SDWebImage (5.1.0)UIView+WebCacheOperation(六)

#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> 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)sd_imageLoadOperationForKey:(nullable NSString *)key  {
    id 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)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 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> 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/