SDWebImage源码阅读-第一篇
一 题外话
之前写过一篇最新版SDWebImage的使用，也简单的介绍了一下原理。这两天正梳理自己的知识网络，觉得有必要再阅读一下源码，一是看具体实现，二是学习一下优秀开源代码的代码风格，比如接口设计，设计模式，变量命名等等。
既然是第一篇，就要制定一个阅读源码的计划，以什么顺序阅读完全部代码。我们从最常见的入口切入sd_setImageWithURL，一路下去，最后再阅读没有设计到的部分。
在开始之前强烈建议先去读我之前的文章：最新版SDWebImage的使用。心里有个大概再去探讨细节，效果更佳。
二 入口
我们为什么使用SDWebImage，是因为他帮我们实现了图片的二级缓存，使我们加载图片更流畅。当然你也可以使用SDWebImage中几个很棒的工具类，比如SDWebImageDownloader，用来下载图片。或者SDImageCache用来缓存图片或者NSData。我们先来看看UIImageView+WebCache中的基本方法：
在UIImageView+WebCache类的最上面，很贴心的贴了一个使用例子，这也是我们很常见的tableViewCell加载图片的场景
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath { static NSString *MyIdentifier = @"MyIdentifier"; UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:MyIdentifier]; if (cell == nil) { cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:MyIdentifier] autorelease]; } // Here we use the provided sd_setImageWithURL: method to load the web image // Ensure you use a placeholder image otherwise cells will be initialized with no image [cell.imageView sd_setImageWithURL:[NSURL URLWithString:@"http://example.com/image.jpg"] placeholderImage:[UIImage imageNamed:@"placeholder"]]; cell.textLabel.text = @"My Text"; return cell; }
而sd_setImageWithURL: placeholderImage:，也是我们最常使用的方法，我们看看除了这个外的其他方法：
//最基本方法 - (void)sd_setImageWithURL:(NSURL *)url; //带placeholder，优先显示placeholder，下载完成后显示原图 - (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder; //多了缓存策略options - (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options; //多了下载完成后的block回调 - (void)sd_setImageWithURL:(NSURL *)url completed:(SDWebImageCompletionBlock)completedBlock; //多了下载完成后的block回调 - (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder completed:(SDWebImageCompletionBlock)completedBlock; //多了下载完成后的block回调 - (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options completed:(SDWebImageCompletionBlock)completedBlock; //多了下载过程progressBlock回调 - (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock; //多了下载过程progressBlock回调 - (void)sd_setImageWithPreviousCachedImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock;
这个接口设计很经典，也是看了这个接口设计，回头重构了自己项目中社交分享模块的接口。而下面这个方法，也可以看做是外观模式的具体体现。
- (void)sd_setImageWithURL:(NSURL *)url;
废话少说，进去看看实现。可以看到，所有方法都指向
- (void)sd_setImageWithPreviousCachedImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock;
具体实现如下
- (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock { [self sd_cancelCurrentImageLoad]; objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC); if (!(options & SDWebImageDelayPlaceholder)) { dispatch_main_async_safe(^{ self.image = placeholder; }); } if (url) { // check if activityView is enabled or not if ([self showActivityIndicatorView]) { [self addActivityIndicator]; } __weak __typeof(self)wself = self; id <SDWebImageOperation> operation = [SDWebImageManager.sharedManager downloadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) { [wself removeActivityIndicator]; if (!wself) return; dispatch_main_sync_safe(^{ if (!wself) return; if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock) { completedBlock(image, error, cacheType, url); return; } else if (image) { wself.image = image; [wself setNeedsLayout]; } else { if ((options & SDWebImageDelayPlaceholder)) { wself.image = placeholder; [wself setNeedsLayout]; } } if (completedBlock && finished) { completedBlock(image, error, cacheType, url); } }); }]; [self sd_setImageLoadOperation:operation forKey:@"UIImageViewImageLoad"]; } else { dispatch_main_async_safe(^{ [self removeActivityIndicator]; if (completedBlock) { NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}]; completedBlock(nil, error, SDImageCacheTypeNone, url); } }); } }
可以看到第一步是取消当前下载，本着不放过任何一个细节的精神，我们看看是怎么取消当前下载的
- (void)sd_cancelCurrentImageLoad { [self sd_cancelImageLoadOperationWithKey:@"UIImageViewImageLoad"]; }
- (void)sd_cancelImageLoadOperationWithKey:(NSString *)key { // Cancel in progress downloader from queue NSMutableDictionary *operationDictionary = [self operationDictionary]; id operations = [operationDictionary objectForKey:key]; if (operations) { if ([operations isKindOfClass:[NSArray class]]) { for (id <SDWebImageOperation> operation in operations) { if (operation) { [operation cancel]; } } } else if ([operations conformsToProtocol:@protocol(SDWebImageOperation)]){ [(id<SDWebImageOperation>) operations cancel]; } [operationDictionary removeObjectForKey:key]; } }
我们看到，operationDictionary就是多个线程的集合。在SDWebImageManager的downloadImageWithURL方法中创建operation并返回，保存在operationDictionary中。然后我们从operationDictionary中通过key "UIImageViewImageLoad" 取出负责下载的所有operation，然后cancel掉。并将下载的所有operation从operationDictionary中移除。特别值得注意的是，当前类是UIImageView的category，我们知道，category不能增加属性，只能增加方法，那么operationDictionary是哪里来的呢。答案是：objc_setAssociatedObject，对象关联，动态的给UIImageView添加新属性。在SDWebImage中有很多这种用法，看到你就要知道，这就是动态增加了属性。
继续看如何cancel，SDWebImageOperation是一个协议，而SDWebImageDownloaderOperation实现了这个协议。我们上面说的从operationDictionary中通过key "UIImageViewImageLoad" 取出负责下载的所有queue，其实就是SDWebImageDownloaderOperation的实例的集合。熟悉NSOperation的都知道，SDWebImageDownloaderOperation继承于NSOperation，每一个SDWebImageDownloaderOperation的实例都是一个新线程。NSOperation其实自己也抽象了cancel方法。- (void)sd_cancelImageLoadOperationWithKey:(NSString *)key方法中，[operation cancel];这里的operation其实是SDWebImageManager的
- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionWithFinishedBlock)completedBlock
这个方法中创建的SDWebImageCombinedOperation实例，
__block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new]; __weak SDWebImageCombinedOperation *weakOperation = operation;
再看看SDWebImageCombinedOperation的定义和实现
@interface SDWebImageCombinedOperation : NSObject <SDWebImageOperation> @property (assign, nonatomic, getter = isCancelled) BOOL cancelled; @property (copy, nonatomic) SDWebImageNoParamsBlock cancelBlock; @property (strong, nonatomic) NSOperation *cacheOperation; @end @implementation SDWebImageCombinedOperation - (void)setCancelBlock:(SDWebImageNoParamsBlock)cancelBlock { // check if the operation is already cancelled, then we just call the cancelBlock if (self.isCancelled) { if (cancelBlock) { cancelBlock(); } _cancelBlock = nil; // don't forget to nil the cancelBlock, otherwise we will get crashes } else { _cancelBlock = [cancelBlock copy]; } } - (void)cancel { self.cancelled = YES; if (self.cacheOperation) { [self.cacheOperation cancel]; self.cacheOperation = nil; } if (self.cancelBlock) { self.cancelBlock(); // TODO: this is a temporary fix to #809. // Until we can figure the exact cause of the crash, going with the ivar instead of the setter // self.cancelBlock = nil; _cancelBlock = nil; } } @end
我们看到，调用该operation的cancel方法，其实是执行cancelBlock，我们就看看，他的cancelBlock传入的是什么东西。在SDWebImageManager的downloadImageWithURL方法中，我们找到了赋值的地方
operation.cancelBlock = ^{ [subOperation cancel]; @synchronized (self.runningOperations) { __strong __typeof(weakOperation) strongOperation = weakOperation; if (strongOperation) { [self.runningOperations removeObject:strongOperation]; } } };
我们看到block里面调用了[subOperation cancel];而这个subOperation,是SDWebImageDownloader的downloadImageWithURL方法创建并返回的SDWebImageDownloaderOperation对象，它是NSOperation的子类。终于到了我们熟悉的对象。究其原因，cancle的时候其实就是SDWebImageDownloaderOperation的实例cancel，具体实现如下：
- (void)cancel { @synchronized (self) { if (self.thread) { [self performSelector:@selector(cancelInternalAndStop) onThread:self.thread withObject:nil waitUntilDone:NO]; } else { [self cancelInternal]; } } }
- (void)cancelInternalAndStop { if (self.isFinished) return; [self cancelInternal]; }
- (void)cancelInternal { if (self.isFinished) return; [super cancel]; if (self.cancelBlock) self.cancelBlock(); if (self.dataTask) { [self.dataTask cancel]; dispatch_async(dispatch_get_main_queue(), ^{ [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self]; }); // As we cancelled the connection, its callback won't be called and thus won't // maintain the isFinished and isExecuting flags. if (self.isExecuting) self.executing = NO; if (!self.isFinished) self.finished = YES; } [self reset]; }
最重要的就是这句：[self.dataTask cancel]。self.dataTask是NSURLSessionTask的实例，这里就是取消网络请求。说这么多，仅仅是取消了下载图片的网络请求。
已经写了不少了，把大头戏放到下一篇。下一篇我们主要分析二级缓存的实现和SDWebImageDownloader的异步加载。
