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:- 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
}