SDWebImage源码剖析（二）

SDWebImageCache管理着SDWebImage的缓存，其中内存缓存采用NSCache，同时会创建一个ioQueue负责对硬盘的读写，并且会添加观察者，在收到内存警告、关闭或进入后台时完成对应的处理:

- (id)init {
     _memCache = [[NSCache alloc] init];
     _ioQueue = dispatch_queue_create("com.hackemist.SDWebImageCache", DISPATCH_QUEUE_SERIAL);
     //收到内存警告时，清除NSCache：[self.memCache removeAllObjects];
     [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(clearMemory)
                                              name:UIApplicationDidReceiveMemoryWarningNotification
                                                   object:nil];
      //程序关闭时，会对硬盘文件做一些处理
      [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(cleanDisk)
                                                     name:UIApplicationWillTerminateNotification
                                                   object:nil];
      //程序进入后台时，也会进行硬盘文件处理
      [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(backgroundCleanDisk)
                                                     name:UIApplicationDidEnterBackgroundNotification
                                                   object:nil];
}

查询图片

每次向SDWebImageCache索取图片的时候，会先根据图片URL对应的key值先检查内存中是否有对应的图片，如果有则直接返回；如果没有则在ioQueue中去硬盘中查找，其中文件名是是根据URL生成的MD5值，找到之后先将图片缓存在内存中，然后在把图片返回：

- (NSOperation *)queryDiskCacheForKey:(NSString *)key done:(SDWebImageQueryCompletedBlock)doneBlock {
    /*...*/
    // 首先查找内存缓存
    UIImage *image = [self imageFromMemoryCacheForKey:key];
    if (image) {
        doneBlock(image, SDImageCacheTypeMemory);
        return nil;
    }
    //硬盘查找
    NSOperation *operation = [NSOperation new];
    dispatch_async(self.ioQueue, ^{
        //创建自动释放池，内存及时释放
        @autoreleasepool {
            UIImage *diskImage = [self diskImageForKey:key];
            if (diskImage) {
                CGFloat cost = diskImage.size.height * diskImage.size.width * diskImage.scale * diskImage.scale;
                //缓存到NSCache中
                [self.memCache setObject:diskImage forKey:key cost:cost];
            }
            dispatch_async(dispatch_get_main_queue(), ^{
                doneBlock(diskImage, SDImageCacheTypeDisk);
            });
        }
    });
    return operation;
}

在硬盘查询的时候，会在后台将NSData转成UIImage，并完成相关的解码工作:

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

保存图片

当下载完图片后，会先将图片保存到NSCache中，并把图片像素大小作为该对象的cost值，同时如果需要保存到硬盘，会先判断图片的格式，PNG或者JPEG，并保存对应的NSData到缓存路径中，文件名为URL的MD5值：

- (NSString *)cachedFileNameForKey:(NSString *)key {
    //根据key生成对应的MD5值作为文件名
    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]];

    return filename;
}

- (void)storeImage:(UIImage *)image recalculateFromImage:(BOOL)recalculate imageData:(NSData *)imageData forKey:(NSString *)key toDisk:(BOOL)toDisk 
{
    //保存到NSCache，cost为像素值
    [self.memCache setObject:image forKey:key cost:image.size.height * image.size.width * image.scale * image.scale];
    if (toDisk) {
        dispatch_async(self.ioQueue, ^{
            NSData *data = imageData;
            if (image && (recalculate || !data)) {
               //判断图片格式
                BOOL imageIsPng = YES;
                // 查看imagedata的前缀是否是PNG的前缀格式
                if ([imageData length] >= [kPNGSignatureData length]) {
                    imageIsPng = ImageDataHasPNGPreffix(imageData);
                }
                if (imageIsPng) {
                    data = UIImagePNGRepresentation(image);
                }
                else {
                    data = UIImageJPEGRepresentation(image, (CGFloat)1.0);
                }
            }
            if (data) {
                if (![_fileManager fileExistsAtPath:_diskCachePath]) {
                    [_fileManager createDirectoryAtPath:_diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL];
                }
                //保存data到指定的路径中
                [_fileManager createFileAtPath:[self defaultCachePathForKey:key] contents:data attributes:nil];
            }
        });
    }
}

硬盘文件的管理

在程序退出或者进入后台时，会出图片文件进行管理，具体的策略：

清除过期的文件，默认一星期
如果设置了最大缓存，并且当前缓存的文件超过了这个限制，则删除最旧的文件，直到当前缓存文件的大小为最大缓存大小的一半

- (void)cleanDiskWithCompletionBlock:(SDWebImageNoParamsBlock)completionBlock {
    dispatch_async(self.ioQueue, ^{
        NSURL *diskCacheURL = [NSURL fileURLWithPath:self.diskCachePath isDirectory:YES];
        NSArray *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.maxCacheAge];
        NSMutableDictionary *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 *urlsToDelete = [[NSMutableArray alloc] init];
        for (NSURL *fileURL in fileEnumerator) {
            NSDictionary *resourceValues = [fileURL resourceValuesForKeys:resourceKeys error:NULL];

            // Skip directories.
            if ([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 setObject:resourceValues forKey:fileURL];
        }
        
        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.maxCacheSize > 0 && currentCacheSize > self.maxCacheSize) {
            // Target half of our maximum cache size for this cleanup pass.
            const NSUInteger desiredCacheSize = self.maxCacheSize / 2;

            // 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();
            });
        }
    });
}

总结

接口设计简单
通常我们使用较多的UIImageView分类：

1 2	[self.imageView sd_setImageWithURL:[NSURL URLWithString:@"url"] placeholderImage:[UIImage imageNamed:@"placeholder"]];

一个简单的接口将其中复杂的实现细节全部隐藏：简单就是美。

采用NSCache作为内存缓
耗时较长的请求，都采用异步形式，在回调函数块中处理请求结果
NSOperation和NSOperationQueue：可以取消任务处理队列中的任务，设置最大并发数，设置operation之间的依赖关系。
图片缓存清理的策略
dispatch_barrier_sync：前面的任务执行结束后它才执行，而且它后面的任务要等它执行完成之后才会执行。
使用weak self strong self 防止retain circle
如果子线程进需要不断处理一些事件，那么设置一个Run Loop是最好的处理方式