SDWebImage 4.x版本源碼分析（二）SDWebImageManager

我們接下來，看 SDWebImage 具體是怎么實現(xiàn)的?？梢詠磉@里下載一下源碼注釋

五、詳細的類的解析和該類的流程

根據(jù)方法調(diào)用的流程來看源碼

1.UIImageView+WebCache

使用類別的設計，把設計模式五大原則之一的接口分離原則體現(xiàn)的淋漓盡致。首先說一下什么是接口分離原則：

接口分離原則：為特定功能提供特定的接口，不要使用單一的總接口包括所有功能，而是應該根據(jù)功能把這些接口分割，減少依賴，不能強迫用戶去依賴那些他們不使用的接口。

我們使用的UIImageView+WebCache中提供了多個不同的方法，這樣調(diào)用者需要什么功能就去調(diào)用特定的API，清晰易擴展.

- (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;

而最終該類別中的總接口又調(diào)用了UIView+WebCache中的方法，所以我們下面來看一下UIView+WebCache。

2.UIView+WebCache

問題：

①.為什么要取消加載任務？
②.dispatch_group_t 在這里的作用是什么？
③.setNeedLayout的作用是什么？

我們來看一下sd_internalSetImageWithURL方法里做了哪些事。

- (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<NSString *, id> *)context
{
1\. 取消當前正在進行的加載任務 operation
2\. 將url作為屬性存起來 set sd_imageURL
3\. 設置 占位圖 
4\. 如果 URL 存在，通過url加載圖片，如果url為nil，直接回調(diào) completedBlock
5\. 設置加載到的圖片 ，然后回調(diào) completedBlock 
}
流程圖：
image.png

3.SDWebImageManager

問題：

①.SDWebImageCombinedOperation的用途？
②.SDScaledImageForKey的作用?
③.weak-strong dance的用途？

核心類

枚舉
//圖片獲取和設置的選項
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文件中的屬性

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

.m文件中的屬性

@property (strong, nonatomic, readwrite, nonnull) SDImageCache *imageCache;
@property (strong, nonatomic, readwrite, nonnull) SDWebImageDownloader *imageDownloader;
//失敗的url數(shù)組
@property (strong, nonatomic, nonnull) NSMutableSet<NSURL *> *failedURLs;
//當前執(zhí)行的任務數(shù)組
@property (strong, nonatomic, nonnull) NSMutableArray<SDWebImageCombinedOperation *> *runningOperations;

.h文件中的方法

+ (nonnull instancetype)sharedManager;

//加載圖片
- (nullable id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
                                              options:(SDWebImageOptions)options
                                             progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                            completed:(nullable SDInternalCompletionBlock)completedBlock;

//保存圖片到緩存中
- (void)saveImageToCache:(nullable UIImage *)image forURL:(nullable NSURL *)url;

//取消當前的operations
- (void)cancelAll;
//檢查operations是否在進行
- (BOOL)isRunning;

//檢查圖片是否存在在緩存里
- (void)cachedImageExistsForURL:(nullable NSURL *)url
                     completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock;
- (void)diskImageExistsForURL:(nullable NSURL *)url
                   completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock;

//根據(jù)url返回緩存的key
 (nullable NSString *)cacheKeyForURL:(nullable NSURL *)url;

.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

//縮放圖片
- (nullable UIImage *)scaledImageForKey:(nullable NSString *)key image:(nullable UIImage *)image

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

//加載圖片
- (id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
                                     options:(SDWebImageOptions)options
                                    progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                   completed:(nullable SDInternalCompletionBlock)completedBlock

//緩存圖片
- (void)saveImageToCache:(nullable UIImage *)image forURL:(nullable NSURL *)url

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

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

//主線程回調(diào)
- (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的核心任務是loadImageWithURL方法實現(xiàn)的：

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

{
1.錯誤檢查
2.創(chuàng)建 SDWebImageCombinedOperation 對象
3.判斷是否是曾經(jīng)下載失敗過的 url
4.如果這個 url 的長度為0，或者曾經(jīng)下載失敗過，并且沒有設置 SDWebImageRetryFailed，就直回調(diào) completedBlock，并且直接返回
5.添加 operation 到 runningOperations 中
6.讀取緩存
6.1如果有緩存就返回 operation
6.2如果沒有就下載圖片，并緩存，然后返回 operation
}
流程圖：
SDWebImageManager.png
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