异步加载图像到 UICollectionView？

我如何将图像加载到UICollectionview异步？ 下面的方法里面？

- (PSTCollectionViewCell *)collectionView:(PSTCollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {

 bookImage = [UIImage imageWithContentsOfFile:[NSString stringWithFormat:@"%@/%@", docPath, [[preview objectAtIndex:indexPath.row] lastPathComponent]]];
 [[cell grid_image] setImage:bookImage];

}

In viewdidload()我正在使用以下异步调用将图像加载到“预览”NSMutablearray

 dispatch_queue_t imageLoadQueue = dispatch_queue_create("com.GMM.assamkar", NULL);

dispatch_async(imageLoadQueue, ^{
    //Wait for 5 seconds...
    usleep(1000000);
    docPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];


    for(int k=0; k <[allImage count] ;k++){


        imgURL = [allImage objectAtIndex:k];
        [imagePreview addObject:imgURL];
        imgData=[NSData dataWithContentsOfURL:[NSURL URLWithString:imgURL]];


        [imgData writeToFile:[NSString stringWithFormat:@"%@/%@", docPath, [allImage lastPathComponent]] atomically:YES];

    }

    [[self collectionView] reloadData];


});

请帮助我..现在加载时间太长了...

尝试回答您的主要问题“我如何将图像加载到UICollectionview异步？”

我建议“娜塔莎·穆拉舍夫" here，这对我来说效果很好而且很简单。

If here imgURL = [allImage objectAtIndex:k]; in allImage属性，您保留 URL 数组，然后更新您的collectionView:cellForItemAtIndexPath:像这样的方法：

- (PSTCollectionViewCell *)collectionView:(PSTCollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    NSURL *url = [NSURL URLWithString:[allImage objectAtIndex:indexPath]];

    [self downloadImageWithURL:url completionBlock:^(BOOL succeeded, NSData *data) {
        if (succeeded) {
            cell.grid_image.image = [[UIImage alloc] initWithData:data];
        }
    }];
}

并添加方法downloadImageWithURL:completionBlock:到您的类，它将异步加载图像并在成功下载图像时自动更新 CollectionView 中的单元格。

- (void)downloadImageWithURL:(NSURL *)url completionBlock:(void (^)(BOOL succeeded, NSData *data))completionBlock
{
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
    [NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) {
        if (!error) {
            completionBlock(YES, data);
        } else {
            completionBlock(NO, nil);
        }
    }];
}

我看到您尝试在视图出现之前预加载图像，所以也许我的解决方案不是您想要的，但从您的问题来看很难说。无论如何，你也可以通过这个实现你想要的。

斯威夫特2.2Swift 中的解决方案。

public typealias ImageFetchCompletionClosure = (image: UIImage?, error: NSError?, imageURL: NSURL?) -> Void

extension String {
   func fetchImage(completionHandler: (image: UIImage?, error: NSError?, imageURL: NSURL?) -> Void) {
        if let imageURL = NSURL(string: self) {
            NSURLSession.sharedSession().dataTaskWithURL(imageURL) { data, response, error in
                guard
                    let httpURLResponse = response as? NSHTTPURLResponse where httpURLResponse.statusCode == 200,
                    let mimeType = response?.MIMEType where mimeType.hasPrefix("image"),
                    let data = data where error == nil,
                    let image = UIImage(data: data)
                    else {
                        if error != nil {
                            completionHandler(image: nil, error: error, imageURL: imageURL)
                        }
                        return
                }
                dispatch_sync(dispatch_get_main_queue()) { () -> Void in
                    completionHandler(image: image, error: nil, imageURL: imageURL)
                }
            }.resume()
        }
    }
}

使用示例：

    "url_string".fetchImage { (image, error, imageURL) in
        // handle different results, either image was downloaded or error received
    }