.h文件

/**
 * 目前緩存里沒有.
 */
SDImageCacheTypeNone,
/**
 * 磁盤緩存.
 */
SDImageCacheTypeDisk,
/**
 * 內(nèi)存緩存.
 */
SDImageCacheTypeMemory

// 查詢緩存圖片完成的回調(diào)block
typedef void(^SDCacheQueryCompletedBlock)(UIImage * _Nullable image, NSData * _Nullable data, SDImageCacheType cacheType);

// 查詢某張圖片是否被緩存的回調(diào)block
typedef void(^SDWebImageCheckCacheCompletionBlock)(BOOL isInCache);

// 計(jì)算disk上所有緩存文件的個(gè)數(shù)和總大小
typedef void(^SDWebImageCalculateSizeBlock)(NSUInteger fileCount, NSUInteger totalSize);

磁盤緩存的寫操作是異步的朴摊，不會(huì)導(dǎo)致UI交互上的延遲

/**

• 緩存配置
  */
  @property (nonatomic, nonnull, readonly) SDImageCacheConfig *config;

/**

• 最大的內(nèi)存緩存的開銷
  */
  @property (assign, nonatomic) NSUInteger maxMemoryCost;

/**

• 最大的內(nèi)存緩存的數(shù)量限制.
  */
  @property (assign, nonatomic) NSUInteger maxMemoryCountLimit;

方法名大家看頭文件都很好懂恢准，沒有生硬的單詞。

.m文件

AutoPurgeCache類繼承NSCache卖丸，用此類創(chuàng)建內(nèi)存緩存龄糊。
在收到內(nèi)存警告的通知后扬绪，自動(dòng)清除內(nèi)存緩存奶甘。在該類里不需要再寫removeAllObjects，因?yàn)樵诟割怤SCache里已經(jīng)提供了重父。

// 內(nèi)聯(lián)函數(shù)花椭，算出一個(gè)UIImage的緩存開銷

FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
#if SD_MAC
    return image.size.height * image.size.width;
#elif SD_UIKIT || SD_WATCH
    return image.size.height * image.size.width * image.scale * image.scale;
#endif
}

nonnull修飾的屬性表示不能傳空，nullable修飾的屬性表示可以傳空房午。

默認(rèn)創(chuàng)建的單例命名空間“default”

獲取SD磁盤緩存的路徑 Library/Caches/default/com.hackemist.SDWebImageCache.default/

- (nullable NSString *)makeDiskCachePath:(nonnull NSString*)fullNamespace {
    NSArray<NSString *> *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
    return [paths[0] stringByAppendingPathComponent:fullNamespace];
}

初始化做了以下操作

1.創(chuàng)建了一個(gè)io操作的串行隊(duì)列 
2.創(chuàng)建緩存配置文件矿辽、內(nèi)存緩存實(shí)例
3.生成磁盤緩存的路徑
4.創(chuàng)建_fileManager實(shí)例,后續(xù)對(duì)_fileManager的所有操作都在ioQueue中執(zhí)行
5.監(jiān)聽一些通知，用來清除內(nèi)存緩存或者刪除磁盤上一些舊的文件郭厌。

檢查當(dāng)前隊(duì)列是否是io隊(duì)列

- (void)checkIfQueueIsIOQueue {
    const char *currentQueueLabel = dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL);
    const char *ioQueueLabel = dispatch_queue_get_label(self.ioQueue);
    if (strcmp(currentQueueLabel, ioQueueLabel) != 0) {
        NSLog(@"This method should be called from the ioQueue");
    }
}

緩存文件名生成規(guī)則：對(duì)圖片url的utf8String進(jìn)行md5,鏈接最后如果有擴(kuò)展名會(huì)拼接上擴(kuò)展名

- (nullable NSString *)cachedFileNameForKey:(nullable NSString *)key {
    const char *str = key.UTF8String;
    if (str == NULL) {
        str = "";
    }
    unsigned char r[CC_MD5_DIGEST_LENGTH];
    CC_MD5(str, (CC_LONG)strlen(str), r);
    NSString *filename = [NSString stringWithFormat:@"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%@",
                          r[0], r[1], r[2], r[3], r[4], r[5], r[6], r[7], r[8], r[9], r[10],
                          r[11], r[12], r[13], r[14], r[15], [key.pathExtension isEqualToString:@""] ? @"" : [NSString stringWithFormat:@".%@", key.pathExtension]];

    return filename;
}

從網(wǎng)絡(luò)上下好一張圖片袋倔，緩存圖片的時(shí)候，先判斷是否開啟了內(nèi)存緩存折柠，默認(rèn)是開啟的宾娜，開啟的話，先存到內(nèi)存里扇售，直接存UIImage對(duì)象前塔。
然后判斷是否需要存到磁盤上，如果需要承冰，把圖片的NSData數(shù)據(jù)存放到磁盤上嘱根。

- (void)storeImage:(nullable UIImage *)image
         imageData:(nullable NSData *)imageData
            forKey:(nullable NSString *)key
            toDisk:(BOOL)toDisk
        completion:(nullable SDWebImageNoParamsBlock)completionBlock {
    if (!image || !key) {
        if (completionBlock) {
            completionBlock();
        }
        return;
    }
    // if memory cache is enabled
    if (self.config.shouldCacheImagesInMemory) {
        NSUInteger cost = SDCacheCostForImage(image);
        [self.memCache setObject:image forKey:key cost:cost];
    }
    
    if (toDisk) {
        dispatch_async(self.ioQueue, ^{
            @autoreleasepool {
                NSData *data = imageData;
                if (!data && image) {
                    SDImageFormat imageFormatFromData = [NSData sd_imageFormatForImageData:data];
                    data = [image sd_imageDataAsFormat:imageFormatFromData];
                }                
                [self storeImageDataToDisk:data forKey:key];
            }
            
            if (completionBlock) {
                dispatch_async(dispatch_get_main_queue(), ^{
                    completionBlock();
                });
            }
        });
    } else {
        if (completionBlock) {
            completionBlock();
        }
    }
}

這個(gè)方法就是緩存到磁盤上，重點(diǎn)講一下 [fileURL setResourceValue:@YES forKey:NSURLIsExcludedFromBackupKey error:nil];這句巷懈，這句的意思是防止這個(gè)本地url所對(duì)應(yīng)的文件被備份。

- (void)storeImageDataToDisk:(nullable NSData *)imageData forKey:(nullable NSString *)key {
    if (!imageData || !key) {
        return;
    }
    
    [self checkIfQueueIsIOQueue];
    
    if (![_fileManager fileExistsAtPath:_diskCachePath]) {
        [_fileManager createDirectoryAtPath:_diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL];
    }
    
    // get cache Path for image key
    NSString *cachePathForKey = [self defaultCachePathForKey:key];
    // transform to NSUrl
    NSURL *fileURL = [NSURL fileURLWithPath:cachePathForKey];
    
    [_fileManager createFileAtPath:cachePathForKey contents:imageData attributes:nil];
    
    // disable iCloud backup
    if (self.config.shouldDisableiCloud) {
        [fileURL setResourceValue:@YES forKey:NSURLIsExcludedFromBackupKey error:nil];
    }
}

檢查磁盤上是否有該key對(duì)應(yīng)的緩存

- (void)diskImageExistsWithKey:(nullable NSString *)key completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock {
    dispatch_async(_ioQueue, ^{
        BOOL exists = [_fileManager fileExistsAtPath:[self defaultCachePathForKey:key]];

        // fallback because of https://github.com/rs/SDWebImage/pull/976 that added the extension to the disk file name
        // checking the key with and without the extension
        if (!exists) {
            exists = [_fileManager fileExistsAtPath:[self defaultCachePathForKey:key].stringByDeletingPathExtension];
        }

        if (completionBlock) {
            dispatch_async(dispatch_get_main_queue(), ^{
                completionBlock(exists);
            });
        }
    });
}

取圖片緩存的時(shí)候慌洪，先取內(nèi)存緩存顶燕，取不到凑保，再取磁盤緩存，所以執(zhí)行該方法說明已經(jīng)找過內(nèi)存緩存了涌攻，沒有找到欧引，所以如果從磁盤上能取到該緩存圖片，說明可能是以前沒開啟內(nèi)存緩存功能恳谎，所以在磁盤上取到了之后芝此，判斷現(xiàn)在是開啟內(nèi)存緩存的狀態(tài)的情況下，需要再往內(nèi)存中存一份因痛。

- (nullable UIImage *)imageFromDiskCacheForKey:(nullable NSString *)key {
    UIImage *diskImage = [self diskImageForKey:key];
    if (diskImage && self.config.shouldCacheImagesInMemory) {
        NSUInteger cost = SDCacheCostForImage(diskImage);
        [self.memCache setObject:diskImage forKey:key cost:cost];
    }

    return diskImage;
}

從所有的可能的磁盤緩存路徑上進(jìn)行查找

- (nullable NSData *)diskImageDataBySearchingAllPathsForKey:(nullable NSString *)key {
    NSString *defaultPath = [self defaultCachePathForKey:key];
    NSData *data = [NSData dataWithContentsOfFile:defaultPath];
    if (data) {
        return data;
    }

    // fallback because of https://github.com/rs/SDWebImage/pull/976 that added the extension to the disk file name
    // checking the key with and without the extension
    data = [NSData dataWithContentsOfFile:defaultPath.stringByDeletingPathExtension];
    if (data) {
        return data;
    }

    NSArray<NSString *> *customPaths = [self.customPaths copy];
    for (NSString *path in customPaths) {
        NSString *filePath = [self cachePathForKey:key inPath:path];
        NSData *imageData = [NSData dataWithContentsOfFile:filePath];
        if (imageData) {
            return imageData;
        }

        // fallback because of https://github.com/rs/SDWebImage/pull/976 that added the extension to the disk file name
        // checking the key with and without the extension
        imageData = [NSData dataWithContentsOfFile:filePath.stringByDeletingPathExtension];
        if (imageData) {
            return imageData;
        }
    }

    return nil;
}

轉(zhuǎn)化成對(duì)應(yīng)scale的圖片

- (nullable UIImage *)scaledImageForKey:(nullable NSString *)key image:(nullable UIImage *)image {
    return SDScaledImageForKey(key, image);
}

按照key值進(jìn)行緩存查詢的操作婚苹，返回operation對(duì)象，便于后續(xù)可以進(jìn)行操作的cancel操作等鸵膏。

- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key done:(nullable SDCacheQueryCompletedBlock)doneBlock {
    if (!key) {
        if (doneBlock) {
            doneBlock(nil, nil, SDImageCacheTypeNone);
        }
        return nil;
    }

    // First check the in-memory cache...
    UIImage *image = [self imageFromMemoryCacheForKey:key];
    if (image) {
        NSData *diskData = nil;
        if ([image isGIF]) {
            diskData = [self diskImageDataBySearchingAllPathsForKey:key];
        }
        if (doneBlock) {
            doneBlock(image, diskData, SDImageCacheTypeMemory);
        }
        return nil;
    }

    NSOperation *operation = [NSOperation new];
    dispatch_async(self.ioQueue, ^{
        if (operation.isCancelled) {
            // do not call the completion if cancelled
            return;
        }

        @autoreleasepool {
            NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
            UIImage *diskImage = [self diskImageForKey:key];
            if (diskImage && self.config.shouldCacheImagesInMemory) {
                NSUInteger cost = SDCacheCostForImage(diskImage);
                [self.memCache setObject:diskImage forKey:key cost:cost];
            }

            if (doneBlock) {
                dispatch_async(dispatch_get_main_queue(), ^{
                    doneBlock(diskImage, diskData, SDImageCacheTypeDisk);
                });
            }
        }
    });

    return operation;
}

刪除舊文件的操作膊升，刪除最后修改時(shí)間是7天前的，SD默認(rèn)的最大緩存時(shí)間是7天谭企，然后判斷如果刪除后剩余的磁盤緩存大于允許緩存的最大開銷廓译，那么設(shè)置最大開銷的一半做為閾值執(zhí)行刪除操作。

- (void)deleteOldFilesWithCompletionBlock:(nullable SDWebImageNoParamsBlock)completionBlock {
    dispatch_async(self.ioQueue, ^{
        NSURL *diskCacheURL = [NSURL fileURLWithPath:self.diskCachePath isDirectory:YES];
        NSArray<NSString *> *resourceKeys = @[NSURLIsDirectoryKey, NSURLContentModificationDateKey, NSURLTotalFileAllocatedSizeKey];

        // This enumerator prefetches useful properties for our cache files.
        NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtURL:diskCacheURL
                                                   includingPropertiesForKeys:resourceKeys
                                                                      options:NSDirectoryEnumerationSkipsHiddenFiles
                                                                 errorHandler:NULL];

        NSDate *expirationDate = [NSDate dateWithTimeIntervalSinceNow:-self.config.maxCacheAge];
        NSMutableDictionary<NSURL *, NSDictionary<NSString *, id> *> *cacheFiles = [NSMutableDictionary dictionary];
        NSUInteger currentCacheSize = 0;

        // Enumerate all of the files in the cache directory.  This loop has two purposes:
        //
        //  1. Removing files that are older than the expiration date.
        //  2. Storing file attributes for the size-based cleanup pass.
        NSMutableArray<NSURL *> *urlsToDelete = [[NSMutableArray alloc] init];
        for (NSURL *fileURL in fileEnumerator) {
            NSError *error;
            NSDictionary<NSString *, id> *resourceValues = [fileURL resourceValuesForKeys:resourceKeys error:&error];

            // Skip directories and errors.
            if (error || !resourceValues || [resourceValues[NSURLIsDirectoryKey] boolValue]) {
                continue;
            }

            // Remove files that are older than the expiration date;
            NSDate *modificationDate = resourceValues[NSURLContentModificationDateKey];
            if ([[modificationDate laterDate:expirationDate] isEqualToDate:expirationDate]) {
                [urlsToDelete addObject:fileURL];
                continue;
            }

            // Store a reference to this file and account for its total size.
            NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
            currentCacheSize += totalAllocatedSize.unsignedIntegerValue;
            cacheFiles[fileURL] = resourceValues;
        }
        
        for (NSURL *fileURL in urlsToDelete) {
            [_fileManager removeItemAtURL:fileURL error:nil];
        }

        // If our remaining disk cache exceeds a configured maximum size, perform a second
        // size-based cleanup pass.  We delete the oldest files first.
        if (self.config.maxCacheSize > 0 && currentCacheSize > self.config.maxCacheSize) {
            // Target half of our maximum cache size for this cleanup pass.
            const NSUInteger desiredCacheSize = self.config.maxCacheSize / 2;

            // Sort the remaining cache files by their last modification time (oldest first).
            NSArray<NSURL *> *sortedFiles = [cacheFiles keysSortedByValueWithOptions:NSSortConcurrent
                                                                     usingComparator:^NSComparisonResult(id obj1, id obj2) {
                                                                         return [obj1[NSURLContentModificationDateKey] compare:obj2[NSURLContentModificationDateKey]];
                                                                     }];

            // Delete files until we fall below our desired cache size.
            for (NSURL *fileURL in sortedFiles) {
                if ([_fileManager removeItemAtURL:fileURL error:nil]) {
                    NSDictionary<NSString *, id> *resourceValues = cacheFiles[fileURL];
                    NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
                    currentCacheSize -= totalAllocatedSize.unsignedIntegerValue;

                    if (currentCacheSize < desiredCacheSize) {
                        break;
                    }
                }
            }
        }
        if (completionBlock) {
            dispatch_async(dispatch_get_main_queue(), ^{
                completionBlock();
            });
        }
    });
}

這個(gè)是應(yīng)用程序在收到進(jìn)入后臺(tái)通知后的處理方法债查，首先在進(jìn)入后臺(tái)后申請(qǐng)更多的時(shí)間進(jìn)行操作非区，用到beginBackgroundTaskWithExpirationHandler。

- (void)backgroundDeleteOldFiles {
    Class UIApplicationClass = NSClassFromString(@"UIApplication");
    if(!UIApplicationClass || ![UIApplicationClass respondsToSelector:@selector(sharedApplication)]) {
        return;
    }
    UIApplication *application = [UIApplication performSelector:@selector(sharedApplication)];
    __block UIBackgroundTaskIdentifier bgTask = [application beginBackgroundTaskWithExpirationHandler:^{
        // Clean up any unfinished task business by marking where you
        // stopped or ending the task outright.
        [application endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }];

    // Start the long-running task and return immediately.
    [self deleteOldFilesWithCompletionBlock:^{
        [application endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }];
}