阅读 SDWebImage

最新推荐文章于 2025-06-09 13:41:42 发布

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本文详细剖析了iOS中流行的异步图像加载库SDWebImage的核心方法sd_setImageWithURL。介绍了SDWebImageCombinedOperation与SDWebImageManager的工作原理，包括如何处理URL加载、缓存策略、失败重试机制等。

[imgBtn sd_setImageWithURL:[NSURL URLWithString:item[@"url"]] forState:UIControlStateNormal];

死磕 sd_setImageWithURL方法

核心方法是

- (id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url

options:(SDWebImageOptions)options

progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock

completed:(nullable SDInternalCompletionBlock)completedBlock

核心的类：SDWebImageCombinedOperation&&SDWebImageManager

@interface SDWebImageCombinedOperation : NSObject <SDWebImageOperation>

@property (assign, nonatomic, getter = isCancelled) BOOL cancelled;

@property (strong, nonatomic, nullable) SDWebImageDownloadToken *downloadToken;

@property (strong, nonatomic, nullable) NSOperation *cacheOperation;

@property (weak, nonatomic, nullable) SDWebImageManager *manager;

@end

以及

@interface SDWebImageManager ()

@property (strong, nonatomic, readwrite, nonnull) SDImageCache *imageCache;

@property (strong, nonatomic, readwrite, nonnull) SDWebImageDownloader *imageDownloader;

@property (strong, nonatomic, nonnull) NSMutableSet<NSURL *> *failedURLs;

@property (strong, nonatomic, nonnull) dispatch_semaphore_t failedURLsLock; // a lock to keep the access to `failedURLs` thread-safe

@property (strong, nonatomic, nonnull) NSMutableSet<SDWebImageCombinedOperation *> *runningOperations;

@property (strong, nonatomic, nonnull) dispatch_semaphore_t runningOperationsLock; // a lock to keep the access to `runningOperations` thread-safe

@end

信号量dispatch_semaphore_t 确保线程安全

001 判断URL是否空

002 创建SDWebImageCombinedOperation *operation

003 通过信号量加解锁将URL添加到失败failedURLs数组中

004 通过信号量加解锁将operation添加到runningOperations集合中

005 获取到当前检查当前URL的对应的key

006 设置缓存策略

007 调用SDImageCache 的queryCacheOperationForKey：options：done方法进行从内存和磁盘中查询图片数据

008 在SDImageCache 的queryCacheOperationForKey：options：done回调block中进行如下操作

1.如果这个operation 为空或者被取消了那就安全移除该操作

2.判断是否需要下载那就执行下载，下载失败就会将URL添加到失败数组中，调用代理方法@selector(imageManager:shouldBlockFailedURL:withError:)

下载成功会将URL从失败数组中移除，判断是否存放到磁盘中

3.block回掉返回completionBlock(image, data, error, cacheType, finished, url);