目标 C：在图像完成之前完成 For 循环

Question

我有一个arrayOfLinks，其中包含很多 url 链接。我需要从这些链接中获取图像。我正在使用以下代码来执行此操作。

- (void)getImages {
    NSArray *links = arrayOfLinks;

    for (NSString *link in links ) {
            [self.picImage sd_setImageWithURL:[NSURL URLWithString:link] placeholderImage:nil options:SDWebImageHighPriority completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
                NSLog(@"pic image set finished");
            }];
        }
           [self finishSettingImages];
           NSLog(@"for loop finished");
    }

这几乎可以工作，但问题是 for 循环在图像设置之前完成。我想在设置每个图像后调用方法finishSettingImages。最好的方法是什么？

编辑：我希望在设置所有图像后调用方法“finishSettingImages”。很抱歉没有澄清这一点。

Answer 1

只需使用 GCD 组。

    NSArray *links = arrayOfLinks;

    dispatch_group_t group = dispatch_group_create();

    for (NSString *link in links) {

        dispatch_group_enter(group);

        [self.picImage sd_setImageWithURL:[NSURL URLWithString:link] placeholderImage:nil options:SDWebImageHighPriority completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
            dispatch_group_leave(group);
        }];
    }

    // you can use dispatch_get_main_queue() or background here
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        // all is done            
    });

如果只需要获取图片，可以使用SDWebImageManager：

    NSArray *links = arrayOfLinks;

    __block NSMutableArray *images = [NSMutableArray new];

    dispatch_group_t group = dispatch_group_create();

    for (NSString *link in links) {

        dispatch_group_enter(group);
        SDWebImageManager *manager = [SDWebImageManager sharedManager];

        [manager downloadImageWithURL:[NSURL URLWithString:link]
                              options:SDWebImageRetryFailed
                             progress:^(NSInteger receivedSize, NSInteger expectedSize) {}
                            completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
                                if (image) {
                                   [images addObject:image];
                                } 
                                dispatch_group_leave(group);
                            }];

    }

    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        // all is done
        for (UIImage *image in images) {
            //do something with images
        }
    });

Swift 3/4 的代码示例：

var arrayOfLinks = ... // you string links are here

let dispatchGroup = DispatchGroup()

for link in arrayOfLinks {
    if let url = URL(string: link) {
        dispatchGroup.enter()
        self.imageView.sd_setImage(with: url) { (image, error, cacheType, url) in
            dispatchGroup.leave()
        }
    }
}

dispatchGroup.notify(queue: .main) {
    print("all is done ")
}

Answer 2

编辑：这个答案很糟糕，正如 cmets 中的 Josh 所指出的，我没有看到 index == total 检查的比赛条件

试试这个：

- (void)getImages 
{
   NSArray *links = arrayOfLinks;
   NSUInteger total = [links count];
   __block NSUInteger index = 0;
   __weak id *weakSelf = self; //if your picImage retains the completion block as a property, 
   //then you will want to capture a weak reference to self, so you don't have a retain cycle
   for (NSString *link in links ) {
      [self.picImage sd_setImageWithURL:[NSURL URLWithString:link] placeholderImage:nil options:SDWebImageHighPriority completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
         NSLog(@"pic image set finished");
         index += 1;
         if( index == total )
         {
            //again if you don't store this block as a property, you can just reference self instead of weakSelf
            [weakSelf finishSettingImages];
         }
      }];
   }
   NSLog(@"for loop finished");
}

__block 属性使块使用指向变量的指针，以便它可以在块的执行过程中修改值。这就是我们可以增加索引变量的方式。 total 不是 __block，因此当您定义块时，total 的当前状态是每次块执行时将使用的状态。因此，即使您在块内将total 从10 设置为100，下次执行该块时，它仍会将total 视为10。