#import "UIView+WebCacheOperation.h"
#import "objc/runtime.h"
static char loadOperationKey;
// key is strong, value is weak because operation instance is retained by SDWebImageManager's runningOperations property
// we should use lock to keep thread-safe because these method may not be acessed from main queue
typedef NSMapTable<NSString *, id<SDWebImageOperation>> SDOperationsDictionary;
@implementation UIView (WebCacheOperation)
//获取任务字典NSMapTable,将NSMapTable关联对象到UIView
- (SDOperationsDictionary *)sd_operationDictionary {
@synchronized(self) {
//如果取到operations就返回否则就创建,保证线程安全
SDOperationsDictionary *operations = objc_getAssociatedObject(self, &loadOperationKey);
if (operations) {
return operations;
}
//https://www.jianshu.com/p/cf4e15b26f64
//NSHashTable与NSMapTable
//用NSMapTable保存operation,NSMapTable可以设置value为弱引用,key为强引用
operations = [[NSMapTable alloc] initWithKeyOptions:NSPointerFunctionsStrongMemory valueOptions:NSPointerFunctionsWeakMemory capacity:0];
objc_setAssociatedObject(self, &loadOperationKey, operations, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
return operations;
}
}
//通过key去拿operation
- (nullable id<SDWebImageOperation>)sd_imageLoadOperationForKey:(nullable NSString *)key {
id<SDWebImageOperation> operation;
if (key) {
SDOperationsDictionary *operationDictionary = [self sd_operationDictionary];
@synchronized (self) {
operation = [operationDictionary objectForKey:key];
}
}
return operation;
}
//设置key和value
- (void)sd_setImageLoadOperation:(nullable id<SDWebImageOperation>)operation forKey:(nullable NSString *)key {
if (key) {
//先把之前的key对应的operation给删除了
[self sd_cancelImageLoadOperationWithKey:key];
//然后添加新的operation
if (operation) {
SDOperationsDictionary *operationDictionary = [self sd_operationDictionary];
@synchronized (self) {
[operationDictionary setObject:operation forKey:key];
}
}
}
}
//通过key把operation给取出来,然后调用cancel方法,取消下载
- (void)sd_cancelImageLoadOperationWithKey:(nullable NSString *)key {
if (key) {
// Cancel in progress downloader from queue
SDOperationsDictionary *operationDictionary = [self sd_operationDictionary];
id<SDWebImageOperation> operation;
@synchronized (self) {
operation = [operationDictionary objectForKey:key];
}
if (operation) {
if ([operation conformsToProtocol:@protocol(SDWebImageOperation)]) {
[operation cancel];
}
@synchronized (self) {
[operationDictionary removeObjectForKey:key];
}
}
}
}
- (void)sd_removeImageLoadOperationWithKey:(nullable NSString *)key {
if (key) {
SDOperationsDictionary *operationDictionary = [self sd_operationDictionary];
@synchronized (self) {
[operationDictionary removeObjectForKey:key];
}
}
}
@end
sd给每个UIView都绑定了一个NSMapTable用于存放当前的下载任务，这就解决了tableview在滑动的时候当cell复用时，image不会错位的问题，这个分类的作用就是解决错位的问题的，当我们给一个imageview下载图片时候，首先去NSMapTable里把当前正在执行的operation给cancel掉然后从NSMapTable移除，然后把新的任务加入到NSMapTable中，这是面试经常被问到的一个考点。
https://sdwebimage.github.io/Categories/UIView(WebCacheOperation).html
然后我们研究下这个NSMapTable，sd为什么不用NSMutableDictionary呢，说明NSMapTable和NSMutableDictionary是有区别的。
key is strong, value is weak because operation instance is retained by SDWebImageManager's runningOperations property we should use lock to keep thread-safe because these method may not be acessed from main queue
typedef NSMapTable<NSString *, id<SDWebImageOperation>> SDOperationsDictionary;
/ key很强，值很弱，因为SDWebImageManager的runningOperations属性保留了操作实例
所以NSMapTable保存的value是弱引用的，当任务完成后自动会从NSMapTable中移除
NSHashTable和NSMapTable相关文章
https://www.jianshu.com/p/39b57ecc99fe
https://objccn.io/issue-7-1/
