SDWebImage这样用会循环引用吗,为什么?

今天有个朋友问我一个问题:

#import "MyView.h"
#import 
@interface MyView()
@end
@implementation MyView

- (void)dealloc{
    NSLog(@"myView Dealloc");
}

- (instancetype)init
{
    self = [super init];
    if (self) {
        [self commonInit];
    }
    return self;
}

- (void)commonInit{
    UIImageView *imageView = [[UIImageView alloc] init];
    [imageView setBackgroundColor:[UIColor yellowColor]];
    imageView.frame = CGRectMake(0, 0, 100, 100);
    imageView.center = self.center;
    [self addSubview:imageView];
    [imageView sd_setImageWithURL:[NSURL URLWithString:@"https://raw.githubusercontent.com/rs/SDWebImage/master/SDWebImage_logo.png"] completed:^(UIImage * _Nullable image, NSError * _Nullable error, SDImageCacheType cacheType, NSURL * _Nullable imageURL) {
        imageView.frame = CGRectMake(100, 100, 200, 200);
        NSLog(@"image download done");
    }];
}
@end

其中self是一个UIView。这样会造成循环引用吗?

答案是不会!

2018-01-16 19:38:31.678455+0800 SDWebImageTest[30248:2155555] myView Dealloc

为什么不会?这里留一个思考题哈。

提示一下看看SDWebImage的源码,到底谁持有的block

====================答案分析====================

如果你产生了困惑,我想可能是因为下面这个方法。

- (void)sd_setImageWithURL:(nullable NSURL *)url
                 completed:(nullable SDExternalCompletionBlock)completedBlock;

这个方法看上去是imageView调用的一个对象方法,那么这个方法中使用的urlcompletedBlock是不是也是imageView这个对象持有呢?

如果imageView持有completedBlockcompletedBlock中又直接访问imageView,那么这样子看上去是存在循环引用的。
实际并不是这样,我们来看一下源码用代码说话吧。
这个方法内部调用经过了基层封装,最后调用的是如下方法。

- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
                  placeholderImage:(nullable UIImage *)placeholder
                           options:(SDWebImageOptions)options
                      operationKey:(nullable NSString *)operationKey
                     setImageBlock:(nullable SDSetImageBlock)setImageBlock
                          progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                         completed:(nullable SDExternalCompletionBlock)completedBlock
                           context:(nullable NSDictionary *)context {
    NSString *validOperationKey = operationKey ?: NSStringFromClass([self class]);
    [self sd_cancelImageLoadOperationWithKey:validOperationKey];
    objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    
    if (!(options & SDWebImageDelayPlaceholder)) {
        dispatch_main_async_safe(^{
            [self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
        });
    }
    
    if (url) {
        // check if activityView is enabled or not
        if ([self sd_showActivityIndicatorView]) {
            [self sd_addActivityIndicator];
        }
        
        __weak __typeof(self)wself = self;
        id  operation = [SDWebImageManager.sharedManager loadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
            __strong __typeof (wself) sself = wself;
            [sself sd_removeActivityIndicator];
            if (!sself) { return; }
            BOOL shouldCallCompletedBlock = finished || (options & SDWebImageAvoidAutoSetImage);
            BOOL shouldNotSetImage = ((image && (options & SDWebImageAvoidAutoSetImage)) ||
                                      (!image && !(options & SDWebImageDelayPlaceholder)));
            SDWebImageNoParamsBlock callCompletedBlockClojure = ^{
                if (!sself) { return; }
                if (!shouldNotSetImage) {
                    [sself sd_setNeedsLayout];
                }
                if (completedBlock && shouldCallCompletedBlock) {
                    completedBlock(image, error, cacheType, url);
                }
            };
            
            // case 1a: we got an image, but the SDWebImageAvoidAutoSetImage flag is set
            // OR
            // case 1b: we got no image and the SDWebImageDelayPlaceholder is not set
            if (shouldNotSetImage) {
                dispatch_main_async_safe(callCompletedBlockClojure);
                return;
            }
            
            UIImage *targetImage = nil;
            NSData *targetData = nil;
            if (image) {
                // case 2a: we got an image and the SDWebImageAvoidAutoSetImage is not set
                targetImage = image;
                targetData = data;
            } else if (options & SDWebImageDelayPlaceholder) {
                // case 2b: we got no image and the SDWebImageDelayPlaceholder flag is set
                targetImage = placeholder;
                targetData = nil;
            }
            BOOL shouldUseGlobalQueue = NO;
            if (context && [context valueForKey:SDWebImageInternalSetImageInGlobalQueueKey]) {
                shouldUseGlobalQueue = [[context valueForKey:SDWebImageInternalSetImageInGlobalQueueKey] boolValue];
            }
            dispatch_queue_t targetQueue = shouldUseGlobalQueue ? dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0) : dispatch_get_main_queue();
            
            dispatch_queue_async_safe(targetQueue, ^{
                [sself sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock];
                dispatch_main_async_safe(callCompletedBlockClojure);
            });
        }];
        [self sd_setImageLoadOperation:operation forKey:validOperationKey];
    } else {
        dispatch_main_async_safe(^{
            [self sd_removeActivityIndicator];
            if (completedBlock) {
                NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
                completedBlock(nil, error, SDImageCacheTypeNone, url);
            }
        });
    }
}

因为这个方法是分类方法,不能直接给imageView添加属性。所以这里用objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);的方式把url添加为ImageView的辅助对象。不过这个和咱们讨论的问题无关。
这里主要看completedBlock是不是imageView持有的

观察下面代码

__weak __typeof(self)wself = self;
id  operation = [SDWebImageManager.sharedManager loadImageWithURL:url options:options progress:progressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
     __strong __typeof (wself) sself = wself;
    [sself sd_removeActivityIndicator];
    if (!sself) { return; }
    BOOL shouldCallCompletedBlock = finished || (options & SDWebImageAvoidAutoSetImage);
    BOOL shouldNotSetImage = ((image && (options & SDWebImageAvoidAutoSetImage)) ||
                                      (!image && !(options & SDWebImageDelayPlaceholder)));
            SDWebImageNoParamsBlock callCompletedBlockClojure = ^{
        if (!sself) { return; }
        if (!shouldNotSetImage) {
             [sself sd_setNeedsLayout];
        }
        if (completedBlock && shouldCallCompletedBlock) {
            completedBlock(image, error, cacheType, url);
        }
     };
    ……
}];

其中SDWebImageManager.sharedManagerself(也就是imageView)存在弱引用关系,便于图片下载完成后给imageView赋值。
completedBlock这里是copySDWebImageManager.sharedManager的如下方法当参数的。

- (nullable id )loadImageWithURL:(nullable NSURL *)url
                                              options:(SDWebImageOptions)options
                                             progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                            completed:(nullable SDInternalCompletionBlock)completedBlock;

再继续深入SDWebImage内部的各种封装传递。completedBlock最终的执行在如下方法中。

- (void)callCompletionBlockForOperation:(nullable SDWebImageCombinedOperation*)operation
                             completion:(nullable SDInternalCompletionBlock)completionBlock
                                  image:(nullable UIImage *)image
                                   data:(nullable NSData *)data
                                  error:(nullable NSError *)error
                              cacheType:(SDImageCacheType)cacheType
                               finished:(BOOL)finished
                                    url:(nullable NSURL *)url {
    dispatch_main_async_safe(^{
        if (operation && !operation.isCancelled && completionBlock) {
            completionBlock(image, data, error, cacheType, finished, url);
        }
    });
}

这个方法正式SDWebImageManager.sharedManager的对象方法。
所以实际上的持有关系如下


所以当 ViewController释放之后。 ViewMyView都会顺序释放。而 imageView因为还在被 completionBlock持有所以不会释放。只有当 completionBlock执行完成后, completionBlockimageView的引用也消失了, imageView才会释放。

思考题*
如果在completionBlock中没有对imageView添加强引用,这时候的释放顺序是怎么样的?

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