SDImageCache

使用的内存缓存是NSCache的子类，主要用于在收到APP内存警告时清空缓存

@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

1、定义默认缓存时间
2、定义判断是否是png图片的方式，具体判断如下：ImageDataHasPNGPreffix 是 kPNGSignatureData 就是用kPNGSignatureBytes创建的NSData，通常判断图片类型都只是判断data的第一位，参考NSData+ImageContentType.h,但此处用了8位进行判断是否是png图片，如果前8位都是一样的，则说明是png图片
3、计算图片内存大小

// 默认缓存时间为一周
static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7; // 1 week
// PNG signature bytes and data (below)
static unsigned char kPNGSignatureBytes[8] = {0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A};
static NSData *kPNGSignatureData = nil;

BOOL ImageDataHasPNGPreffix(NSData *data);
BOOL ImageDataHasPNGPreffix(NSData *data) {
    NSUInteger pngSignatureLength = [kPNGSignatureData length];
    if ([data length] >= pngSignatureLength) {
        if ([[data subdataWithRange:NSMakeRange(0, pngSignatureLength)] isEqualToData:kPNGSignatureData]) {
            return YES;
        }
    }

    return NO;
}
// 作为参照判断图片类型，在"NSData+ImageContentType.h"中
+ (NSString *)sd_contentTypeForImageData:(NSData *)data {
    uint8_t c;
    [data getBytes:&c length:1];
    switch (c) {
        case 0xFF:
            return @"image/jpeg";
        case 0x89:
            return @"image/png";
        case 0x47:
            return @"image/gif";
        case 0x49:
        case 0x4D:
            return @"image/tiff";
        case 0x52:
            // R as RIFF for WEBP
            if ([data length] < 12) {
                return nil;
            }

            NSString *testString = [[NSString alloc] initWithData:[data subdataWithRange:NSMakeRange(0, 12)] encoding:NSASCIIStringEncoding];
            if ([testString hasPrefix:@"RIFF"] && [testString hasSuffix:@"WEBP"]) {
                return @"image/webp";
            }

            return nil;
    }
    return nil;
}

// 计算图片内存大小
FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
    return image.size.height * image.size.width * image.scale * image.scale;
}

主要成员变量有内存缓存已经FileManager

@interface SDImageCache ()

@property (strong, nonatomic) NSCache *memCache;
@property (strong, nonatomic) NSString *diskCachePath;
@property (strong, nonatomic) NSMutableArray *customPaths;
@property (SDDispatchQueueSetterSementics, nonatomic) dispatch_queue_t ioQueue;

@end


@implementation SDImageCache {
    NSFileManager *_fileManager;
}

通过命名空间来初始化缓存的名称和磁盘存储的路径，同时添加接收内存警告、应用退出、应用进入后台通知

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

        // initialise PNG signature data
        kPNGSignatureData = [NSData dataWithBytes:kPNGSignatureBytes length:8];

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

        // Init default values
        _maxCacheAge = kDefaultCacheMaxCacheAge;

        // 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;
        }

        // Set decompression to YES
        _shouldDecompressImages = YES;

        // memory cache enabled
        _shouldCacheImagesInMemory = YES;

        // 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);
}

将字符串转换为md5的文件名

- (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;
}

存储图片到内存和磁盘中
1、如果允许存储在内存缓存中，则计算出图片大小，存储在内存中- (void)setObject:(ObjectType)obj forKey:(KeyType)key cost:(NSUInteger)g;
2、将图片存到磁盘上，先判断图片是png还是jpeg
判断png用UIImage判断alpha通道或者NSData判断前8个字节，如果是png图片用UIImagePNGRepresentation(image);获取data，如果是其他图片，用UIImageJPEGRepresentation(image, (CGFloat)1.0);获取data
3、将data存入磁盘

- (void)storeImage:(UIImage *)image recalculateFromImage:(BOOL)recalculate imageData:(NSData *)imageData forKey:(NSString *)key toDisk:(BOOL)toDisk {
    if (!image || !key) {
        return;
    }
    // if memory cache is enabled
    if (self.shouldCacheImagesInMemory) {
        NSUInteger cost = SDCacheCostForImage(image);
        [self.memCache setObject:image forKey:key cost:cost];
    }

    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

                // 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
                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];
        });
    }
}

NSData存入磁盘的方法

- (void)storeImageDataToDisk:(NSData *)imageData forKey:(NSString *)key {
    
    if (!imageData) {
        return;
    }
    
    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.shouldDisableiCloud) {
        [fileURL setResourceValue:[NSNumber numberWithBool:YES] forKey:NSURLIsExcludedFromBackupKey error:nil];
    }
}

查找在硬盘上是否存在，先用url转化的key用FileManager查找，如果找不到，把后缀去掉再找一遍

- (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
    // checking the key with and without the extension
    if (!exists) {
        exists = [[NSFileManager defaultManager] fileExistsAtPath:[[self defaultCachePathForKey:key] stringByDeletingPathExtension]];
    }
    
    return exists;
}

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

从硬盘里把图片读取出来

- (UIImage *)diskImageForKey:(NSString *)key {
    NSData *data = [self diskImageDataBySearchingAllPathsForKey:key];
    if (data) {
    // 如果是gif图片，将图片里包含的图片一一取出来[UIImage animatedImageWithImages:images duration:duration];来创建
// 如果是一般图片用[[UIImage alloc] initWithData:data];[UIImage imageWithCGImage:image.CGImage
                                        scale:image.scale
                                  orientation:orientation];创建
// 如果是非gif图片根据屏幕比例伸缩图片

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

在内存查找，则直接在cache里用key查找

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

- (UIImage *)imageFromDiskCacheForKey:(NSString *)key {

    // 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) {
        NSUInteger cost = SDCacheCostForImage(diskImage);
        [self.memCache setObject:diskImage forKey:key cost:cost];
    }

    return diskImage;
}