三、SDWebImage源码解读UIImageView+WebCache

UIImageView+WebCache这个类别是我们常用到的，我相信绝大部分的程序员都看过其中的内容,直接上代码

- (void)sd_setImageWithURL:(nullable NSURL *)url
- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options
- (void)sd_setImageWithURL:(nullable NSURL *)url
                 completed:(nullable SDExternalCompletionBlock)completedBlock
- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                 completed:(nullable SDExternalCompletionBlock)completedBlock
- (void)sd_setImageWithURL:(nullable NSURL *)url
          placeholderImage:(nullable UIImage *)placeholder
                   options:(SDWebImageOptions)options
                 completed:(nullable SDExternalCompletionBlock)completedBlock
- (void)sd_setImageWithPreviousCachedImageWithURL:(nullable NSURL *)url
                                 placeholderImage:(nullable UIImage *)placeholder
                                          options:(SDWebImageOptions)options
                                         progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                        completed:(nullable SDExternalCompletionBlock)completedBlock

这里的设计思路值得我们学习，当你写一个对外的接口或者方法的时候，可以根据不能的需求定制不同的方法，这样可以满足不同需求的人，而且显得美观和整洁。

- (void)sd_setAnimationImagesWithURLs:(nonnull NSArray *)arrayOfURLs {
    [self sd_cancelCurrentAnimationImagesLoad];
    __weak __typeof(self)wself = self;

    NSPointerArray *operationsArray = [self sd_animationOperationArray];
    //这个方法,遍历字典和量大的数组和遍历字典时性能比 for in 好,且代码更加优雅;对于单个数组使用for in 新能更好
    [arrayOfURLs enumerateObjectsUsingBlock:^(NSURL *logoImageURL, NSUInteger idx, BOOL * _Nonnull stop) {//遍历urls
        id  operation = [[SDWebImageManager sharedManager] loadImageWithURL:logoImageURL options:0 progress:nil completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
            if (!wself) return;
            dispatch_main_async_safe(^{
                __strong UIImageView *sself = wself;
                [sself stopAnimating];
                if (sself && image) {
                    NSMutableArray *currentImages = [[sself animationImages] mutableCopy];//处理gif图像
                    if (!currentImages) {
                        currentImages = [[NSMutableArray alloc] init];
                    }
                    
                    // We know what index objects should be at when they are returned so
                    // we will put the object at the index, filling any empty indexes
                    // with the image that was returned too "early". These images will
                    // be overwritten. (does not require additional sorting datastructure)
                    while ([currentImages count] < idx) { //若gif图片将静态数组添加到数组中
                        [currentImages addObject:image];
                    }
                    
                    currentImages[idx] = image;

                    sself.animationImages = currentImages;
                    [sself setNeedsLayout];
                }
                [sself startAnimating];
            });
        }];
        @synchronized (self) {
            [operationsArray addPointer:(__bridge void *)(operation)]; //添加operation
        }
    }];
}

static char animationLoadOperationKey;

// element 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
- (NSPointerArray *)sd_animationOperationArray {
    @synchronized(self) {
        NSPointerArray *operationsArray = objc_getAssociatedObject(self, &animationLoadOperationKey);//关联对象
        if (operationsArray) {
            return operationsArray;
        }
        operationsArray = [NSPointerArray weakObjectsPointerArray]; //建立弱连接
        objc_setAssociatedObject(self, &animationLoadOperationKey, operationsArray, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
        return operationsArray;
    }
}

- (void)sd_cancelCurrentAnimationImagesLoad {
    NSPointerArray *operationsArray = [self sd_animationOperationArray];
    if (operationsArray) {
        @synchronized (self) {//同步锁，防止多线程同时访问
            for (id operation in operationsArray) {
                if ([operation conformsToProtocol:@protocol(SDWebImageOperation)]) { // operation 是否遵守SDWebImageOperation协议
                    [operation cancel]; //取消
                }
            }
            operationsArray.count = 0;
        }
    }
}

扩展知识
1.NSMapTable， NSHashTable NSPointerArray
现在我们虽然都在用ARC,但是有时候我们需要精确的知道我们是不是在dic，set，和array中持有还是不持有对象，但是NSDictionary，NSSet，NSArray，所以我们需要NSMapTable， NSHashTable NSPointerArray
以NSPointerArray为例

+ (NSPointerArray *)strongObjectsPointerArray //建立强连接
+ (NSPointerArray *)weakObjectsPointerArray //建立弱连接

2. enumerateObjectsUsingBlock 这个方法,遍历字典和量大的数组和遍历字典时性能比 for in 好,且代码更加优雅;对于单个数组使用for in 新能更好 在很多框架中都被使用，以利用到多核cpu的优势
   用法：
   NSArray 和 NSSet 都可以使用

• (void)enumerateObjectsUsingBlock:(void (NS_NOESCAPE ^)(ObjectType obj, NSUInteger idx, BOOL *stop))block
• (void)enumerateObjectsWithOptions:(NSEnumerationOptions)opts usingBlock:(void (NS_NOESCAPE ^)(ObjectType obj, NSUInteger idx, BOOL *stop))block

NSDictionary 使用

• (void)enumerateKeysAndObjectsUsingBlock:(void (NS_NOESCAPE ^)(KeyType key, ObjectType obj, BOOL *stop))block
• (void)enumerateKeysAndObjectsWithOptions:(NSEnumerationOptions)opts usingBlock:(void (NS_NOESCAPE ^)(KeyType key, ObjectType obj, BOOL *stop))block

NSEnumerationConcurrent 顺序
NSEnumerationReverse 倒序

例子：

NSArray :
NSArray *array = @[@"a", @"b", @"c"];
    for (NSString *str in [array reverseObjectEnumerator]) { //reverseObjectEnumerator倒序 objectEnumerator 正序
    }
 [array enumerateObjectsWithOptions:NSEnumerationReverse usingBlock:^(Sark *sark, NSUInteger idx, BOOL *stop) {
    }];

NSDictionary:
NSDictionary *dict = @{@"a": @"1", @"b": @"2"};
    [dict enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
    NSLog(@"key: %@, value: %@", key, obj);
    }];

**
遍历执行block会分配在多核cpu上执行，底层可能是GCD的并发queue，对于耗时的任务来说是不错的选择，同事，对于遍历的外部是保持同步的，遍历都完成后才继续执行下一行。
**