前言
上一篇中分析了实际上UIImageView+WebCache调用的是
[self sd_internalSetImageWithURL:url
placeholderImage:placeholder
options:0
operationKey:nil
setImageBlock:nil
progress:nil
completed:nil];
将它的实现方法分为4部分
- (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 {
// 1.
NSString *validOperationKey = NSStringFromClass([self class]);
[self sd_cancelImageLoadOperationWithKey:validOperationKey];
// 2.
objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
//3.
if (!(options & SDWebImageDelayPlaceholder)) {
dispatch_main_async_safe(^{
[self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
});
}
// 4.
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) {
[sself sd_setImage:image imageData:data basedOnClassOrViaCustomSetImageBlock:setImageBlock];
[sself sd_setNeedsLayout];
} else {
if ((options & SDWebImageDelayPlaceholder)) {
[sself sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
[sself sd_setNeedsLayout];
}
}
});
}];
[self sd_setImageLoadOperation:operation forKey:validOperationKey];
}
}
1. 取消正在进行的loading操作
NSString *validOperationKey = NSStringFromClass([self class]); // part1
[self sd_cancelImageLoadOperationWithKey:validOperationKey];// part2
part1. 其中validOperationKey在当前的例子中就是@"UIImageView",因此
NSString *validOperationKey = @"UIImageView";
2. 为这个对象绑定一个属性，key 名为imageURLKey，value就是我们传入的网址url
objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
3. 如果设置的Options值不包含SDWebImageDelayPlaceholder,那么先将imageView的image设置为placeHolder
if (!(options & SDWebImageDelayPlaceholder)) { // a.
dispatch_main_async_safe(^{ // b.
[self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock]; // c.
});
}
简单分析一下
a. options & SDWebImageDelayPlaceholder 表示options为NS_OPTIONS 枚举变量是否包含SDWebImageDelayPlaceholder值，如果是返回 True,否则返回False.显然我们这里返回False,取反后为True.
b. dispatch_main_async_safe()是一个宏函数
#ifndef dispatch_main_async_safe
#define dispatch_main_async_safe(block)\
if (strcmp(dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL), dispatch_queue_get_label(dispatch_get_main_queue())) == 0) {\
block();\
} else {\
dispatch_async(dispatch_get_main_queue(), block);\
}
#endif
如果当前线程是主线程 ，调用 [self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];在主线程上，直接同步顺序执行，否则异步提交到主线程上执行。因为在主线程上异步提交一个任务到主线程上会引起随机的崩溃。
c. [self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock]对应的方法实现如下
- (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
}
在我们的代码调用中，setImageBlock为空，所以 下面代码不进行任何操作
if (setImageBlock) {
setImageBlock(image, imageData);
return;
}
SD_UIKIT ，SD_MAC两个简单的宏，前者表示在运行的设备是iOS设备或者是iwatch.后者表示平台是mac平台，对应的定义在SDWebImageCompat文件中。如下
#if !TARGET_OS_IPHONE && !TARGET_OS_IOS && !TARGET_OS_TV && !TARGET_OS_WATCH
#define SD_MAC 1
#else
#define SD_MAC 0
#endif
// iOS and tvOS are very similar, UIKit exists on both platforms
// Note: watchOS also has UIKit, but it's very limited
#if TARGET_OS_IOS || TARGET_OS_TV
#define SD_UIKIT 1
#else
#define SD_UIKIT 0
#endif
所以翻译一下，如果self 是 UIImageView就设置imageView.image = image,如果是iOS平台设置UIbutton的normal状态的image
4.调用SDWebImageManager.sharedManager的下载方法在回调中处理返回的数据
a.我们先研究一下回调做了些什么事情
{
__strong __typeof (wself) sself = wself;
if (!sself) {
return;
}
dispatch_main_async_safe(^{
if (!sself) {
return;
}
if (image) {
[sself sd_setImage:image imageData:data basedOnClassOrViaCustomSetImageBlock:setImageBlock];
[sself sd_setNeedsLayout];
} else {
if ((options & SDWebImageDelayPlaceholder)) {
[sself sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
[sself sd_setNeedsLayout];
}
}
});
}
如果当前对象已经被释放，那么不执行任何操作，如果获取到图片，那么设置图片当imageView.image上。如果没有且options有SDWebImageDelayPlaceholder值，那么用placeHolder图片设置最终显示的imageview.image.
b. 最后一个方法 [self sd_setImageLoadOperation:operation forKey:validOperationKey];实现如下
- (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;
}
}
}
- (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;
}
上面两个方法都是UIView+WebCacheOperation的方法
做了两件事情，1.是取消当前正在下载的操作 2.设置保存一个键值对在loadOperationKey对应的字典中。
key就是@"UIImageView"，value是id <SDWebImageOperation>类型的对象
后续
后续我们顺着这个思路研究一下SDWebImageManager.sharedManager的下载方法- (id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url options:(SDWebImageOptions)options progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock completed:(nullable SDInternalCompletionBlock)completedBlock做了什么事情
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