该文章阅读的SDWebImage的版本为4.3.3。
这个类是下载器类，管理着图像的下载。
1.公共枚举
/**
这个位枚举定义了下载器的可选项
*/
typedef NS_OPTIONS(NSUInteger, SDWebImageDownloaderOptions) {
/**
低优先级
*/
SDWebImageDownloaderLowPriority = 1 << 0,
/**
渐进式下载
*/
SDWebImageDownloaderProgressiveDownload = 1 << 1,
/**
默认情况下，请求阻止使用NSURLCache。设置这个选项就会使用NSURLCache
*/
SDWebImageDownloaderUseNSURLCache = 1 << 2,
/**
这个选项要与上面那个选项配合使用，如果图像是从NSURLCache读取的，
那么回调block中返回的image和imageData参数就是nil
*/
SDWebImageDownloaderIgnoreCachedResponse = 1 << 3,
/**
应用进入后台时继续下载
*/
SDWebImageDownloaderContinueInBackground = 1 << 4,
/**
通过设置NSMutableURLRequest.HTTPShouldHandleCookies = YES获取保存在NSHTTPCookieStore中的cookies
*/
SDWebImageDownloaderHandleCookies = 1 << 5,
/**
允许接受不受信任的SSL证书
*/
SDWebImageDownloaderAllowInvalidSSLCertificates = 1 << 6,
/**
高优先级
*/
SDWebImageDownloaderHighPriority = 1 << 7,
/**
缩小图片
*/
SDWebImageDownloaderScaleDownLargeImages = 1 << 8,
};
/**
这个枚举定义了下载执行顺序
*/
typedef NS_ENUM(NSInteger, SDWebImageDownloaderExecutionOrder) {
/**
先进先出，默认情况
*/
SDWebImageDownloaderFIFOExecutionOrder,
/**
后进先出
*/
SDWebImageDownloaderLIFOExecutionOrder
};
2.公共常量
/**
下载开始通知名称
*/
FOUNDATION_EXPORT NSString * _Nonnull const SDWebImageDownloadStartNotification;
/**
下载停止通知名称
*/
FOUNDATION_EXPORT NSString * _Nonnull const SDWebImageDownloadStopNotification;
3.公共类型定义
/**
下载进度回调block
*/
typedef void(^SDWebImageDownloaderProgressBlock)(NSInteger receivedSize, NSInteger expectedSize, NSURL * _Nullable targetURL);
/**
下载完成回调block
*/
typedef void(^SDWebImageDownloaderCompletedBlock)(UIImage * _Nullable image, NSData * _Nullable data, NSError * _Nullable error, BOOL finished);
/**
HTTP请求头字典
*/
typedef NSDictionary<NSString *, NSString *> SDHTTPHeadersDictionary;
/**
HTTP请求头可变字典
*/
typedef NSMutableDictionary<NSString *, NSString *> SDHTTPHeadersMutableDictionary;
/**
HTTP请求头过滤算法block
*/
typedef SDHTTPHeadersDictionary * _Nullable (^SDWebImageDownloaderHeadersFilterBlock)(NSURL * _Nullable url, SDHTTPHeadersDictionary * _Nullable headers);
4.SDWebImageDownloadToken类
4.1 公共属性
/**
用于下载的url
*/
@property (nonatomic, strong, nullable) NSURL *url;
/**
用于取消的token
*/
@property (nonatomic, strong, nullable) id downloadOperationCancelToken;
4.2 类扩展属性
/**
下载操作对象
*/
@property (nonatomic, weak, nullable) NSOperation<SDWebImageDownloaderOperationInterface> *downloadOperation;
4.3 实现
- (void)cancel {
// 有下载操作对象
if (self.downloadOperation) {
// 有取消token
SDWebImageDownloadToken *cancelToken = self.downloadOperationCancelToken;
if (cancelToken) {
// 取消下载操作
[self.downloadOperation cancel:cancelToken];
}
}
}
5.公共属性
/**
解压图像可以提高性能，但会占用大量内存。
默认为YES。如果由于过多的内存消耗而遇到崩溃，请将此项设置为NO。
*/
@property (assign, nonatomic) BOOL shouldDecompressImages;
/**
最大并发下载数量
*/
@property (assign, nonatomic) NSInteger maxConcurrentDownloads;
/**
仍需要下载的载量
*/
@property (readonly, nonatomic) NSUInteger currentDownloadCount;
/**
下载请求超时的时间，默认15秒
*/
@property (assign, nonatomic) NSTimeInterval downloadTimeout;
/**
会话配置对象
*/
@property (readonly, nonatomic, nonnull) NSURLSessionConfiguration *sessionConfiguration;
/**
下载顺序
*/
@property (assign, nonatomic) SDWebImageDownloaderExecutionOrder executionOrder;
/**
请求操作的证书
*/
@property (strong, nonatomic, nullable) NSURLCredential *urlCredential;
/**
证书的用户名
*/
@property (strong, nonatomic, nullable) NSString *username;
/**
证书的密码
*/
@property (strong, nonatomic, nullable) NSString *password;
/**
为HTTP请求头设置过滤器
*/
@property (nonatomic, copy, nullable) SDWebImageDownloaderHeadersFilterBlock headersFilter;
6.公共方法
/**
获取单例对象
*/
+ (nonnull instancetype)sharedDownloader;
/**
以指定会话配置对象初始化
*/
- (nonnull instancetype)initWithSessionConfiguration:(nullable NSURLSessionConfiguration *)sessionConfiguration NS_DESIGNATED_INITIALIZER;
/**
为HTTP请求头添加字段
*/
- (void)setValue:(nullable NSString *)value forHTTPHeaderField:(nullable NSString *)field;
/**
获取HTTP请求头指定字段的值
*/
- (nullable NSString *)valueForHTTPHeaderField:(nullable NSString *)field;
/**
设置下载操作对象
*/
- (void)setOperationClass:(nullable Class)operationClass;
/**
下载指定url的图像
*/
- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
options:(SDWebImageDownloaderOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock;
/**
取消指定token的操作
*/
- (void)cancel:(nullable SDWebImageDownloadToken *)token;
/**
设置下载的暂停状态
*/
- (void)setSuspended:(BOOL)suspended;
/**
取消所有的下载
*/
- (void)cancelAllDownloads;
/**
以指定会话配置对象创建新的会话对象
*/
- (void)createNewSessionWithConfiguration:(nonnull NSURLSessionConfiguration *)sessionConfiguration;
/**
使会话对象无效并取消任务
*/
- (void)invalidateSessionAndCancel:(BOOL)cancelPendingOperations;
7.私有宏
/**
利用GCD的信号量实现加锁的效果
*/
#define LOCK(lock) dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER);
/**
利用GCD的信号量实现解锁的效果
*/
#define UNLOCK(lock) dispatch_semaphore_signal(lock);
8.类扩展属性
/**
下载操作队列
*/
@property (strong, nonatomic, nonnull) NSOperationQueue *downloadQueue;
/**
最后添加的操作对象
*/
@property (weak, nonatomic, nullable) NSOperation *lastAddedOperation;
/**
下载操作类
*/
@property (assign, nonatomic, nullable) Class operationClass;
/**
保存url和其对应的操作对象
*/
@property (strong, nonatomic, nonnull) NSMutableDictionary<NSURL *, SDWebImageDownloaderOperation *> *URLOperations;
/**
HTTP请求头
*/
@property (strong, nonatomic, nullable) SDHTTPHeadersMutableDictionary *HTTPHeaders;
/**
用于保证对属性URLOperations操作线程安全的锁
*/
@property (strong, nonatomic, nonnull) dispatch_semaphore_t operationsLock;
/**
用于保证对属性HTTPHeaders操作线程安全的锁
*/
@property (strong, nonatomic, nonnull) dispatch_semaphore_t headersLock;
/**
会话对象
*/
@property (strong, nonatomic) NSURLSession *session;
9.实现
9.1 生命周期方法
+ (void)initialize {
// 如果获取到了SDNetworkActivityIndicator类
if (NSClassFromString(@"SDNetworkActivityIndicator")) {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
// 获取SDNetworkActivityIndicator类的单例对象
id activityIndicator = [NSClassFromString(@"SDNetworkActivityIndicator") performSelector:NSSelectorFromString(@"sharedActivityIndicator")];
#pragma clang diagnostic pop
// 移除之前添加的观察者
[[NSNotificationCenter defaultCenter] removeObserver:activityIndicator name:SDWebImageDownloadStartNotification object:nil];
[[NSNotificationCenter defaultCenter] removeObserver:activityIndicator name:SDWebImageDownloadStopNotification object:nil];
// 添加观察者
[[NSNotificationCenter defaultCenter] addObserver:activityIndicator
selector:NSSelectorFromString(@"startActivity")
name:SDWebImageDownloadStartNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:activityIndicator
selector:NSSelectorFromString(@"stopActivity")
name:SDWebImageDownloadStopNotification object:nil];
}
}
- (nonnull instancetype)init {
// 以默认会话配置对象初始化
return [self initWithSessionConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]];
}
- (void)dealloc {
// 取消任务并使会话对象无效
[self.session invalidateAndCancel];
// 保存会话对象的属性置空
self.session = nil;
// 取消操作队列中所有操作
[self.downloadQueue cancelAllOperations];
}
9.2 私有方法
/**
添加下载操作
*/
- (nullable SDWebImageDownloadToken *)addProgressCallback:(SDWebImageDownloaderProgressBlock)progressBlock
completedBlock:(SDWebImageDownloaderCompletedBlock)completedBlock
forURL:(nullable NSURL *)url
createCallback:(SDWebImageDownloaderOperation *(^)(void))createCallback {
// url是必传的
if (url == nil) {
if (completedBlock != nil) {
completedBlock(nil, nil, nil, NO);
}
return nil;
}
// 加锁
LOCK(self.operationsLock);
// 从缓存中获取url对应的操作对象
SDWebImageDownloaderOperation *operation = [self.URLOperations objectForKey:url];
// 如果没获取到，防止同一图像重复下载
if (!operation) {
// 从参数中获取操作对象
operation = createCallback();
__weak typeof(self) wself = self;
operation.completionBlock = ^{
// 操作完成时，从缓存中移除该url对应的操作对象
__strong typeof(wself) sself = wself;
if (!sself) {
return;
}
LOCK(sself.operationsLock);
[sself.URLOperations removeObjectForKey:url];
UNLOCK(sself.operationsLock);
};
// 缓存url和其对应的操作对象
[self.URLOperations setObject:operation forKey:url];
// 将操作对象添加到操作队列中
[self.downloadQueue addOperation:operation];
}
// 解锁
UNLOCK(self.operationsLock);
// 生成下载操作取消token
id downloadOperationCancelToken = [operation addHandlersForProgress:progressBlock completed:completedBlock];
// 生成下载token
SDWebImageDownloadToken *token = [SDWebImageDownloadToken new];
token.downloadOperation = operation;
token.url = url;
token.downloadOperationCancelToken = downloadOperationCancelToken;
// 返回token
return token;
}
/**
获取操作队列中task对应的下载操作对象
*/
- (SDWebImageDownloaderOperation *)operationWithTask:(NSURLSessionTask *)task {
// 创建变量奥村下载操作对象
SDWebImageDownloaderOperation *returnOperation = nil;
// 遍历找到task对应的下载操作对象
for (SDWebImageDownloaderOperation *operation in self.downloadQueue.operations) {
if (operation.dataTask.taskIdentifier == task.taskIdentifier) {
returnOperation = operation;
break;
}
}
// 返回下载操作对象
return returnOperation;
}
9.3 公共方法
+ (nonnull instancetype)sharedDownloader {
// 获取该类的单例对象
static dispatch_once_t once;
static id instance;
dispatch_once(&once, ^{
instance = [self new];
});
return instance;
}
- (nonnull instancetype)initWithSessionConfiguration:(nullable NSURLSessionConfiguration *)sessionConfiguration {
if ((self = [super init])) {
// 操作对象类默认是SDWebImageDownloaderOperation
_operationClass = [SDWebImageDownloaderOperation class];
// 默认是解压缩图像
_shouldDecompressImages = YES;
// 默认顺序是先进先出
_executionOrder = SDWebImageDownloaderFIFOExecutionOrder;
// 初始化操作队列对象
_downloadQueue = [NSOperationQueue new];
// 默认最大并发下载数为6
_downloadQueue.maxConcurrentOperationCount = 6;
// 设置操作队列对象的名称
_downloadQueue.name = @"com.hackemist.SDWebImageDownloader";
// 初始化字典
_URLOperations = [NSMutableDictionary new];
#ifdef SD_WEBP
// 如果有webp格式的图像的HTTP请求头
_HTTPHeaders = [@{@"Accept": @"image/webp,image/*;q=0.8"} mutableCopy];
#else
// 如果没有webp格式的图像的HTTP请求头
_HTTPHeaders = [@{@"Accept": @"image/*;q=0.8"} mutableCopy];
#endif
// 利用GCD的信号量初始化锁
_operationsLock = dispatch_semaphore_create(1);
// 利用GCD的信号量初始化锁
_headersLock = dispatch_semaphore_create(1);
// 默认15秒下载请求超时
_downloadTimeout = 15.0;
// 创建会话对象
[self createNewSessionWithConfiguration:sessionConfiguration];
}
return self;
}
- (void)createNewSessionWithConfiguration:(NSURLSessionConfiguration *)sessionConfiguration {
// 取消所有下载
[self cancelAllDownloads];
// 如果当前有会话就对象就取消会话对象的任务并使会话对象无效
if (self.session) {
[self.session invalidateAndCancel];
}
// 设置会话配置对象的请求超时时间
sessionConfiguration.timeoutIntervalForRequest = self.downloadTimeout;
// 创建会话对象
self.session = [NSURLSession sessionWithConfiguration:sessionConfiguration
delegate:self
delegateQueue:nil];
}
- (void)invalidateSessionAndCancel:(BOOL)cancelPendingOperations {
// 当前类是SDWebImageDownloader的单例对象就不继续向下执行
if (self == [SDWebImageDownloader sharedDownloader]) {
return;
}
if (cancelPendingOperations) {
// 取消任务并使会话对象无效
[self.session invalidateAndCancel];
} else {
// 等待任务完成并使会话对象无效
[self.session finishTasksAndInvalidate];
}
}
- (void)setValue:(nullable NSString *)value forHTTPHeaderField:(nullable NSString *)field {
// 加锁
LOCK(self.headersLock);
if (value) {
// 如果传入了值，就给指定字段赋值
self.HTTPHeaders[field] = value;
} else {
// 如果未传值，就删除指定字段
[self.HTTPHeaders removeObjectForKey:field];
}
// 解锁
UNLOCK(self.headersLock);
}
- (nullable NSString *)valueForHTTPHeaderField:(nullable NSString *)field {
// 获取HTTP头中指定字段的值
if (!field) {
return nil;
}
return [[self allHTTPHeaderFields] objectForKey:field];
}
- (void)setOperationClass:(nullable Class)operationClass {
// 操作对象类必须是NSOperation类的子类，并且遵守了SDWebImageDownloaderOperationInterface协议，否则就设置为SDWebImageDownloaderOperation类
if (operationClass && [operationClass isSubclassOfClass:[NSOperation class]] && [operationClass conformsToProtocol:@protocol(SDWebImageDownloaderOperationInterface)]) {
_operationClass = operationClass;
} else {
_operationClass = [SDWebImageDownloaderOperation class];
}
}
- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
options:(SDWebImageDownloaderOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock {
__weak SDWebImageDownloader *wself = self;
// 添加任务相关参数
return [self addProgressCallback:progressBlock completedBlock:completedBlock forURL:url createCallback:^SDWebImageDownloaderOperation *{
__strong __typeof (wself) sself = wself;
// 获取下载请求超时时间，默认15秒
NSTimeInterval timeoutInterval = sself.downloadTimeout;
if (timeoutInterval == 0.0) {
timeoutInterval = 15.0;
}
// 根据设置的选项不同，设置不同的请求缓存策略
NSURLRequestCachePolicy cachePolicy = options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData;
// 创建请求对象
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url
cachePolicy:cachePolicy
timeoutInterval:timeoutInterval];
// Cookies处理方式
request.HTTPShouldHandleCookies = (options & SDWebImageDownloaderHandleCookies);
// 默认使用管线化
request.HTTPShouldUsePipelining = YES;
// 设置HTTP请求头字段
if (sself.headersFilter) {
request.allHTTPHeaderFields = sself.headersFilter(url, [sself allHTTPHeaderFields]);
}
else {
request.allHTTPHeaderFields = [sself allHTTPHeaderFields];
}
// 创建下载操作对象
SDWebImageDownloaderOperation *operation = [[sself.operationClass alloc] initWithRequest:request inSession:sself.session options:options];
// 设置下载操作对象是否解压缩
operation.shouldDecompressImages = sself.shouldDecompressImages;
// 设置下载操作对象的证书
if (sself.urlCredential) {
operation.credential = sself.urlCredential;
} else if (sself.username && sself.password) {
operation.credential = [NSURLCredential credentialWithUser:sself.username password:sself.password persistence:NSURLCredentialPersistenceForSession];
}
// 设置下载操作对象的优先级
if (options & SDWebImageDownloaderHighPriority) {
operation.queuePriority = NSOperationQueuePriorityHigh;
} else if (options & SDWebImageDownloaderLowPriority) {
operation.queuePriority = NSOperationQueuePriorityLow;
}
// 如果设置了后进先出就通过添加依赖的方式实现
if (sself.executionOrder == SDWebImageDownloaderLIFOExecutionOrder) {
[sself.lastAddedOperation addDependency:operation];
sself.lastAddedOperation = operation;
}
// 将生成的操作对象作为参数传递
return operation;
}];
}
- (void)cancel:(nullable SDWebImageDownloadToken *)token {
// 获取token中的url，如果没有就不继续执行了
NSURL *url = token.url;
if (!url) {
return;
}
// 加锁
LOCK(self.operationsLock);
// 获取到url对应的操作对象
SDWebImageDownloaderOperation *operation = [self.URLOperations objectForKey:url];
// 取消操作
if (operation) {
BOOL canceled = [operation cancel:token.downloadOperationCancelToken];
// 移除url对应的操作对象
if (canceled) {
[self.URLOperations removeObjectForKey:url];
}
}
// 解锁
UNLOCK(self.operationsLock);
}
- (void)setSuspended:(BOOL)suspended {
// 设置暂停状态
self.downloadQueue.suspended = suspended;
}
- (void)cancelAllDownloads {
// 取消所有操作
[self.downloadQueue cancelAllOperations];
}
9.4 自定义getting/setting
- (void)setMaxConcurrentDownloads:(NSInteger)maxConcurrentDownloads {
// 直接给下载操作队列对象的最大并发数赋值
_downloadQueue.maxConcurrentOperationCount = maxConcurrentDownloads;
}
- (NSInteger)maxConcurrentDownloads {
// 返回下载操作队列对象的最大并发数
return _downloadQueue.maxConcurrentOperationCount;
}
- (NSUInteger)currentDownloadCount {
// 返回当前下载操作队列中操作对象数量
return _downloadQueue.operationCount;
}
- (NSURLSessionConfiguration *)sessionConfiguration {
// 返回会话对象配置对象
return self.session.configuration;
}
9.5 NSURLSessionDataDelegate方法实现
- (void)URLSession:(NSURLSession *)session
dataTask:(NSURLSessionDataTask *)dataTask
didReceiveResponse:(NSURLResponse *)response
completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler {
// 如果调用这个代理方法的dataTask是SDWebImageDownloaderOperation生成的，就将代理方法实现转交给SDWebImageDownloaderOperation类对象去实现
SDWebImageDownloaderOperation *dataOperation = [self operationWithTask:dataTask];
if ([dataOperation respondsToSelector:@selector(URLSession:dataTask:didReceiveResponse:completionHandler:)]) {
[dataOperation URLSession:session dataTask:dataTask didReceiveResponse:response completionHandler:completionHandler];
} else {
// 如果不是的话就默认响应意向为允许
if (completionHandler) {
completionHandler(NSURLSessionResponseAllow);
}
}
}
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data {
// 如果调用这个代理方法的dataTask是SDWebImageDownloaderOperation生成的，就将代理方法实现转交给SDWebImageDownloaderOperation类对象去实现
SDWebImageDownloaderOperation *dataOperation = [self operationWithTask:dataTask];
if ([dataOperation respondsToSelector:@selector(URLSession:dataTask:didReceiveData:)]) {
[dataOperation URLSession:session dataTask:dataTask didReceiveData:data];
}
}
- (void)URLSession:(NSURLSession *)session
dataTask:(NSURLSessionDataTask *)dataTask
willCacheResponse:(NSCachedURLResponse *)proposedResponse
completionHandler:(void (^)(NSCachedURLResponse *cachedResponse))completionHandler {
// 如果调用这个代理方法的dataTask是SDWebImageDownloaderOperation生成的，就将代理方法实现转交给SDWebImageDownloaderOperation类对象去实现
SDWebImageDownloaderOperation *dataOperation = [self operationWithTask:dataTask];
if ([dataOperation respondsToSelector:@selector(URLSession:dataTask:willCacheResponse:completionHandler:)]) {
[dataOperation URLSession:session dataTask:dataTask willCacheResponse:proposedResponse completionHandler:completionHandler];
} else {
if (completionHandler) {
completionHandler(proposedResponse);
}
}
}
9.6 NSURLSessionTaskDelegate方法实现
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
// 如果调用这个代理方法的dataTask是SDWebImageDownloaderOperation生成的，就将代理方法实现转交给SDWebImageDownloaderOperation类对象去实现
SDWebImageDownloaderOperation *dataOperation = [self operationWithTask:task];
if ([dataOperation respondsToSelector:@selector(URLSession:task:didCompleteWithError:)]) {
[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 {
// 如果调用这个代理方法的dataTask是SDWebImageDownloaderOperation生成的，就将代理方法实现转交给SDWebImageDownloaderOperation类对象去实现
SDWebImageDownloaderOperation *dataOperation = [self operationWithTask:task];
if ([dataOperation respondsToSelector:@selector(URLSession:task:willPerformHTTPRedirection:newRequest:completionHandler:)]) {
[dataOperation URLSession:session task:task willPerformHTTPRedirection:response newRequest:request completionHandler:completionHandler];
} else {
if (completionHandler) {
completionHandler(request);
}
}
}
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler {
// 如果调用这个代理方法的dataTask是SDWebImageDownloaderOperation生成的，就将代理方法实现转交给SDWebImageDownloaderOperation类对象去实现
SDWebImageDownloaderOperation *dataOperation = [self operationWithTask:task];
if ([dataOperation respondsToSelector:@selector(URLSession:task:didReceiveChallenge:completionHandler:)]) {
[dataOperation URLSession:session task:task didReceiveChallenge:challenge completionHandler:completionHandler];
} else {
if (completionHandler) {
completionHandler(NSURLSessionAuthChallengePerformDefaultHandling, nil);
}
}
}
10.总结
这个类从字面上理解是一个下载器类，实际看代码，发现这个类并没有做实际的下载，而是维护着一个NSURLSession对象和一个NSOperationQueue对象，管理着一些SDWebImageDownloaderOperation对象。
SDWebImageDownloader通过传入的参数生成一个SDWebImageDownloaderOperation对象，并添加到操作队列中。具体的网络下载操作则由SDWebImageDownloaderOperation对象自己处理。
源码阅读系列：SDWebImage
源码阅读：SDWebImage（二）——SDWebImageCompat
源码阅读：SDWebImage（三）——NSData+ImageContentType
源码阅读：SDWebImage（四）——SDWebImageCoder
源码阅读：SDWebImage（五）——SDWebImageFrame
源码阅读：SDWebImage（六）——SDWebImageCoderHelper
源码阅读：SDWebImage（七）——SDWebImageImageIOCoder
源码阅读：SDWebImage（八）——SDWebImageGIFCoder
源码阅读：SDWebImage（九）——SDWebImageCodersManager
源码阅读：SDWebImage（十）——SDImageCacheConfig
源码阅读：SDWebImage（十一）——SDImageCache
源码阅读：SDWebImage（十二）——SDWebImageDownloaderOperation
源码阅读：SDWebImage（十三）——SDWebImageDownloader
源码阅读：SDWebImage（十四）——SDWebImageManager
源码阅读：SDWebImage（十五）——SDWebImagePrefetcher
源码阅读：SDWebImage（十六）——SDWebImageTransition
源码阅读：SDWebImage（十七）——UIView+WebCacheOperation
源码阅读：SDWebImage（十八）——UIView+WebCache
源码阅读：SDWebImage（十九）——UIImage+ForceDecode/UIImage+GIF/UIImage+MultiFormat
源码阅读：SDWebImage（二十）——UIButton+WebCache
源码阅读：SDWebImage（二十一）——UIImageView+WebCache/UIImageView+HighlightedWebCache
