SDWebImage 4.X 源码分析 （一）

SDWebImage UML 类图

SDWebImage 流程图

1.UIImageView+WebCache

接口分离原则：为特定功能提供特定的接口，不要使用单一的总接口包括所有功能，而是应该根据功能把这些接口分割，减少依赖，不能强迫用户去依赖那些他们不使用的接口。

- (void)sd_setImageWithURL:(nullable NSURL *)url
                 completed:(nullable SDExternalCompletionBlock)completedBlock;

- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options
                 completed:(nullable SDExternalCompletionBlock)completedBlock;

- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options
                  progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                 completed:(nullable SDExternalCompletionBlock)completedBlock;

2.UIView+WebCache

- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
                  placeholderImage:(nullable UIImage *)placeholder
                           options:(SDWebImageOptions)options
                      operationKey:(nullable NSString *)operationKey
                     setImageBlock:(nullable SDSetImageBlock)setImageBlock
                          progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                         completed:(nullable SDExternalCompletionBlock)completedBlock
                           context:(nullable NSDictionary *)context
{
1\. 取消当前正在进行的加载任务 operation
2\. 将url作为属性存起来 set sd_imageURL
3\. 设置 占位图 
4\. 如果 URL 存在，通过url加载图片，如果url为nil，直接回调 completedBlock
5\. 设置加载到的图片 ，然后回调 completedBlock 
}

3.SDWebImageManager

枚举
//图片获取和设置的选项
typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
    SDWebImageRetryFailed = 1 << 0,
    SDWebImageLowPriority = 1 << 1,
    SDWebImageCacheMemoryOnly = 1 << 2,
    SDWebImageProgressiveDownload = 1 << 3,
    SDWebImageRefreshCached = 1 << 4,
    SDWebImageContinueInBackground = 1 << 5,
    SDWebImageHandleCookies = 1 << 6,
    SDWebImageAllowInvalidSSLCertificates = 1 << 7,
    SDWebImageHighPriority = 1 << 8,
    SDWebImageDelayPlaceholder = 1 << 9,
    SDWebImageTransformAnimatedImage = 1 << 10,
    SDWebImageAvoidAutoSetImage = 1 << 11,
    SDWebImageScaleDownLargeImages = 1 << 12,
    SDWebImageQueryDataWhenInMemory = 1 << 13,
    SDWebImageQueryDiskSync = 1 << 14,
    SDWebImageFromCacheOnly = 1 << 15,
    SDWebImageForceTransition = 1 << 16
};

.h文件中的属性

@property (weak, nonatomic, nullable) id  delegate;
@property (strong, nonatomic, readonly, nullable) SDImageCache *imageCache;
@property (strong, nonatomic, readonly, nullable) SDWebImageDownloader *imageDownloader;
@property (nonatomic, copy, nullable) SDWebImageCacheKeyFilterBlock cacheKeyFilter;

.h文件中的方法

+ (nonnull instancetype)sharedManager;

//加载图片
- (nullable id )loadImageWithURL:(nullable NSURL *)url
                                              options:(SDWebImageOptions)options
                                             progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                            completed:(nullable SDInternalCompletionBlock)completedBlock;

//保存图片到缓存中
- (void)saveImageToCache:(nullable UIImage *)image forURL:(nullable NSURL *)url;

//取消当前的operations
- (void)cancelAll;
//检查operations是否在进行
- (BOOL)isRunning;

//检查图片是否存在在缓存里
- (void)cachedImageExistsForURL:(nullable NSURL *)url
                     completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock;
- (void)diskImageExistsForURL:(nullable NSURL *)url
                   completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock;

//根据url返回缓存的key
 (nullable NSString *)cacheKeyForURL:(nullable NSURL *)url;

.m文件中的属性

@property (strong, nonatomic, readwrite, nonnull) SDImageCache *imageCache;
@property (strong, nonatomic, readwrite, nonnull) SDWebImageDownloader *imageDownloader;
//失败的url数组
@property (strong, nonatomic, nonnull) NSMutableSet *failedURLs;
//当前执行的任务数组
@property (strong, nonatomic, nonnull) NSMutableArray *runningOperations;

.m文件中的方法

//初始化
+ (nonnull instancetype)sharedManager;
- (nonnull instancetype)init
- (nonnull instancetype)initWithCache:(nonnull SDImageCache *)cache downloader:(nonnull SDWebImageDownloader *)downloader

- (nullable NSString *)cacheKeyForURL:(nullable NSURL *)url

//缩放图片
- (nullable UIImage *)scaledImageForKey:(nullable NSString *)key image:(nullable UIImage *)image

//查找图片
- (void)cachedImageExistsForURL:(nullable NSURL *)url
                     completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock
- (void)diskImageExistsForURL:(nullable NSURL *)url
                   completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock

//加载图片
- (id )loadImageWithURL:(nullable NSURL *)url
                                     options:(SDWebImageOptions)options
                                    progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                   completed:(nullable SDInternalCompletionBlock)completedBlock

//缓存图片
- (void)saveImageToCache:(nullable UIImage *)image forURL:(nullable NSURL *)url

//operation 相关
- (void)cancelAll
- (BOOL)isRunning

//从runningOperations中移除operation
- (void)safelyRemoveOperationFromRunning:(nullable SDWebImageCombinedOperation*)operation

//主线程回调
- (void)callCompletionBlockForOperation:(nullable SDWebImageCombinedOperation*)operation
                             completion:(nullable SDInternalCompletionBlock)completionBlock
                                  error:(nullable NSError *)error
                                    url:(nullable NSURL *)url 
- (void)callCompletionBlockForOperation:(nullable SDWebImageCombinedOperation*)operation
                             completion:(nullable SDInternalCompletionBlock)completionBlock
                                  image:(nullable UIImage *)image
                                   data:(nullable NSData *)data
                                  error:(nullable NSError *)error
                              cacheType:(SDImageCacheType)cacheType
                               finished:(BOOL)finished
                                    url:(nullable NSURL *)url

SDWebImageManager的核心任务是loadImageWithURL方法实现的：

- (id )loadImageWithURL:(nullable NSURL *)url
                                     options:(SDWebImageOptions)options
                                    progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                   completed:(nullable SDInternalCompletionBlock)completedBlock

{
1.错误检查
2.创建 SDWebImageCombinedOperation 对象
3.判断是否是曾经下载失败过的 url
4.如果这个 url 的长度为0，或者曾经下载失败过，并且没有设置 SDWebImageRetryFailed，就直回调 completedBlock，并且直接返回
5.添加 operation 到 runningOperations 中
6.读取缓存
6.1如果有缓存就返回 operation
6.2如果没有就下载图片，并缓存，然后返回 operation
}