该类库所提供的功能

1、带有管理网络图片下载和缓存的UIImageView类别；
2、支持异步下载；
3、支持异步内存＋磁盘缓存，自动清理过期缓存；
4、支持Gif；
5、支持WebP；
6、支持图片后台解压；
7、确保同样的URL不会下载多次；
8、伪造的URL不会尝试重新下载；
9、确保主线程不会堵塞；
10、高性能；
11、使用GCD和ARC；
12、支持64位；

目录结构

1、Downloader：图片下载相关类
2、Cache：图片缓存处理相关类
3、Utils：SDWebImageManager管理SDWebImageDownloader和SDImageCache；SDWebImageDecoder用来解压图片； SDWebImagePrefetcher预加载图片；
4、Categories:控件类目扩展

F884FC26-13A3-4781-8094-2733EC2A7FF8.png

SDImageCache类结构

SDImageCache:主要提供三种缓存方式（ SDImageCacheTypeNone、SDImageCacheTypeDisk、SDImageCacheTypeMemory），其中图片的磁盘缓存写操作采用异步处理方式。一般情况下，磁盘缓存的操作都采用异步处理。下面是 文件#import “SDImageCache.m”
中类实现的结构：

屏幕快照 2016-07-06 上午10.55.46.png

屏幕快照 2016-07-06 上午10.56.14.png

缓存图片包括4个方法：

- (void)storeImage:(UIImage *)image forKey:(NSString *)key;
- (void)storeImage:(UIImage *)image forKey:(NSString *)key toDisk:(BOOL)toDisk;
- (void)storeImage:(UIImage *)image recalculateFromImage:(BOOL)recalculate imageData:(NSData *)imageData forKey:(NSString *)key toDisk:(BOOL)toDisk;
- (void)storeImageDataToDisk:(NSData *)imageData forKey:(NSString *)key;

从缓存中获取图片包括3个方法（第一个方法异步获取）：

- (NSOperation *)queryDiskCacheForKey:(NSString *)key done:(SDWebImageQueryCompletedBlock)doneBlock;
- (UIImage *)imageFromMemoryCacheForKey:(NSString *)key;
- (UIImage *)imageFromDiskCacheForKey:(NSString *)key;

根据键值从缓存中清除图片包括4个方法（全部方法都是异步操作）：

- (void)removeImageForKey:(NSString *)key;
- (void)removeImageForKey:(NSString *)key withCompletion:(SDWebImageNoParamsBlock)completion;
- (void)removeImageForKey:(NSString *)key fromDisk:(BOOL)fromDisk;
- (void)removeImageForKey:(NSString *)key fromDisk:(BOOL)fromDisk withCompletion:(SDWebImageNoParamsBlock)completion;

清理缓存包括5个方法：

- (void)clearMemory;
- (void)clearDiskOnCompletion:(SDWebImageNoParamsBlock)completion;
- (void)clearDisk;
- (void)cleanDiskWithCompletionBlock:(SDWebImageNoParamsBlock)completionBlock;
- (void)cleanDisk;

根据键值判断图片是否存在磁盘中（第一个方法异步获取）：

- (void)diskImageExistsWithKey:(NSString *)key completion:(SDWebImageCheckCacheCompletionBlock)completionBlock;
- (BOOL)diskImageExistsWithKey:(NSString *)key;

其它方法，计算磁盘缓存大小、缓存中图片数、缓存中是否存在键值对应的图片等。

代码注释

/*
 * This file is part of the SDWebImage package.
 * (c) Olivier Poitrey <rs@dailymotion.com>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

#import "SDImageCache.h"
#import "SDWebImageDecoder.h"
#import "UIImage+MultiFormat.h"
#import <CommonCrypto/CommonDigest.h>

// See https://github.com/rs/SDWebImage/pull/1141 for discussion
#pragma mark -----内存缓存类：添加观察者，内存警告时，清空内存缓存-----
@interface AutoPurgeCache : NSCache
@end

@implementation AutoPurgeCache

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

- (void)dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self name:UIApplicationDidReceiveMemoryWarningNotification object:nil];

}

@end

// 缓存生命期1周：1 week
static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7;
// PNG signature bytes and data (below)
static unsigned char kPNGSignatureBytes[8] = {0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A};
// PNG签名的数据：装载kPNGSignatureBytes
static NSData *kPNGSignatureData = nil;

BOOL ImageDataHasPNGPreffix(NSData *data);

/**
 *  ImageData是否含有PNG前缀
 *
 *  @param data 图片数据
 *
 *  @return 图片是否含有PNG前缀
 */
BOOL ImageDataHasPNGPreffix(NSData *data) {
    NSUInteger pngSignatureLength = [kPNGSignatureData length];
    if ([data length] >= pngSignatureLength) {
        if ([[data subdataWithRange:NSMakeRange(0, pngSignatureLength)] isEqualToData:kPNGSignatureData]) {
            return YES;
        }
    }

    return NO;
}

/**
 *  计算图片所占内存大小
 *
 *  @param image <#image description#>
 *
 *  @return <#return value description#>
 */
FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
    // 计算图片所消耗的内存注意乘以屏幕的比例因子
    return image.size.height * image.size.width * image.scale * image.scale;
}

#pragma mark -----图片缓存类-----
@interface SDImageCache ()

// 内存缓存
@property (strong, nonatomic) NSCache *memCache;
// 磁盘缓存路径
@property (strong, nonatomic) NSString *diskCachePath;
// 保存缓存路径的数组
@property (strong, nonatomic) NSMutableArray *customPaths;
// 执行处理输入输出的等待队列
@property (SDDispatchQueueSetterSementics, nonatomic) dispatch_queue_t ioQueue;

@end


@implementation SDImageCache {
    NSFileManager *_fileManager;
}

#pragma mark -- 单例
+ (SDImageCache *)sharedImageCache {
    static dispatch_once_t once;
    static id instance;
    dispatch_once(&once, ^{
        instance = [self new];
    });
    return instance;
}

#pragma mark -- 初始化
- (id)init {
    // 默认命名空间default，也可以自定义命名空间
    return [self initWithNamespace:@"default"];
}

- (id)initWithNamespace:(NSString *)ns {
    // path = "/var/mobile/Containers/Data/Application/382D0176-0BC4-44E3-98FF-095D02B85D38/Library/Caches/default"
    NSString *path = [self makeDiskCachePath:ns];
    return [self initWithNamespace:ns diskCacheDirectory:path];
}

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

        // 初始化PNG签名数据initialise PNG signature data
        kPNGSignatureData = [NSData dataWithBytes:kPNGSignatureBytes length:8];

        // 初始化执行处理输入输出的等待队列：Create IO serial queue
        _ioQueue = dispatch_queue_create("com.hackemist.SDWebImageCache", DISPATCH_QUEUE_SERIAL);

        // 初始化缓存生命期： Init default values
        _maxCacheAge = kDefaultCacheMaxCacheAge;

        // 初始化内存缓存：Init the memory cache
        _memCache = [[AutoPurgeCache alloc] init];
        _memCache.name = fullNamespace;

        // 初始化硬盘缓存：Init the disk cache
        if (directory != nil) {
            // /var/mobile/Containers/Data/Application/3EC2CFBB-6B30-4E4C-8CFE-11B717DEC9BE/Library/Caches/default/com.hackemist.SDWebImageCache.default
            _diskCachePath = [directory stringByAppendingPathComponent:fullNamespace];
        } else {
            NSString *path = [self makeDiskCachePath:ns];
            _diskCachePath = path;
        }

        // 是否解压图片：Set decompression to YES
        _shouldDecompressImages = YES;

        // 是否开启内存缓存：memory cache enabled
        _shouldCacheImagesInMemory = YES;

        // 设置默认不使用iCloud：Disable iCloud
        _shouldDisableiCloud = YES;

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

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

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

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

    return self;
}

- (void)dealloc {
    [[NSNotificationCenter defaultCenter] removeObserver:self];
    SDDispatchQueueRelease(_ioQueue);
}

#pragma mark -- 图片路径操作
/**
 *  往数组中添加唯一缓存路径
 *
 *  @param path 路径
 */
- (void)addReadOnlyCachePath:(NSString *)path {
    if (!self.customPaths) {
        self.customPaths = [NSMutableArray new];
    }

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

/**
 *  返回缓存完整路径，其中文件名是根据key值生成的MD5值
 *
 *  @param key  key
 *  @param path 路径
 *
 *  @return <#return value description#>
 */
- (NSString *)cachePathForKey:(NSString *)key inPath:(NSString *)path {
    NSString *filename = [self cachedFileNameForKey:key];// 508746268d43f962e80def6392aff711.35786508303135633
    return [path stringByAppendingPathComponent:filename];// /var/mobile/Containers/Data/Application/111DBEDD-3D5E-457D-867F-EC0E08260A46/Library/Caches/default/com.hackemist.SDWebImageCache.default/508746268d43f962e80def6392aff711.35786508303135633
}

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

#pragma mark SDImageCache (private)
/**
 *  根据key值生成文件名：采用MD5
 *
 *  @param key key
 *
 *  @return 文件名
 */
- (NSString *)cachedFileNameForKey:(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;
}

#pragma mark ImageCache
#pragma mark -- 初始化磁盘缓存路径
/**
 *  初始化磁盘缓存路径
 *
 *  @param fullNamespace 磁盘缓存命名空间：默认为default
 *
 *  @return 磁盘缓存路径
 */
-(NSString *)makeDiskCachePath:(NSString*)fullNamespace{
    // /var/mobile/Containers/Data/Application/382D0176-0BC4-44E3-98FF-095D02B85D38/Library/Caches
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
    return [paths[0] stringByAppendingPathComponent:fullNamespace];
}

#pragma mark -- 缓存图片
/**
 *  缓存图片
 *
 *  @param image       图片
 *  @param recalculate 是否重新计算
 *  @param imageData   imageData
 *  @param key         缓存的key
 *  @param toDisk      是否缓存到磁盘
 */
- (void)storeImage:(UIImage *)image recalculateFromImage:(BOOL)recalculate imageData:(NSData *)imageData forKey:(NSString *)key toDisk:(BOOL)toDisk {
    if (!image || !key) {
        return;
    }
    
    // 缓存到内存
    if (self.shouldCacheImagesInMemory) {
        NSUInteger cost = SDCacheCostForImage(image);
        [self.memCache setObject:image forKey:key cost:cost];
    }

    // 缓存到磁盘，采用异步操作
    if (toDisk) {
        dispatch_async(self.ioQueue, ^{
            NSData *data = imageData;

            if (image && (recalculate || !data)) {
#if TARGET_OS_IPHONE
                // We need to determine if the image is a PNG or a JPEG
                // PNGs are easier to detect because they have a unique signature (http://www.w3.org/TR/PNG-Structure.html)
                // The first eight bytes of a PNG file always contain the following (decimal) values:
                // 137 80 78 71 13 10 26 10
                // PNG图片有统一的签名较易甄别，前8个字节通常包含：137 80 78 71 13 10 26 10

                // If the imageData is nil (i.e. if trying to save a UIImage directly or the image was transformed on download)
                // and the image has an alpha channel, we will consider it PNG to avoid losing the transparency
                // 如图片的imageData为空（如果试图直接保存一个UIImage 或者 图片是由下载转换得来），且图片含有alpha通道，将进行PNG缓存处理避免失去透明度
                int alphaInfo = CGImageGetAlphaInfo(image.CGImage);
                BOOL hasAlpha = !(alphaInfo == kCGImageAlphaNone ||
                                  alphaInfo == kCGImageAlphaNoneSkipFirst ||
                                  alphaInfo == kCGImageAlphaNoneSkipLast);
                BOOL imageIsPng = hasAlpha;

                // But if we have an image data, we will look at the preffix
                if ([imageData length] >= [kPNGSignatureData length]) {
                    imageIsPng = ImageDataHasPNGPreffix(imageData);
                }

                if (imageIsPng) {
                    data = UIImagePNGRepresentation(image);
                }
                else {
                    data = UIImageJPEGRepresentation(image, (CGFloat)1.0);
                }
#else
                data = [NSBitmapImageRep representationOfImageRepsInArray:image.representations usingType: NSJPEGFileType properties:nil];
#endif
            }

            [self storeImageDataToDisk:data forKey:key];
        });
    }
}

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

- (void)storeImage:(UIImage *)image forKey:(NSString *)key toDisk:(BOOL)toDisk {
    [self storeImage:image recalculateFromImage:YES imageData:nil forKey:key toDisk:toDisk];
}

/**
 *  保存图片imageData到磁盘
 *
 *  @param imageData imageData
 *  @param key       缓存的key
 */
- (void)storeImageDataToDisk:(NSData *)imageData forKey:(NSString *)key {
    
    if (!imageData) {
        return;
    }
    
    // 如果不存在该路径，则创建
    if (![_fileManager fileExistsAtPath:_diskCachePath]) {
        [_fileManager createDirectoryAtPath:_diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL];
    }
    
    // 根据key获取缓存路径：get cache Path for image key
    // /var/mobile/Containers/Data/Application/752F97A5-F9EB-4C6C-A96B-2C785087B4A6/Library/Caches/default/com.hackemist.SDWebImageCache.default/fcc1e50837ec9a5d0cca14cda3339ca7.webp
    NSString *cachePathForKey = [self defaultCachePathForKey:key];
    // 将缓存路径转换为URL：transform to NSUrl
    // file:///var/mobile/Containers/Data/Application/752F97A5-F9EB-4C6C-A96B-2C785087B4A6/Library/Caches/default/com.hackemist.SDWebImageCache.default/fcc1e50837ec9a5d0cca14cda3339ca7.webp
    NSURL *fileURL = [NSURL fileURLWithPath:cachePathForKey];
    
    // 保存数据
    [_fileManager createFileAtPath:cachePathForKey contents:imageData attributes:nil];
    
    // 关闭iCloud备份：disable iCloud backup
    if (self.shouldDisableiCloud) {
        [fileURL setResourceValue:[NSNumber numberWithBool:YES] forKey:NSURLIsExcludedFromBackupKey error:nil];
    }
}

#pragma mark -- 磁盘中是否存在图片
/**
 *  根据key判断是否Image是否存在磁盘中
 *
 *  @param key key
 *
 *  @return <#return value description#>
 */
- (BOOL)diskImageExistsWithKey:(NSString *)key {
    BOOL exists = NO;
    
    // this is an exception to access the filemanager on another queue than ioQueue, but we are using the shared instance
    // from apple docs on NSFileManager: The methods of the shared NSFileManager object can be called from multiple threads safely.
    exists = [[NSFileManager defaultManager] fileExistsAtPath:[self defaultCachePathForKey:key]];

    // fallback because of https://github.com/rs/SDWebImage/pull/976 that added the extension to the disk file name
    // 去掉路径的扩展名后在进行检查：checking the key with and without the extension
    if (!exists) {
        exists = [[NSFileManager defaultManager] fileExistsAtPath:[[self defaultCachePathForKey:key] stringByDeletingPathExtension]];
    }
    
    return exists;
}

/**
 *  根据key判断是否Image是否存在磁盘中：异步处理
 *
 *  @param key             key
 *  @param completionBlock Image是否存在磁盘中回调
 */
- (void)diskImageExistsWithKey:(NSString *)key completion:(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);
            });
        }
    });
}

#pragma mark -- 根据key从磁盘或内存中获取图片
/**
 *  根据key从内存中获取图片
 *
 *  @param key key
 *
 *  @return <#return value description#>
 */
- (UIImage *)imageFromMemoryCacheForKey:(NSString *)key {
    return [self.memCache objectForKey:key];
}

/**
 *  根据key从磁盘缓存中获取图片
 *
 *  @param key key
 *
 *  @return <#return value description#>
 */
- (UIImage *)imageFromDiskCacheForKey:(NSString *)key {
    // 首先查看内存的缓存中是否含有该key所对应的图片，在检查磁盘缓存
    
    // First check the in-memory cache...
    UIImage *image = [self imageFromMemoryCacheForKey:key];
    if (image) {
        return image;
    }

    // Second check the disk cache...
    UIImage *diskImage = [self diskImageForKey:key];
    if (diskImage && self.shouldCacheImagesInMemory) {
        // 将图片写入内存缓存
        NSUInteger cost = SDCacheCostForImage(diskImage);
        [self.memCache setObject:diskImage forKey:key cost:cost];
    }

    return diskImage;
}

/**
 *  根据key在磁盘缓存中搜索图片
 *
 *  @param key key
 *
 *  @return 图片数据
 */
- (NSData *)diskImageDataBySearchingAllPathsForKey:(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 *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;
}

- (UIImage *)diskImageForKey:(NSString *)key {
    NSData *data = [self diskImageDataBySearchingAllPathsForKey:key];
    if (data) {
        UIImage *image = [UIImage sd_imageWithData:data];
        // 根据图片的scale 或 图片中的图片组 重新计算返回图片
        image = [self scaledImageForKey:key image:image];
        if (self.shouldDecompressImages) {
            image = [UIImage decodedImageWithImage:image];
        }
        return image;
    }
    else {
        return nil;
    }
}

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

/**
 *  根据key从磁盘缓存中获取图片：异步操作
 *
 *  @param key       key
 *  @param doneBlock 根据key从磁盘缓存中获取图片回调
 *
 *  @return <#return value description#>
 */
- (NSOperation *)queryDiskCacheForKey:(NSString *)key done:(SDWebImageQueryCompletedBlock)doneBlock {
    if (!doneBlock) {
        return nil;
    }

    if (!key) {
        doneBlock(nil, SDImageCacheTypeNone);
        return nil;
    }

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

    NSOperation *operation = [NSOperation new];
    dispatch_async(self.ioQueue, ^{
        if (operation.isCancelled) {
            return;
        }

        // 这里使用自动释放池，自己的理解：里面的操作内存消耗过大，需要在系统回收的时候，优先、及时回收自动释放池中分配的对象，优化内存使用。
        @autoreleasepool {
            UIImage *diskImage = [self diskImageForKey:key];
            if (diskImage && self.shouldCacheImagesInMemory) {
                // 将图片写入内存缓存
                NSUInteger cost = SDCacheCostForImage(diskImage);
                [self.memCache setObject:diskImage forKey:key cost:cost];
            }

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

    return operation;
}

#pragma mark -- 根据key从缓存中清除图片
- (void)removeImageForKey:(NSString *)key {
    [self removeImageForKey:key withCompletion:nil];
}

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

- (void)removeImageForKey:(NSString *)key fromDisk:(BOOL)fromDisk {
    [self removeImageForKey:key fromDisk:fromDisk withCompletion:nil];
}

/**
 *  根据key从缓存中清除图片
 *
 *  @param key        key
 *  @param fromDisk   是否磁盘缓存
 *  @param completion 缓存图片清除回调
 */
- (void)removeImageForKey:(NSString *)key fromDisk:(BOOL)fromDisk withCompletion:(SDWebImageNoParamsBlock)completion {
    
    if (key == nil) {
        return;
    }

    // 内存缓存清除
    if (self.shouldCacheImagesInMemory) {
        [self.memCache removeObjectForKey:key];
    }

    // 磁盘缓存清除
    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();
    }
    
}

#pragma mark -- 设置
- (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;
}

#pragma mark -- 清空 或清除 缓存
/**
 *  清空内存缓存
 */
- (void)clearMemory {
    [self.memCache removeAllObjects];
}

/**
 *  清空磁盘缓存
 */
- (void)clearDisk {
    [self clearDiskOnCompletion:nil];
}

/**
 *  清空磁盘缓存
 *
 *  @param completion 磁盘缓存清空回调
 */
- (void)clearDiskOnCompletion:(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();
            });
        }
    });
}

/**
 *  清除磁盘缓存
 */
- (void)cleanDisk {
    [self cleanDiskWithCompletionBlock:nil];
}

/**
 *  清除磁盘缓存，异步操作：1、清除过期的缓存文件；2、缓存超过设定阈值时，按时间顺序删除缓存中的图片，直到缓存剩余空间达阈值的一半
 *
 *  @param completion 磁盘缓存清除回调
 */
- (void)cleanDiskWithCompletionBlock:(SDWebImageNoParamsBlock)completionBlock {
    dispatch_async(self.ioQueue, ^{
        // dickCacheURL：file:///var/mobile/Containers/Data/Application/E42DF859-9AF1-46B3-B518-FB62ECE73D33/Library/Caches/default/com.hackemist.SDWebImageCache.default/
        NSURL *diskCacheURL = [NSURL fileURLWithPath:self.diskCachePath isDirectory:YES];
        // resourceKeys包含要了解的属性：允许判断遍历到的URL所指对象是否是目录、允许判断遍历返回的URL所指项目的最后修改时间、URL目录中所分配的空间大小
        NSArray *resourceKeys = @[NSURLIsDirectoryKey, NSURLContentModificationDateKey, NSURLTotalFileAllocatedSizeKey];

        // 使用目录枚举器获取缓存文件：This enumerator prefetches useful properties for our cache files.
        // 使用NSdirectoryEnumerator遍历所有的缓存文件，获取文件属性，如我们需要的文件大小信息，是不会有性能问题的。NSdirectoryEnumerator获取文件属性是通过查看文件的inode数据，并不需要想象中的fileopen和fileclose。
        NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtURL:diskCacheURL
                                                   includingPropertiesForKeys:resourceKeys
                                                                      options:NSDirectoryEnumerationSkipsHiddenFiles
                                                                 errorHandler:NULL];

        // 计算过期时间，默认1周以前的缓存文件为过期
        NSDate *expirationDate = [NSDate dateWithTimeIntervalSinceNow:-self.maxCacheAge];
        NSMutableDictionary *cacheFiles = [NSMutableDictionary dictionary]; // 当前保存下来文件的fileURL
        NSUInteger currentCacheSize = 0; // 当前保存下来文件的总内存大小

        // 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.
        // 遍历目录枚举器：1、清除过期的文件；2、
        NSMutableArray *urlsToDelete = [[NSMutableArray alloc] init]; // 将会删除的文件fileURL数组
        for (NSURL *fileURL in fileEnumerator) {
            NSDictionary *resourceValues = [fileURL resourceValuesForKeys:resourceKeys error:NULL];

            // 跳过目录：Skip directories.
            if ([resourceValues[NSURLIsDirectoryKey] boolValue]) {
                continue;
            }

            // 记录超过过期日期文件的fileURL：Remove files that are older than the expiration date;
            NSDate *modificationDate = resourceValues[NSURLContentModificationDateKey];
            if ([[modificationDate laterDate:expirationDate] isEqualToDate:expirationDate]) {
                [urlsToDelete addObject:fileURL];
                continue;
            }

            // 保存保留下来的文件的引用并计算文件总的大小：Store a reference to this file and account for its total size.
            NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
            currentCacheSize += [totalAllocatedSize unsignedIntegerValue];
            [cacheFiles setObject:resourceValues forKey:fileURL];
        }
        
        // 删除缓存中过期的文件
        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.
        // 当设置的maxCacheSize > 0 且 当前缓存中的文件所占内存大于设置的最大缓存时，按时间顺序删除文件，直到缓存剩余空间达阈值的一半
        if (self.maxCacheSize > 0 && currentCacheSize > self.maxCacheSize) {
            // 清除缓存目标是最大缓存的一半：Target half of our maximum cache size for this cleanup pass.
            const NSUInteger desiredCacheSize = self.maxCacheSize / 2;

            // 缓存文件排序，最老的文件先清除：Sort the remaining cache files by their last modification time (oldest first).
            NSArray *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 *resourceValues = cacheFiles[fileURL];
                    NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
                    currentCacheSize -= [totalAllocatedSize unsignedIntegerValue];

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

/**
 *  后台清理磁盘缓存：收到UIApplicationDidEnterBackgroundNotification时，在手机系统后台进行如上面描述的异步磁盘缓存清理
 */
- (void)backgroundCleanDisk {
    Class UIApplicationClass = NSClassFromString(@"UIApplication");
    if(!UIApplicationClass || ![UIApplicationClass respondsToSelector:@selector(sharedApplication)]) {
        return;
    }
    
    // 使用sharedApplication开启后台任务cleanDiskWithCompletionBlock:
    UIApplication *application = [UIApplication performSelector:@selector(sharedApplication)];
    __block UIBackgroundTaskIdentifier bgTask = [application beginBackgroundTaskWithExpirationHandler:^{
        // 清理任何未完成的任务作业，标记完全停止或结束任务：Clean up any unfinished task business by marking where you stopped or ending the task outright.
        [application endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }];

    // 开始长时间后台运行的任务并且立即return：Start the long-running task and return immediately.
    [self cleanDiskWithCompletionBlock:^{
        [application endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }];
}

#pragma mark -- 计算缓存大小、缓存中图片数
/**
 *  同步操作：获取缓存中所有文件的大小
 *
 *  @return <#return value description#>
 */
- (NSUInteger)getSize {
    __block NSUInteger size = 0;
    
    // 需要同步操作：等待队列self.ioQueue中的任务执行完后（有可能队列中的任务正在添加图片或者删除图片操作），再进行获取文件大小计算
    dispatch_sync(self.ioQueue, ^{
        NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtPath:self.diskCachePath];
        for (NSString *fileName in fileEnumerator) {
            NSString *filePath = [self.diskCachePath stringByAppendingPathComponent:fileName];
            NSDictionary *attrs = [[NSFileManager defaultManager] attributesOfItemAtPath:filePath error:nil];
            size += [attrs fileSize];
        }
    });
    return size;
}

/**
 *  同步操作：获取磁盘缓存中图片数
 *
 *  @return <#return value description#>
 */
- (NSUInteger)getDiskCount {
    __block NSUInteger count = 0;
    dispatch_sync(self.ioQueue, ^{
        NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtPath:self.diskCachePath];
        count = [[fileEnumerator allObjects] count];
    });
    return count;
}

/**
 *  异步操作：获取缓存中图片数及图片所占内存总大小
 *
 *  @param completionBlock <#completionBlock description#>
 */
- (void)calculateSizeWithCompletionBlock:(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);
            });
        }
    });
}

@end