sdwebimage 取缓存图片源码分析

    NSURL* avatarURL = [KLUtil getImageDisplayUrl:channelMessage.avatar width:50 height:50];
    NSString* key = [[SDWebImageManager sharedManager] cacheKeyForURL:avatarURL];
    //此方法会先从memory中取。
    UIImage *image = [[SDImageCache sharedImageCache] imageFromDiskCacheForKey:key];
   _ivAvatar.image = image;

- (nullable NSString *)cacheKeyForURL:(nullable NSURL *)url {
    if (!url) {
        return @"";
    }

    if (self.cacheKeyFilter) {
        return self.cacheKeyFilter(url);
    } else {
        return url.absoluteString;
    }
}

- (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;
}

- (nullable UIImage *)diskImageForKey:(nullable NSString *)key {
    NSData *data = [self diskImageDataBySearchingAllPathsForKey:key];
    return [self diskImageForKey:key data:data];
}

- (nullable NSData *)diskImageDataBySearchingAllPathsForKey:(nullable NSString *)key {
    NSString *defaultPath = [self defaultCachePathForKey:key];
    NSData *data = [NSData dataWithContentsOfFile:defaultPath options:self.config.diskCacheReadingOptions error:nil];
    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 options:self.config.diskCacheReadingOptions error:nil];
    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 options:self.config.diskCacheReadingOptions error:nil];
        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 options:self.config.diskCacheReadingOptions error:nil];
        if (imageData) {
            return imageData;
        }
    }

    return nil;
}

// `setObject:forKey:` just call this with 0 cost. Override this is enough
- (void)setObject:(id)obj forKey:(id)key cost:(NSUInteger)g {
    [super setObject:obj forKey:key cost:g];
    if (key && obj) {
        // Store weak cache
        LOCK(self.weakCacheLock);
        [self.weakCache setObject:obj forKey:key];
        UNLOCK(self.weakCacheLock);
    }
}