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。