SDWebImage之UIView+WebCache

UIView+WebCache是我们能很方便的使用sd_setImageWithURL:系列方法来加载图片的关键类。UIButton(WebCache)、MKAnnotationView(WebCache)、UIImageView(HighlightedWebCache)、FLAnimatedImageView(WebCache)都会调用UIView(WebCache)的sd_internalSetImageWithURL:方法来做图片加载请求，实现了视图本身加载图片的方式。它和 UIView+WebCacheOperation配合使用。下面我们来看一下它的实现。

1.核心方法

/**
 * 所有UIView及其子类都是通过这个方法来加载图片
 *
 * 异步下载和缓存
 *
 * @param url            图片URL
 * @param placeholder   占位图
 * @param options        加载选项
 * @param operationKey   操作（operation）的 key，如果为空时，将使用类名。这个主要使用来取消一个 operation，结合 UIView+WebCacheOperation.h 使用
 * @param setImageBlock  如果不想使用 SD 加载完图片后显示到视图上，可以使用这个 Block 自定义加载图片，这样就可以在调用加载图片的方法中加载图片。
 * @param progressBlock  进度回调
 * @param completedBlock 图片加载完成后的回调
 */
- (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 {
    NSString *validOperationKey = operationKey ?: NSStringFromClass([self class]);
    //取消当前类对应的所有下载Operation对象
    [self sd_cancelImageLoadOperationWithKey:validOperationKey];
    //把UIImageView的加载图片操作和它自身用关联对象关联起来，方便后面取消等操作。关联的key就是UIImageView对应的类名
    objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    //如果有设置占位图，则先显示占位图
    if (!(options & SDWebImageDelayPlaceholder)) {
        dispatch_main_async_safe(^{
            [self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
        });
    }
    
    if (url) {
        //如果UIImageView对象有设置添加转动菊花数据，加载的时候添加转动的菊花
        if ([self sd_showActivityIndicatorView]) {
            [self sd_addActivityIndicator];
        }
        
        __weak __typeof(self)wself = self;
        //operation是一个`SDWebImageCombinedOperation`对象，通过这个对象来取消下载等操作
        id <SDWebImageOperation> 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;
            //停止菊花
            [sself sd_removeActivityIndicator];
            if (!sself) {
                return;
            }
            dispatch_main_async_safe(^{
                if (!sself) {
                    return;
                }
                //如果设置了不自动显示图片，则直接调用completedBlock，让调用者处理图片的显示
                if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock) {
                    completedBlock(image, error, cacheType, url);
                    return;
                } else if (image) {
                    //自动显示图片
                    [sself sd_setImage:image imageData:data basedOnClassOrViaCustomSetImageBlock:setImageBlock];
                    [sself sd_setNeedsLayout];
                } else {
                    if ((options & SDWebImageDelayPlaceholder)) {
                        //如果设置了延迟显示占位图，则图片加载失败的情况下显示占位图
                        [sself sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
                        [sself sd_setNeedsLayout];
                    }
                }
                //完成回调
                if (completedBlock && finished) {
                    completedBlock(image, error, cacheType, url);
                }
            });
        }];
        //关联Operationkey与Operation对象。方便后面根据key取消operation操作等
        [self sd_setImageLoadOperation:operation forKey:validOperationKey];
    } else {
        //加载失败的情况
        dispatch_main_async_safe(^{
            //移除菊花
            [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);
            }
        });
    }
}

/**
 取消当前Class对应的所有加载请求
 */
- (void)sd_cancelCurrentImageLoad {
    [self sd_cancelImageLoadOperationWithKey:NSStringFromClass([self class])];
}

- (void)sd_setImage:(UIImage *)image imageData:(NSData *)imageData basedOnClassOrViaCustomSetImageBlock:(SDSetImageBlock)setImageBlock {
    if (setImageBlock) {
        setImageBlock(image, imageData);
        return;
    }
    
#if SD_UIKIT || SD_MAC
    if ([self isKindOfClass:[UIImageView class]]) {
        UIImageView *imageView = (UIImageView *)self;
        imageView.image = image;
    }
#endif
    
#if SD_UIKIT
    if ([self isKindOfClass:[UIButton class]]) {
        UIButton *button = (UIButton *)self;
        [button setImage:image forState:UIControlStateNormal];
    }
#endif
}

2.旋转的菊花

#pragma mark 通过关联对象来实现菊花的添加
#if SD_UIKIT
- (UIActivityIndicatorView *)activityIndicator {
    return (UIActivityIndicatorView *)objc_getAssociatedObject(self, &TAG_ACTIVITY_INDICATOR);
}

- (void)setActivityIndicator:(UIActivityIndicatorView *)activityIndicator {
    objc_setAssociatedObject(self, &TAG_ACTIVITY_INDICATOR, activityIndicator, OBJC_ASSOCIATION_RETAIN);
}
#endif

#pragma mark 是否显示旋转菊花

- (void)sd_setShowActivityIndicatorView:(BOOL)show {
    objc_setAssociatedObject(self, &TAG_ACTIVITY_SHOW, @(show), OBJC_ASSOCIATION_RETAIN);
}

- (BOOL)sd_showActivityIndicatorView {
    return [objc_getAssociatedObject(self, &TAG_ACTIVITY_SHOW) boolValue];
}

#if SD_UIKIT

#pragma mark 旋转菊花的样式

- (void)sd_setIndicatorStyle:(UIActivityIndicatorViewStyle)style{
    objc_setAssociatedObject(self, &TAG_ACTIVITY_STYLE, [NSNumber numberWithInt:style], OBJC_ASSOCIATION_RETAIN);
}

- (int)sd_getIndicatorStyle{
    return [objc_getAssociatedObject(self, &TAG_ACTIVITY_STYLE) intValue];
}
#endif

- (void)sd_addActivityIndicator {
#if SD_UIKIT
    dispatch_main_async_safe(^{
        if (!self.activityIndicator) {
            self.activityIndicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:[self sd_getIndicatorStyle]];
            self.activityIndicator.translatesAutoresizingMaskIntoConstraints = NO;
        
            [self addSubview:self.activityIndicator];
            
            [self addConstraint:[NSLayoutConstraint constraintWithItem:self.activityIndicator
                                                             attribute:NSLayoutAttributeCenterX
                                                             relatedBy:NSLayoutRelationEqual
                                                                toItem:self
                                                             attribute:NSLayoutAttributeCenterX
                                                            multiplier:1.0
                                                              constant:0.0]];
            [self addConstraint:[NSLayoutConstraint constraintWithItem:self.activityIndicator
                                                             attribute:NSLayoutAttributeCenterY
                                                             relatedBy:NSLayoutRelationEqual
                                                                toItem:self
                                                             attribute:NSLayoutAttributeCenterY
                                                            multiplier:1.0
                                                              constant:0.0]];
        }
        [self.activityIndicator startAnimating];
    });
#endif
}

- (void)sd_removeActivityIndicator {
#if SD_UIKIT
    dispatch_main_async_safe(^{
        if (self.activityIndicator) {
            [self.activityIndicator removeFromSuperview];
            self.activityIndicator = nil;
        }
    });
#endif
}