SDWebImage 4.x版本源码分析（三）SDWebImageDownloader

可以来这里下载一下源码注释

4.SDWebImageDownloader

问题：

①. clang diagnostic 是什么？
②. 添加监听前，为什么要先移除监听？
③. 为什么用单例？何时需要用单例？？？
④. SDWebImageDownloadToken的用途？

枚举

//下载选项
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,
};

//下载执行的顺序
typedef NS_ENUM(NSInteger, SDWebImageDownloaderExecutionOrder) {
    SDWebImageDownloaderFIFOExecutionOrder,
    SDWebImageDownloaderLIFOExecutionOrder
};

.h文件中的属性

 //是否解压图片
@property (assign, nonatomic) BOOL shouldDecompressImages;
 //设置最大并发数量
@property (assign, nonatomic) NSInteger maxConcurrentDownloads;
 //获取当前下载的数量
@property (readonly, nonatomic) NSUInteger currentDownloadCount;
//下载时间 默认15秒
@property (assign, nonatomic) NSTimeInterval downloadTimeout;
 //NSURLSession配置一些请求所需要的策略
@property (readonly, nonatomic, nonnull) NSURLSessionConfiguration *sessionConfiguration;
 //设置下载顺序
@property (assign, nonatomic) SDWebImageDownloaderExecutionOrder executionOrder;

 //生成单例 并返回当前实例
+ (nonnull instancetype)sharedDownloader;

//设置身份认证
@property (strong, nonatomic, nullable) NSURLCredential *urlCredential;
 //设置用户名
@property (strong, nonatomic, nullable) NSString *username;
//设置密码
@property (strong, nonatomic, nullable) NSString *password;

 //针对header进行过滤的block
@property (nonatomic, copy, nullable) SDWebImageDownloaderHeadersFilterBlock headersFilter;

.m中的属性

//定义下载队列
@property (strong, nonatomic, nonnull) NSOperationQueue *downloadQueue;
//定义下载的上个operation 作用是为了后面的下载依赖
@property (weak, nonatomic, nullable) NSOperation *lastAddedOperation;

// 图片下载操作类
@property (assign, nonatomic, nullable) Class operationClass;
//下载url作为key value是具体的下载operation 用字典来存储，方便cancel等操作
@property (strong, nonatomic, nonnull) NSMutableDictionary *URLOperations;

//HTTP请求头
@property (strong, nonatomic, nullable) SDHTTPHeadersMutableDictionary *HTTPHeaders;

//保证线程安全设置的信号量
@property (strong, nonatomic, nonnull) dispatch_semaphore_t operationsLock; // a lock to keep the access to `URLOperations` thread-safe
@property (strong, nonatomic, nonnull) dispatch_semaphore_t headersLock; // a lock to keep the access to `HTTPHeaders` thread-safe

// The session in which data tasks will run
@property (strong, nonatomic) NSURLSession *session;

.h中的方法

//生成一个实例，利用特定的配置sessionConfiguration
- (nonnull instancetype)initWithSessionConfiguration:(nullable NSURLSessionConfiguration *)sessionConfiguration NS_DESIGNATED_INITIALIZER;

 //针对request添加HTTP header
- (void)setValue:(nullable NSString *)value forHTTPHeaderField:(nullable NSString *)field;

 //返回某个header field的value
- (nullable NSString *)valueForHTTPHeaderField:(nullable NSString *)field;

 //设置一个SDWebImageDownloaderOperation的子类赋值给_operationClass
- (void)setOperationClass:(nullable Class)operationClass;

//根据指定的url异步加载图片
- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
                                                   options:(SDWebImageDownloaderOptions)options
                                                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                                 completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock;

//取消指定token的下载
- (void)cancel:(nullable SDWebImageDownloadToken *)token;
 //设置下载队列是否挂起
- (void)setSuspended:(BOOL)suspended;
 //取消所有的下载
- (void)cancelAllDownloads;

 //强制给self设置一个新的NSURLSession
- (void)createNewSessionWithConfiguration:(nonnull NSURLSessionConfiguration *)sessionConfiguration;

 //取消operation并且session设置为Invalidates
- (void)invalidateSessionAndCancel:(BOOL)cancelPendingOperations;

.m中的方法

//init和dealloc
+ (void)initialize
+ (nonnull instancetype)sharedDownloader
- (nonnull instancetype)init
- (nonnull instancetype)initWithSessionConfiguration:(nullable NSURLSessionConfiguration *)sessionConfiguration
- (void)createNewSessionWithConfiguration:(NSURLSessionConfiguration *)sessionConfiguration
- (void)invalidateSessionAndCancel:(BOOL)cancelPendingOperations
- (void)dealloc

//setter和getter方法
- (void)setValue:(nullable NSString *)value forHTTPHeaderField:(nullable NSString *)field
- (nullable NSString *)valueForHTTPHeaderField:(nullable NSString *)field
- (nonnull SDHTTPHeadersDictionary *)allHTTPHeaderFields
- (void)setMaxConcurrentDownloads:(NSInteger)maxConcurrentDownloads
- (NSUInteger)currentDownloadCount
- (NSInteger)maxConcurrentDownloads
- (NSURLSessionConfiguration *)sessionConfiguration
- (void)setOperationClass:(nullable Class)operationClass

//下载和取消下载
- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
                                                   options:(SDWebImageDownloaderOptions)options
                                                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                                 completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock
- (void)cancel:(nullable SDWebImageDownloadToken *)token
- (nullable SDWebImageDownloadToken *)addProgressCallback:(SDWebImageDownloaderProgressBlock)progressBlock
                                           completedBlock:(SDWebImageDownloaderCompletedBlock)completedBlock
                                                   forURL:(nullable NSURL *)url
                                           createCallback:(SDWebImageDownloaderOperation *(^)(void))createCallback
- (void)setSuspended:(BOOL)suspended
- (void)cancelAllDownloads
- (SDWebImageDownloaderOperation *)operationWithTask:(NSURLSessionTask *)task

这个类具体做了什么呢，首先看下+ (void)initialize方法

+ (void)initialize
{
    if (NSClassFromString(@"SDNetworkActivityIndicator")) {
        //clang diagnostic 是什么用途？？？

        //为了让 SDNetworkActivityIndicator 文件可以不用导入项目中来（如果不需要的话），这里使用了 runtime 的方式来实现动态创建类以及调用方法，如果导入了这个类，就能设置 Network Activity Indicator，如果没有导入，就不用管他，但也能编译通过
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"

        id activityIndicator = [NSClassFromString(@"SDNetworkActivityIndicator") performSelector:NSSelectorFromString(@"sharedActivityIndicator")];

#pragma clang diagnostic pop

        //移除通知观察者。以防它之前被添加。 
        //再添加通知观察者。
    }
}

+shareDownloader 方法调用- init创建了一个单例，在-init方法中,调用了-initWithSessionConfiguration和-createNewSessionWithConfiguration来进行一些初始化和默认值设置，如 创建session，设置默认下载超时时长 15s，OperationQueue的最大并发数为6等

SDWebImageDownloader中，最核心的方法是downloadImageWithURLoptions 方法，我们来具体看下

- (nullable SDWebImageDownloadToken *)downloadImageWithURL:(nullable NSURL *)url
                                                   options:(SDWebImageDownloaderOptions)options
                                                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                                 completed:(nullable SDWebImageDownloaderCompletedBlock)completedBlock 
{
   1.调用addProgressCallback方法 return token，addProgressCallback的回调里进行以下操作
    {
        1.1设置下载时间
        1.2创建request
        1.3创建operation对象 传入 request session options
        1.4设置身份认证
        1.5设置下载优先级
        1.6设置下载顺序
    }
}

原来这个方法里只调用了addProgressCallback方法，那么 addProgressCallback 究竟做了哪些事情？

- (nullable SDWebImageDownloadToken *)addProgressCallback:(SDWebImageDownloaderProgressBlock)progressBlock
                                           completedBlock:(SDWebImageDownloaderCompletedBlock)completedBlock
                                                   forURL:(nullable NSURL *)url
                                           createCallback:(SDWebImageDownloaderOperation *(^)(void))createCallback 
{
    1.生成URLOperations字典 下载url作为key value是具体的下载operation
    2.将操作添加到操作队列中
    3.将进度progressBlock和下载结束completedBlock封装成字典SDCallbacksDictionary，装入数组callbackBlocks，
    4.生成token标识，并返回token
}

流程图：

image.png