前言

SDWebImageManager 是 SDWebImage中最核心的類了，但是源代碼卻是非常簡單的。之所以能做到這一點，都歸功于功能的良好分類。

有了SDWebImageManager這個基石，我們就能做很多其他的有意思的事情。比如給各種view綁定一個URL，就能顯示圖片的功能，有了Options，就能滿足多種應用場景的圖片下載任務。

讀源碼既能讓我們更好地使用該框架，又能讓我們學到很多知識，還能讓我們懂得如何去擴充現(xiàn)有的功能。

SDWebImageOptions

SDWebImageOptions作為下載的選項提供了非常多的子項，用法和注意事項我都寫在代碼的注釋中了：

typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
/**
 * By default, when a URL fail to be downloaded, the URL is blacklisted so the library won't keep trying.
 * This flag disable this blacklisting.
 */
/// 每一個下載都會提供一個URL，如果這個URL是錯誤，SD就會把它放入到黑名單之中，
/// 黑名單中的URL是不會再次進行下載的,但是，當設置了該選項時，SD會將其在黑名單中移除，重新下載該URL，
SDWebImageRetryFailed = 1 << 0,

/**
 * By default, image downloads are started during UI interactions, this flags disable this feature,
 * leading to delayed download on UIScrollView deceleration for instance.
 */
/// 一般來說，下載都是按照一定的先后順序開始的，但是該選項能夠延遲下載，也就說他的權(quán)限比較低，權(quán)限比他高的在他前邊下載
SDWebImageLowPriority = 1 << 1,

/**
 * This flag disables on-disk caching
 */
/// 該選項要求SD只把圖片緩存到內(nèi)存中，不緩存到disk中
SDWebImageCacheMemoryOnly = 1 << 2,

/**
 * This flag enables progressive download, the image is displayed progressively during download as a browser would do.
 * By default, the image is only displayed once completely downloaded.
 */
/// 給下載添加進度
SDWebImageProgressiveDownload = 1 << 3,

/**
 * Even if the image is cached, respect the HTTP response cache control, and refresh the image from remote location if needed.
 * The disk caching will be handled by NSURLCache instead of SDWebImage leading to slight performance degradation.
 * This option helps deal with images changing behind the same request URL, e.g. Facebook graph api profile pics.
 * If a cached image is refreshed, the completion block is called once with the cached image and again with the final image.
 *
 * Use this flag only if you can't make your URLs static with embedded cache busting parameter.
 */
/// 有這么一種使用場景，如果一個圖片的資源發(fā)生了改變。但是url并沒有變，我們就可以使用該選項來刷新數(shù)據(jù)了
SDWebImageRefreshCached = 1 << 4,

/**
 * In iOS 4+, continue the download of the image if the app goes to background. This is achieved by asking the system for
 * extra time in background to let the request finish. If the background task expires the operation will be cancelled.
 */
/// 支持切換到后臺也能下載
SDWebImageContinueInBackground = 1 << 5,

/**
 * Handles cookies stored in NSHTTPCookieStore by setting
 * NSMutableURLRequest.HTTPShouldHandleCookies = YES;
 */
/// 使用Cookies
SDWebImageHandleCookies = 1 << 6,

/**
 * Enable to allow untrusted SSL certificates.
 * Useful for testing purposes. Use with caution in production.
 */
/// 允許驗證證書
SDWebImageAllowInvalidSSLCertificates = 1 << 7,

/**
 * By default, images are loaded in the order in which they were queued. This flag moves them to
 * the front of the queue.
 */
/// 高權(quán)限
SDWebImageHighPriority = 1 << 8,

/**
 * By default, placeholder images are loaded while the image is loading. This flag will delay the loading
 * of the placeholder image until after the image has finished loading.
 */
/// 一般情況下，placeholder image 都會在圖片下載完成前顯示，該選項將設置placeholder image在下載完成之后才能顯示
SDWebImageDelayPlaceholder = 1 << 9,

/**
 * We usually don't call transformDownloadedImage delegate method on animated images,
 * as most transformation code would mangle it.
 * Use this flag to transform them anyway.
 */
/// 使用該屬性來自由改變圖片，但需要使用transformDownloadedImage delegate
SDWebImageTransformAnimatedImage = 1 << 10,

/**
 * By default, image is added to the imageView after download. But in some cases, we want to
 * have the hand before setting the image (apply a filter or add it with cross-fade animation for instance)
 * Use this flag if you want to manually set the image in the completion when success
 */
/// 該選項允許我們在圖片下載完成后不會立刻給view設置圖片，比較常用的使用場景是給賦值的圖片添加動畫
SDWebImageAvoidAutoSetImage = 1 << 11,

/**
 * By default, images are decoded respecting their original size. On iOS, this flag will scale down the
 * images to a size compatible with the constrained memory of devices.
 * If `SDWebImageProgressiveDownload` flag is set the scale down is deactivated.
 */
/// 壓縮大圖片
SDWebImageScaleDownLargeImages = 1 << 12
};

Block

typedef void(^SDExternalCompletionBlock)(UIImage * _Nullable image, NSError * _Nullable error, SDImageCacheType cacheType, NSURL * _Nullable imageURL);

typedef void(^SDInternalCompletionBlock)(UIImage * _Nullable image, NSData * _Nullable data, NSError * _Nullable error, SDImageCacheType cacheType, BOOL finished, NSURL * _Nullable imageURL);

typedef NSString * _Nullable (^SDWebImageCacheKeyFilterBlock)(NSURL * _Nullable url);

我們在平時的開發(fā)中，使用Block可以參考上邊的使用方法，XXXxxxCompletionBlock這種命名應該是Apple的風格。

SDWebImageManagerDelegate

使用協(xié)議增加了我們編程的靈活性，SDWebImageManagerDelegate提供了兩個方法

• 在緩存中沒發(fā)現(xiàn)圖片，控制是不是下載該圖片
• 自由轉(zhuǎn)換圖片

代碼：

@protocol SDWebImageManagerDelegate <NSObject>

@optional

/**
 * Controls which image should be downloaded when the image is not found in the cache.
 *
 * @param imageManager The current `SDWebImageManager`
 * @param imageURL     The url of the image to be downloaded
 *
 * @return Return NO to prevent the downloading of the image on cache misses. If not implemented, YES is implied.
 */
- (BOOL)imageManager:(nonnull SDWebImageManager *)imageManager shouldDownloadImageForURL:(nullable NSURL *)imageURL;

/**
 * Allows to transform the image immediately after it has been downloaded and just before to cache it on disk and memory.
 * NOTE: This method is called from a global queue in order to not to block the main thread.
 *
 * @param imageManager The current `SDWebImageManager`
 * @param image        The image to transform
 * @param imageURL     The url of the image to transform
 *
 * @return The transformed image object.
 */
- (nullable UIImage *)imageManager:(nonnull SDWebImageManager *)imageManager transformDownloadedImage:(nullable UIImage *)image withURL:(nullable NSURL *)imageURL;

@end

SDWebImageCombinedOperation

SDWebImageCombinedOperation是對每一個下載任務的封裝，重要的是它提供了一個取消功能。

@interface SDWebImageCombinedOperation : NSObject <SDWebImageOperation>

@property (assign, nonatomic, getter = isCancelled) BOOL cancelled;
@property (copy, nonatomic, nullable) SDWebImageNoParamsBlock cancelBlock;
@property (strong, nonatomic, nullable) NSOperation *cacheOperation;

@end

SDWebImageManager

1.屬性

@interface SDWebImageManager ()

@property (strong, nonatomic, readwrite, nonnull) SDImageCache *imageCache;
@property (strong, nonatomic, readwrite, nonnull) SDWebImageDownloader *imageDownloader;
@property (strong, nonatomic, nonnull) NSMutableSet<NSURL *> *failedURLs;
@property (strong, nonatomic, nonnull) NSMutableArray<SDWebImageCombinedOperation *> *runningOperations;

@end

2.初始化（非常典型的初始化）

+ (nonnull instancetype)sharedManager {
static dispatch_once_t once;
static id instance;
dispatch_once(&once, ^{
    instance = [self new];
});
return instance;
}

- (nonnull instancetype)init {
SDImageCache *cache = [SDImageCache sharedImageCache];
SDWebImageDownloader *downloader = [SDWebImageDownloader sharedDownloader];
return [self initWithCache:cache downloader:downloader];
}

- (nonnull instancetype)initWithCache:(nonnull SDImageCache *)cache downloader:(nonnull SDWebImageDownloader *)downloader {
if ((self = [super init])) {
    _imageCache = cache;
    _imageDownloader = downloader;
    _failedURLs = [NSMutableSet new];
    _runningOperations = [NSMutableArray new];
}
return self;
}

3.URL => key

- (nullable NSString *)cacheKeyForURL:(nullable NSURL *)url {
if (!url) {
    return @"";
}

if (self.cacheKeyFilter) {
    return self.cacheKeyFilter(url);
} else {
    return url.absoluteString;
  }
}

4.查看圖片是否已經(jīng)緩存

現(xiàn)在內(nèi)容中讀取，沒有的話再去disk讀取。

- (void)cachedImageExistsForURL:(nullable NSURL *)url
                 completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock {
NSString *key = [self cacheKeyForURL:url];

BOOL isInMemoryCache = ([self.imageCache imageFromMemoryCacheForKey:key] != nil);

if (isInMemoryCache) {
    // making sure we call the completion block on the main queue
    dispatch_async(dispatch_get_main_queue(), ^{
        if (completionBlock) {
            completionBlock(YES);
        }
    });
    return;
}

[self.imageCache diskImageExistsWithKey:key completion:^(BOOL isInDiskCache) {
    // the completion block of checkDiskCacheForImageWithKey:completion: is always called on the main queue, no need to further dispatch
    if (completionBlock) {
        completionBlock(isInDiskCache);
    }
}];
}

5.查看是否已經(jīng)緩存到了硬盤

- (void)diskImageExistsForURL:(nullable NSURL *)url
               completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock {
NSString *key = [self cacheKeyForURL:url];

[self.imageCache diskImageExistsWithKey:key completion:^(BOOL isInDiskCache) {
    // the completion block of checkDiskCacheForImageWithKey:completion: is always called on the main queue, no need to further dispatch
    if (completionBlock) {
        completionBlock(isInDiskCache);
    }
}];
}

6.核心下載方法

對于一個比較復雜的函數(shù)，我們往往只需要左下邊三件事就可以了：

• 處理參數(shù)相關(guān)的異常
• 處理復雜的邏輯
• 返回數(shù)據(jù)

這三條適合所有的函數(shù)，由于這個方法的邏輯比較復雜，我把詳細的注釋都寫到代碼中了

- (id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
                                 options:(SDWebImageOptions)options
                                progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                               completed:(nullable SDInternalCompletionBlock)completedBlock {
// Invoking this method without a completedBlock is pointless
/// 1.如果想預先下載圖片，使用[SDWebImagePrefetcher prefetchURLs]取代本方法
/// 預下載圖片是有很多種使用場景的，當我們使用SDWebImagePrefetcher下載圖片后，之后使用該圖片時就不用再網(wǎng)絡上下載了。
NSAssert(completedBlock != nil, @"If you mean to prefetch the image, use -[SDWebImagePrefetcher prefetchURLs] instead");

// Very common mistake is to send the URL using NSString object instead of NSURL. For some strange reason, XCode won't
// throw any warning for this type mismatch. Here we failsafe this error by allowing URLs to be passed as NSString.
/// 2.XCode有時候經(jīng)常會犯一些錯誤，當用戶給url賦值了字符串的時候，XCode也沒有報錯，因此這里提供一種
/// 錯誤修正的處理。
if ([url isKindOfClass:NSString.class]) {
    url = [NSURL URLWithString:(NSString *)url];
}

// Prevents app crashing on argument type error like sending NSNull instead of NSURL
/// 3.防止參數(shù)的其他錯誤
if (![url isKindOfClass:NSURL.class]) {
    url = nil;
}

/// 4.operation會被作為該方法的返回值，但operation的類型是SDWebImageCombinedOperation，是一個封裝的對象，并不是一個NSOperation
__block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
__weak SDWebImageCombinedOperation *weakOperation = operation;

/// 5.在圖片的下載中，會有一些下載失敗的情況，這時候我們把這些下載失敗的url放到一個集合中去，
/// 也就是加入了黑名單中，默認是不會再繼續(xù)下載黑名單中的url了，但是也有例外，當options被設置為
/// SDWebImageRetryFailed的時候，會嘗試進行重新下載。
BOOL isFailedUrl = NO;
if (url) {
    @synchronized (self.failedURLs) {
        isFailedUrl = [self.failedURLs containsObject:url];
    }
}

/// 6.會有兩種情況讓我們停止下載這個url指定的圖片：
/// - url的長度為0
/// - options并沒有選擇SDWebImageRetryFailed(重新下載錯誤url)且這個url在黑名單之中
/// 調(diào)用完成Block，返回operation
if (url.absoluteString.length == 0 || (!(options & SDWebImageRetryFailed) && isFailedUrl)) {
    [self callCompletionBlockForOperation:operation completion:completedBlock error:[NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorFileDoesNotExist userInfo:nil] url:url];
    return operation;
}

/// 7.排除了所有的錯誤可能后，我們就先把這個operation添加到正在運行操作的數(shù)組中
/// 這里沒有判斷self.runningOperations是不是包含了operation，
/// 說明肯定會在下邊的代碼中做判斷，如果存在就刪除operation
@synchronized (self.runningOperations) {
    [self.runningOperations addObject:operation];
}
NSString *key = [self cacheKeyForURL:url];

/// 8.self.imageCache的queryCacheOperationForKey方法是異步的獲取指定key的圖片，
/// 但是這個方法的operation是同步返回的，也就是說下邊的代碼會直接執(zhí)行到return那里。
operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
    /// (8.1).這個Block會在查詢完指定的key的圖片后調(diào)用，由` dispatch_async(self.ioQueue, ^{`
    /// 這個可以看出，實在異步線程采用串行的方式在調(diào)用，任務在self.imageCache的ioQueue中一個一個執(zhí)行，是線程安全的

    /// (8.2).如果每次調(diào)用loadImage方法都會生成一個operation，如果我們想取消某個下載任務
    /// 再設計上來說，只要把響應的operation.isCancelled設置為NO，那么下載就會被取消。
    if (operation.isCancelled) {
        [self safelyRemoveOperationFromRunning:operation];
        return;
    }

    /// (8.3).代碼來到這里，我們就要根據(jù)是否有緩存的圖片來做出響應的處理
    /// 如果沒有獲取到緩存圖片或者需要刷新緩存圖片我們應該通過網(wǎng)絡獲取圖片，但是這里增加了一個額外的控制
    /// 根據(jù)delegate的imageManager:shouldDownloadImageForURL:獲取是否下載的權(quán)限，返回YES，就繼續(xù)下載
    if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
        /// (8.3.1).這里需要注意了，當圖片已經(jīng)下載了，Options又選擇了SDWebImageRefreshCached
        /// 就會觸發(fā)一次completionBlock回調(diào)，這說明這個下載的回調(diào)不是只觸發(fā)一次的
        /// 如果使用了dispatch_group_enter和dispatch_group_leave就一定要注意了
        if (cachedImage && options & SDWebImageRefreshCached) {
            // If image was found in the cache but SDWebImageRefreshCached is provided, notify about the cached image
            // AND try to re-download it in order to let a chance to NSURLCache to refresh it from server.
            [self callCompletionBlockForOperation:weakOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
        }

        // download if no image or requested to refresh anyway, and download allowed by delegate
        /// (8.3.2).這里是SDWebImageOptions到SDWebImageDownloaderOptions的轉(zhuǎn)換
        /// 其實就是0 |= xxx
        SDWebImageDownloaderOptions downloaderOptions = 0;
        if (options & SDWebImageLowPriority) downloaderOptions |= SDWebImageDownloaderLowPriority;
        if (options & SDWebImageProgressiveDownload) downloaderOptions |= SDWebImageDownloaderProgressiveDownload;
        if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderUseNSURLCache;
        if (options & SDWebImageContinueInBackground) downloaderOptions |= SDWebImageDownloaderContinueInBackground;
        if (options & SDWebImageHandleCookies) downloaderOptions |= SDWebImageDownloaderHandleCookies;
        if (options & SDWebImageAllowInvalidSSLCertificates) downloaderOptions |= SDWebImageDownloaderAllowInvalidSSLCertificates;
        if (options & SDWebImageHighPriority) downloaderOptions |= SDWebImageDownloaderHighPriority;
        if (options & SDWebImageScaleDownLargeImages) downloaderOptions |= SDWebImageDownloaderScaleDownLargeImages;

        /// (8.3.3).已經(jīng)緩存且SDWebImageRefreshCached的比較特殊
        if (cachedImage && options & SDWebImageRefreshCached) {
            // force progressive off if image already cached but forced refreshing
            /// (8.3.3.1).SDWebImageDownloaderProgressiveDownload為 1<<1 ,
            /*
                由于當options == SDWebImageRefreshCached時，downloaderOptions |= SDWebImageDownloaderUseNSURLCache（1 << 2）
                00000000 | 00000100 =>  00000100
                ~SDWebImageDownloaderProgressiveDownload : ~ 00000010 => 111111101
                00000100 & 11111101 => 00000100
             */
            downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;
            // ignore image read from NSURLCache if image if cached but force refreshing
            /// (8.3.3.2). 00000100 | 00001000 => 00001100
            /// 通過這種位的運算，就能夠給同一個值賦值兩種轉(zhuǎn)態(tài)
            downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
        }

        /// (8.3.4).下載圖片
        SDWebImageDownloadToken *subOperationToken = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
            __strong __typeof(weakOperation) strongOperation = weakOperation;
            if (!strongOperation || strongOperation.isCancelled) {
                // Do nothing if the operation was cancelled
                // See #699 for more details
                // if we would call the completedBlock, there could be a race condition between this block and another completedBlock for the same object, so if this one is called second, we will overwrite the new data
            } else if (error) {
                /// (8.3.4.1).發(fā)生錯誤就返回
                [self callCompletionBlockForOperation:strongOperation completion:completedBlock error:error url:url];

                /// (8.3.4.2).除了下邊這幾種情況之外的情況則把url加入黑名單
                if (   error.code != NSURLErrorNotConnectedToInternet
                    && error.code != NSURLErrorCancelled
                    && error.code != NSURLErrorTimedOut
                    && error.code != NSURLErrorInternationalRoamingOff
                    && error.code != NSURLErrorDataNotAllowed
                    && error.code != NSURLErrorCannotFindHost
                    && error.code != NSURLErrorCannotConnectToHost) {
                    @synchronized (self.failedURLs) {
                        [self.failedURLs addObject:url];
                    }
                }
            }
            else {

                /// (8.3.4.3).如果是SDWebImageRetryFailed就在黑名單中移除，不管有沒有
                if ((options & SDWebImageRetryFailed)) {
                    @synchronized (self.failedURLs) {
                        [self.failedURLs removeObject:url];
                    }
                }

                BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);

                if (options & SDWebImageRefreshCached && cachedImage && !downloadedImage) {
                    // Image refresh hit the NSURLCache cache, do not call the completion block
                } else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) { // 要不要修改圖片，這個修改圖片完全由代理來操作
                    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
                        UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url];

                        if (transformedImage && finished) {
                            BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
                            // pass nil if the image was transformed, so we can recalculate the data from the image
                            [self.imageCache storeImage:transformedImage imageData:(imageWasTransformed ? nil : downloadedData) forKey:key toDisk:cacheOnDisk completion:nil];
                        }

                        [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:transformedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                    });
                } else {
                    if (downloadedImage && finished) {
                        [self.imageCache storeImage:downloadedImage imageData:downloadedData forKey:key toDisk:cacheOnDisk completion:nil];
                    }
                    [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:downloadedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                }
            }

            /// (8.3.4.4).完成了就把該任務在正運行著的數(shù)組中刪除
            if (finished) {
                [self safelyRemoveOperationFromRunning:strongOperation];
            }
        }];

        /// (8.3.5).設置取消的回調(diào)
        operation.cancelBlock = ^{
            [self.imageDownloader cancel:subOperationToken];
            __strong __typeof(weakOperation) strongOperation = weakOperation;
            [self safelyRemoveOperationFromRunning:strongOperation];
        };
    } else if (cachedImage) {
        __strong __typeof(weakOperation) strongOperation = weakOperation;
        [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
        [self safelyRemoveOperationFromRunning:operation];
    } else {
        // Image not in cache and download disallowed by delegate
        /// (8.4).既沒有緩存也下載了代理不允許的圖片
        __strong __typeof(weakOperation) strongOperation = weakOperation;
        [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:nil data:nil error:nil cacheType:SDImageCacheTypeNone finished:YES url:url];
        [self safelyRemoveOperationFromRunning:operation];
    }
}];

return operation;
}

7.保存圖片到緩存區(qū)

- (void)saveImageToCache:(nullable UIImage *)image forURL:(nullable NSURL *)url {
if (image && url) {
    NSString *key = [self cacheKeyForURL:url];
    [self.imageCache storeImage:image forKey:key toDisk:YES completion:nil];
}
}

8.取消所有的下載

- (void)cancelAll {
@synchronized (self.runningOperations) {
    NSArray<SDWebImageCombinedOperation *> *copiedOperations = [self.runningOperations copy];
    [copiedOperations makeObjectsPerformSelector:@selector(cancel)];
    [self.runningOperations removeObjectsInArray:copiedOperations];
}
}

9.查看是否下載完畢

- (BOOL)isRunning {
BOOL isRunning = NO;
@synchronized (self.runningOperations) {
    isRunning = (self.runningOperations.count > 0);
}
return isRunning;
}

10.安全移除任務

要想安全的操作數(shù)組，只要給數(shù)組本身加個鎖就行了，但@synchronized的性能不是很好

- (void)safelyRemoveOperationFromRunning:(nullable SDWebImageCombinedOperation*)operation {
@synchronized (self.runningOperations) {
    if (operation) {
        [self.runningOperations removeObject:operation];
    }
}
}

11.回調(diào)

- (void)callCompletionBlockForOperation:(nullable SDWebImageCombinedOperation*)operation
                         completion:(nullable SDInternalCompletionBlock)completionBlock
                              error:(nullable NSError *)error
                                url:(nullable NSURL *)url {
[self callCompletionBlockForOperation:operation completion:completionBlock image:nil data:nil error:error cacheType:SDImageCacheTypeNone finished:YES url: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 {
dispatch_main_async_safe(^{
    if (operation && !operation.isCancelled && completionBlock) {
        completionBlock(image, data, error, cacheType, finished, url);
    }
});
}

12.SDWebImageCombinedOperation實現(xiàn)方法

- (void)setCancelBlock:(nullable SDWebImageNoParamsBlock)cancelBlock {
// check if the operation is already cancelled, then we just call the cancelBlock
if (self.isCancelled) {
    if (cancelBlock) {
        cancelBlock();
    }
    _cancelBlock = nil; // don't forget to nil the cancelBlock, otherwise we will get crashes
} else {
    _cancelBlock = [cancelBlock copy];
}
}

- (void)cancel {
self.cancelled = YES;
if (self.cacheOperation) {
    [self.cacheOperation cancel];
    self.cacheOperation = nil;
}
if (self.cancelBlock) {
    self.cancelBlock();

    // TODO: this is a temporary fix to #809.
    // Until we can figure the exact cause of the crash, going with the ivar instead of the setter
    //        self.cancelBlock = nil;
    _cancelBlock = nil;
}
}

總結(jié)

SD的源碼讀了大部分了，也收獲很大，現(xiàn)在寫代碼的風格也越來越靠近這種框架風格，我們應該為寫出的每一個函數(shù)負責。

在上一篇SDWebImage源碼解讀之SDWebImageDownloader的評論區(qū)，有小伙伴提出了SD在特定使用場景會崩潰的情況，我也做了一些實驗。

- (void)test {
NSURL *url = [NSURL URLWithString:@"http://upload-images.jianshu.io/upload_images/1432482-dcc38746f56a89ab.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240"];

SDWebImageManager *manager = [SDWebImageManager sharedManager];

dispatch_group_t group = dispatch_group_create();

dispatch_group_enter(group);
[manager loadImageWithURL:url options:SDWebImageRefreshCached progress:nil completed:^(UIImage * _Nullable image, NSData * _Nullable data, NSError * _Nullable error, SDImageCacheType cacheType, BOOL finished, NSURL * _Nullable imageURL) {
    dispatch_group_leave(group);
}];

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    NSLog(@"下載完成了");
});
}

在上邊的函數(shù)中，我使用了dispatch_group_t dispatch_group_enter dispatch_group_leave目的是等待所有的異步任務完成。

enter和leave方法必須成對出現(xiàn)，如果調(diào)用leave的次數(shù)多于enter就會崩潰，當我們使用SD時，如果Options設置為SDWebImageRefreshCached，那么這個completionBlock至少會調(diào)用兩次，首先返回緩存中的圖片。其次在下載完成后再次調(diào)用Block，這也就是崩潰的原因。

要想重現(xiàn)上邊方法的崩潰，等圖片下載完之后，再從新調(diào)用該方法就行。

SD默認下載的圖片會緩存進內(nèi)存和disk中。