第十一篇
前言
我们知道SDWebImageManager是用来管理图片下载的，但我们平时的开发更多的是使用UIImageView和UIButton这两个控件显示图片。
按照正常的想法，我们只需要在他们的分类中，通过SDWebImageManager把图片下载下载之后，再进行赋值就行了。但这样的设计并不是最好的设计，我们在准备提供一项功能的时候，应该要尽可能的弄明白这个功能的使用者是谁？这些使用者的共同点是什么？
UIImageView和UIButton这两个控件都继承自UIView。那么我们是不是可以给UIView赋予加载图片的功能，然后，UIImageView和UIButton不就都有这个能力了吗？
除了讲解这些之外，在本文的最后，我们给view的layer也添加这个能力。
UIView+WebCache
/**
* Set the imageView `image` with an `url` and optionally a placeholder image.
*
* The download is asynchronous and cached.
*
* @param url The url for the image.
* @param placeholder The image to be set initially, until the image request finishes.
* @param options The options to use when downloading the image. @see SDWebImageOptions for the possible values.
* @param operationKey A string to be used as the operation key. If nil, will use the class name
* @param setImageBlock Block used for custom set image code
* @param progressBlock A block called while image is downloading
* @note the progress block is executed on a background queue
* @param completedBlock A block called when operation has been completed. This block has no return value
* and takes the requested UIImage as first parameter. In case of error the image parameter
* is nil and the second parameter may contain an NSError. The third parameter is a Boolean
* indicating if the image was retrieved from the local cache or from the network.
* The fourth parameter is the original image url.
*/
- (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;
实现方法：
- (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 {
/// 每一个view都给他绑定一个operation，通过一个key来获取这个operation
/// 如果operationKey为nil，就采用他自身的类的字符串作为key
NSString *validOperationKey = operationKey ?: NSStringFromClass([self class]);
/// 这是一个view的分类方法，目的是取消之前绑定的operation
[self sd_cancelImageLoadOperationWithKey:validOperationKey];
/// 为自身绑定一个URL
objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
/// 如果不是延迟显示placeholder的情况
if (!(options & SDWebImageDelayPlaceholder)) {
dispatch_main_async_safe(^{
/// 在图片下载下来之前，先给自身赋值一个placeholder
[self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
});
}
if (url) {
// check if activityView is enabled or not
/// 如果设置了显示加载动画，就添加ActivityIndicatorView
if ([self sd_showActivityIndicatorView]) {
[self sd_addActivityIndicator];
}
/// 开始下载图片
__weak __typeof(self)wself = self;
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;
/// 下载完成后关闭动画
[sself sd_removeActivityIndicator];
if (!sself) {
return;
}
dispatch_main_async_safe(^{
if (!sself) {
return;
}
if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock) {
completedBlock(image, error, cacheType, url);
return;
} else if (image) {
[sself sd_setImage:image imageData:data basedOnClassOrViaCustomSetImageBlock:setImageBlock];
[sself sd_setNeedsLayout];
} else {
/// 如果是设置了SDWebImageDelayPlaceholder，那么就会在下载完成之后给自身赋值placeholder
if ((options & SDWebImageDelayPlaceholder)) {
[sself sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
[sself sd_setNeedsLayout];
}
}
if (completedBlock && finished) {
completedBlock(image, error, cacheType, url);
}
});
}];
/// 绑定operation
[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);
}
});
}
}其实这个核心的实现方法也很简单，逻辑也不复杂，需要注意一下下边的方法：
- (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
}这个方法说明如果我们设置了setImageBlock,那么view的显示图片的逻辑应该写到这个Block中，如果setImageBlock为nil，就判断是不是UIImageView和UIButton。
UIImageView的实现
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDExternalCompletionBlock)completedBlock {
[self sd_internalSetImageWithURL:url
placeholderImage:placeholder
options:options
operationKey:nil
setImageBlock:nil
progress:progressBlock
completed:completedBlock];
}CALay的实现
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDExternalCompletionBlock)completedBlock {
NSString *validOperationKey = @"CALayerImages" ?: NSStringFromClass([self class]);
[self sd_cancelImageLoadOperationWithKey:validOperationKey];
objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
if (!(options & SDWebImageDelayPlaceholder)) {
dispatch_main_async_safe(^{
self.contents = (__bridge id _Nullable)placeholder.CGImage;
});
}
if (url) {
__weak __typeof(self)wself = self;
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;
if (!sself) {
return;
}
dispatch_main_async_safe(^{
if (!sself) {
return;
}
if (image && (options & SDWebImageAvoidAutoSetImage) && completedBlock) {
completedBlock(image, error, cacheType, url);
return;
} else if (image) {
self.contents = (__bridge id _Nullable)image.CGImage;
} else {
if ((options & SDWebImageDelayPlaceholder)) {
self.contents = (__bridge id _Nullable)placeholder.CGImage;
}
}
if (completedBlock && finished) {
completedBlock(image, error, cacheType, url);
}
});
}];
[self sd_setImageLoadOperation:operation forKey:validOperationKey];
} else {
dispatch_main_async_safe(^{
if (completedBlock) {
NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
completedBlock(nil, error, SDImageCacheTypeNone, url);
}
});
}
}
- (SDOperationsDictionary *)operationDictionary {
SDOperationsDictionary *operations = objc_getAssociatedObject(self, &loadOperationKey);
if (operations) {
return operations;
}
operations = [NSMutableDictionary dictionary];
objc_setAssociatedObject(self, &loadOperationKey, operations, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
return operations;
}
- (void)sd_setImageLoadOperation:(nullable id)operation forKey:(nullable NSString *)key {
if (key) {
[self sd_cancelImageLoadOperationWithKey:key];
if (operation) {
SDOperationsDictionary *operationDictionary = [self operationDictionary];
operationDictionary[key] = operation;
}
}
}
- (void)sd_cancelImageLoadOperationWithKey:(nullable NSString *)key {
// Cancel in progress downloader from queue
SDOperationsDictionary *operationDictionary = [self operationDictionary];
id operations = operationDictionary[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];
}
}
- (void)sd_removeImageLoadOperationWithKey:(nullable NSString *)key {
if (key) {
SDOperationsDictionary *operationDictionary = [self operationDictionary];
[operationDictionary removeObjectForKey:key];
}
}实现思路跟给view添加该能力完全相同。
[self.imageView.layer sd_setImageWithURL:url placeholderImage:[UIImage imageNamed:@"4-3397163ecdb3855a0a4139c34a695885.jpg"] options:0 progress:nil completed:nil];总结
SDWebImage的源码就看到这里了，下一篇我会把这十一篇中用到的知识点进行一个总结。