURLSessionDataTask的下载任务,并由其全权负责下载工作,接着使用
NSOperationQueue实现多线程的多图片下载。源码中值得我们学习的地方有很多,比如,在设计第三方库时要设计全面的通知,为了防止竞争条件可以使用一个串行队列或是barrier方法来执行一些可能会产生多线程异常的代码,还有很多设计代码的细节需要我们自行体会。
备注
由于作者水平有限,难免出现纰漏,如有问题还请不吝赐教。相关文章推荐
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