SDWebImage源码解读之SDWebImageCache(下)

第六篇

前言

我们在SDWebImageCache(上)中了解了这个缓存类大概的功能是什么？那么接下来就要看看这些功能是如何实现的？ 再次强调，不管是图片的缓存还是其他各种不同形式的缓存，在原理上都极其相似，我们通过SDWebImageCache,来看看作者是如何实现这个功能的。

在业务中，经常要缓存数据，通过本篇的学习，我们写出的缓存管理者这个管理者对象，就能够有所进步。

NSCache

对于很多开发者来说，NSCache是一个陌生人，因为大家往往对NSMutableDictionary情有独钟。可怜的 NSCache 一直处于 NSMutableDictionary 的阴影之下。就好像没有人知道它提供了垃圾处理的功能，而开发者们却费劲力气地去自己实现它。

没错，NSCache 基本上就是一个会自动移除对象来释放内存的 NSMutableDictionary。无需响应内存警告或者使用计时器来清除缓存。唯一的不同之处是键对象不会像 NSMutableDictionary 中那样被复制，这实际上是它的一个优点（键不需要实现 NSCopying 协议）。

当有缓存数据到内存的业务的时候，就应该考虑NSCache了，有缓存就有清楚缓存。

NSCache 每个方法和属性的具体作用，请参考这篇文章NSCache

AutoPurgeCache

NSCache在收到内存警告的时候会释放自身的一部分资源，设计AutoPurgeCache的目的是在收到警告时，释放缓存的所有资源。

通过继承自NSCache,监听UIApplicationDidReceiveMemoryWarningNotification来实现。

@interface AutoPurgeCache : NSCache
@end

@implementation AutoPurgeCache

- (nonnull instancetype)init {
    self = [super init];
    if (self) {
#if SD_UIKIT
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(removeAllObjects) name:UIApplicationDidReceiveMemoryWarningNotification object:nil];
#endif
    }
    return self;
}

- (void)dealloc {
#if SD_UIKIT
    [[NSNotificationCenter defaultCenter] removeObserver:self name:UIApplicationDidReceiveMemoryWarningNotification object:nil];
#endif
}

@end

计算一个UIImage的SDCacheCost

图片在该缓存中的大小是通过像素来衡量的。

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
}

** 注意：FOUNDATION_STATIC_INLINE表示该函数是一个具有文件内部访问权限的内联函数，所谓的内联函数就是建议编译器在调用时将函数展开。建议的意思就是说编译器不一定会按照你的建议做。因此内联函数尽量不要写的太复杂。**

Properties

SDWebImageCache实现部分有下边几个属性：

#pragma mark - Properties
@property (strong, nonatomic, nonnull) NSCache *memCache;
@property (strong, nonatomic, nonnull) NSString *diskCachePath;
@property (strong, nonatomic, nullable) NSMutableArray<NSString *> *customPaths;
@property (SDDispatchQueueSetterSementics, nonatomic, nullable) dispatch_queue_t ioQueue;

@end

@implementation SDImageCache {
    NSFileManager *_fileManager;
}

• memCache 内存容器
• diskCachePath 硬盘缓存路径
• customPaths 自定义的读取路径，这是一个数组，我们可以通过addReadOnlyCachePath:这个方法往里边添加路径。当我们读取读片的时候，这个数组的路径也会作为数据源
• ioQueue 称作输入输出队列，队列往往可以当做一种“锁”来使用，我们把某些任务按照顺利一步一步的进行，必须考虑线程是否安全
• _fileManager 文件管理者，这个就不多说了，大家都知道怎么用

初始化

这一部分关系到Singleton, init, dealloc这三个方面的代码，初始化有四个方法，我们重点讲解最后一个初始化方法（这也是作者建议的方法，其他方法通过该方法实现）：

+ (nonnull instancetype)sharedImageCache
- (instancetype)init
- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns
- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns
                       diskCacheDirectory:(nonnull NSString *)directory

- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns
                       diskCacheDirectory:(nonnull NSString *)directory {
    if ((self = [super init])) {
        NSString *fullNamespace = [@"com.hackemist.SDWebImageCache." stringByAppendingString:ns];

        // Create IO serial queue
        _ioQueue = dispatch_queue_create("com.hackemist.SDWebImageCache", DISPATCH_QUEUE_SERIAL);

        _config = [[SDImageCacheConfig alloc] init];

        // Init the memory cache
        _memCache = [[AutoPurgeCache alloc] init];
        _memCache.name = fullNamespace;

        // Init the disk cache
        if (directory != nil) {
            _diskCachePath = [directory stringByAppendingPathComponent:fullNamespace];
        } else {
            NSString *path = [self makeDiskCachePath:ns];
            _diskCachePath = path;
        }

        dispatch_sync(_ioQueue, ^{
            _fileManager = [NSFileManager new];
        });

#if SD_UIKIT
        // Subscribe to app events
        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(clearMemory)
                                                     name:UIApplicationDidReceiveMemoryWarningNotification
                                                   object:nil];

        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(deleteOldFiles)
                                                     name:UIApplicationWillTerminateNotification
                                                   object:nil];

        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(backgroundDeleteOldFiles)
                                                     name:UIApplicationDidEnterBackgroundNotification
                                                   object:nil];
#endif
    }

    return self;
}

这个方法其实就做了两件事：1.初始化自身的属性 2.添加通知监听。其他的初始化代码在这里就不写了。

Cache paths

1.添加自定义路径

- (void)addReadOnlyCachePath:(nonnull NSString *)path {
    if (!self.customPaths) {
        self.customPaths = [NSMutableArray new];
    }

    if (![self.customPaths containsObject:path]) {
        [self.customPaths addObject:path];
    }
}

2.文件名（MD5）

- (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;
}

3.默认的某个图片的路径

- (nullable NSString *)defaultCachePathForKey:(nullable NSString *)key {
    return [self cachePathForKey:key inPath:self.diskCachePath];
}

4.根据名称和路径拼接路径

- (nullable NSString *)cachePathForKey:(nullable NSString *)key inPath:(nonnull NSString *)path {
    NSString *filename = [self cachedFileNameForKey:key];
    return [path stringByAppendingPathComponent:filename];
}

Store Image

保存图片也有四个方法，我们按照顺序来看：

1.保存数据到Disk

- (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];
    }
}

• 检查imageData或者key是否为nil
• 检查是否在自身的队列中进行的操作
• 创建Disk缓存文件夹
• 根据key获取默认的缓存路径
• 将数据写入到上边获取的路径中
• 根据配置文件设置是否禁用iCloud的备份功能

2.参数最多的保存图片的方法

- (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, ^{
            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();
        }
    }
}

• 检查image或者key是否为nil
• 根据配置文件中是否设置了缓存到内存，保存image到缓存中，这个过程是非常快的，因此不用考虑线程
• 如果保存到Disk，创建异步串行队列 我们把数据保存到Disk，其实保存的应该是数据的二进制文件
• 保存二进制数据到Disk，如果不存在，需要把image转换成NSData
• 调用Block

3.其他两个保存的方法

- (void)storeImage:(nullable UIImage *)image
            forKey:(nullable NSString *)key
        completion:(nullable SDWebImageNoParamsBlock)completionBlock {
    [self storeImage:image imageData:nil forKey:key toDisk:YES completion:completionBlock];
}

- (void)storeImage:(nullable UIImage *)image
            forKey:(nullable NSString *)key
            toDisk:(BOOL)toDisk
        completion:(nullable SDWebImageNoParamsBlock)completionBlock {
    [self storeImage:image imageData:nil forKey:key toDisk:toDisk completion:completionBlock];
}

Query and Retrieve 数据

1.根据key判断Disk中的数据是否存在

- (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);
            });
        }
    });
}

2.获取缓存到内存中的数据

- (nullable UIImage *)imageFromMemoryCacheForKey:(nullable NSString *)key {
    return [self.memCache objectForKey:key];
}

3.获取Disk中的数据

在Disk中获取数据跟在内存中获取不一样，内存中直接保存的是UIImage，而Disk中保存的是NSData，因此肯定需要一个NSData -> UIImage 的转换过程。接下来我们看看这个转换过程：

• 根据key获取Disk中的NSData数据，总体思路就是先从默认的路径获取，如果没有获取到，再从自定义的路径获取，值得注意的是，要考虑没有pathExtention的情况

    - (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;
    }
  

• 根据NSData 获取 UIImage，需要scaled图片，根据配置文件的设置，是否解压图片

    - (nullable UIImage *)diskImageForKey:(nullable NSString *)key {
        NSData *data = [self diskImageDataBySearchingAllPathsForKey:key];
        if (data) {
            UIImage *image = [UIImage sd_imageWithData:data];
            image = [self scaledImageForKey:key image:image];
            if (self.config.shouldDecompressImages) {
                image = [UIImage decodedImageWithImage:image];
            }
            return image;
        }
        else {
            return nil;
        }
    }
  

• 将UIImage 放入内存，返回图片

    - (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;
    }
  

4.先获取内存的数据，如果没有，在获取Disk的数据

- (nullable UIImage *)imageFromCacheForKey:(nullable NSString *)key {
    // First check the in-memory cache...
    UIImage *image = [self imageFromMemoryCacheForKey:key];
    if (image) {
        return image;
    }

    // Second check the disk cache...
    image = [self imageFromDiskCacheForKey:key];
    return image;
}

5.异步获取数据

上边1.2.3.4 中获取数据的方法都不是异步获取的，在SDWebImageCache中，涉及到异步获取的，都会通过Block来回调的。

这个异步获取值得说的有两点：

• 如果在内存中获取到的图片是GIF，那么要去Disk中获取
• 为什么要返回一个NSOperation对象呢？ 其实我们可以通过这个NSOperation对象取消获取任务

代码：

- (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;
}

Remove 数据

- (void)removeImageForKey:(nullable NSString *)key withCompletion:(nullable SDWebImageNoParamsBlock)completion {
    [self removeImageForKey:key fromDisk:YES withCompletion:completion];
}

- (void)removeImageForKey:(nullable NSString *)key fromDisk:(BOOL)fromDisk withCompletion:(nullable SDWebImageNoParamsBlock)completion {
    if (key == nil) {
        return;
    }

    if (self.config.shouldCacheImagesInMemory) {
        [self.memCache removeObjectForKey:key];
    }

    if (fromDisk) {
        dispatch_async(self.ioQueue, ^{
            [_fileManager removeItemAtPath:[self defaultCachePathForKey:key] error:nil];

            if (completion) {
                dispatch_async(dispatch_get_main_queue(), ^{
                    completion();
                });
            }
        });
    } else if (completion){
        completion();
    }

}

Mem Cache settings

- (void)setMaxMemoryCost:(NSUInteger)maxMemoryCost {
    self.memCache.totalCostLimit = maxMemoryCost;
}

- (NSUInteger)maxMemoryCost {
    return self.memCache.totalCostLimit;
}

- (NSUInteger)maxMemoryCountLimit {
    return self.memCache.countLimit;
}

- (void)setMaxMemoryCountLimit:(NSUInteger)maxCountLimit {
    self.memCache.countLimit = maxCountLimit;
}

清空数据

清空数据有值得我们注意的地方，我们一个一个方法的看：

1.清空内存缓存数据

- (void)clearMemory {
    [self.memCache removeAllObjects];
}

2.清空Disk数据

- (void)clearDiskOnCompletion:(nullable SDWebImageNoParamsBlock)completion {
    dispatch_async(self.ioQueue, ^{
        [_fileManager removeItemAtPath:self.diskCachePath error:nil];
        [_fileManager createDirectoryAtPath:self.diskCachePath
                withIntermediateDirectories:YES
                                 attributes:nil
                                      error:NULL];

        if (completion) {
            dispatch_async(dispatch_get_main_queue(), ^{
                completion();
            });
        }
    });
}

3.清空旧数据

对于清空旧数据而言，我们需要考虑两个方面：

• 首先要清空掉所有的过期的数据
• 过期的数据清空后，缓存的数据比我们设置的最大缓存量还大，我们要继续清空数据，直到满足我们的需求为止

这里边大概用到的思路就是上边说的两点，关键是如何实现上边所说的内容。有一些我们平时可能不太接触的知识点，在这里做一些简要的讲解

首先我们需要遍历Disk缓存路径下的所有文件，那么我们怎么遍历呢？NSFileManager有一个很好地方法：

• 返回一个NSDirectoryEnumerator<NSURL *> * 这个对象中存放的是NSURLs

• url 需要遍历的路径

• (nullable NSArray<NSURLResourceKey> \*)keys 这个需要传入一个数组，表示想获取的NSURLResourceKeys，我们来看看这个NSURLResourceKey: 点进去看了下。太长了，在这里就不copy了。有兴趣的同学，自己去看看，太长了。我们就说说着这个清空方法中用到的把：

  • NSURLIsDirectoryKey 是否是文件夹
  • NSURLContentModificationDateKey 最后修改时间
  • NSURLTotalFileAllocatedSizeKey 分配的尺寸

• options:(NSDirectoryEnumerationOptions)mask 传入过滤参数，这里NSDirectoryEnumerationSkipsHiddenFiles 是指忽略隐藏文件

其次，我们有了这些参数了。在根据最后修改日期是否过期，删除掉过期的数据就行了。还有一个值得我们注意的就是如何对一个字典进行排序：

NSArray<NSURL *> *sortedFiles = [cacheFiles keysSortedByValueWithOptions:NSSortConcurrent
                                                                     usingComparator:^NSComparisonResult(id obj1, id obj2) {
                                                                         return [obj1[NSURLContentModificationDateKey] compare:obj2[NSURLContentModificationDateKey]];
                                                                     }];

这个会返回排好序的字典的所有的key。NSSortConcurrent是并发排序，效率高，但可能不稳定，NSSortStable 稳定，但可能效率不如NSSortConcurrent高。排序的规则通过Block指定。

好了，基本要注意的就这些。这个函数的实现也是基于这种思路。

- (void)deleteOldFiles {
    [self deleteOldFilesWithCompletionBlock:nil];
}

- (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();
            });
        }
    });
}

申请一段时间在后台删除旧数据

- (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;
    }];
}

Cache Info

- (NSUInteger)getSize {
    __block NSUInteger size = 0;
    dispatch_sync(self.ioQueue, ^{
        NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtPath:self.diskCachePath];
        for (NSString *fileName in fileEnumerator) {
            NSString *filePath = [self.diskCachePath stringByAppendingPathComponent:fileName];
            NSDictionary<NSString *, id> *attrs = [[NSFileManager defaultManager] attributesOfItemAtPath:filePath error:nil];
            size += [attrs fileSize];
        }
    });
    return size;
}

- (NSUInteger)getDiskCount {
    __block NSUInteger count = 0;
    dispatch_sync(self.ioQueue, ^{
        NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtPath:self.diskCachePath];
        count = fileEnumerator.allObjects.count;
    });
    return count;
}

- (void)calculateSizeWithCompletionBlock:(nullable SDWebImageCalculateSizeBlock)completionBlock {
    NSURL *diskCacheURL = [NSURL fileURLWithPath:self.diskCachePath isDirectory:YES];

    dispatch_async(self.ioQueue, ^{
        NSUInteger fileCount = 0;
        NSUInteger totalSize = 0;

        NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtURL:diskCacheURL
                                                   includingPropertiesForKeys:@[NSFileSize]
                                                                      options:NSDirectoryEnumerationSkipsHiddenFiles
                                                                 errorHandler:NULL];

        for (NSURL *fileURL in fileEnumerator) {
            NSNumber *fileSize;
            [fileURL getResourceValue:&fileSize forKey:NSURLFileSizeKey error:NULL];
            totalSize += fileSize.unsignedIntegerValue;
            fileCount += 1;
        }

        if (completionBlock) {
            dispatch_async(dispatch_get_main_queue(), ^{
                completionBlock(fileCount, totalSize);
            });
        }
    });
}

总结

SDWebImageCache 就写完了，本篇算是长文了，对于那种图片比较多的app，实现一个自己的缓存类还是很有必要的。

由于个人知识有限，如有错误之处，还望各路大侠给予指出啊

1. SDWebImage源码解读 之 NSData+ImageContentType 简书 博客园
2. SDWebImage源码解读 之 UIImage+GIF 简书 博客园
3. SDWebImage源码解读 之 SDWebImageCompat 简书 博客园
4. SDWebImage源码解读 之SDWebImageDecoder 简书 博客园
5. SDWebImage源码解读 之SDWebImageCache(上) 简书 博客园