UIImage initWithData:阻止UI线程进行异步分派? [英] UIImage initWithData: blocking UI thread from async dispatch?

问题描述

以下代码阻止了UI(在图像加载完成之前无法点击其他选项卡).

The following code is blocking UI (can't tap another tab until images finish loading).

我已经通过注释该行并保留下载(我知道这是异步发生并在主线程上调用完成处理程序)来验证了是导致罪魁祸首的原因是UIImage *imageToShow = [[UIImage alloc] initWithData:data];调用.

I have verified it is the UIImage *imageToShow = [[UIImage alloc] initWithData:data]; call that is the culprit by commenting that line out and keeping the download (which I know happens asynchronously and calls completion handler on main thread).

在仅注释initWithData:行的情况下，UI响应很好.但是，如果明确在后台分派该行，那怎么可能是支撑UI的行呢?

In the case where only the initWithData: line is commented, the UI response is fine. But how can that line be the line holding up the UI if it is clearly dispatched in background?

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    ...

    [objectStore getFileInContainer:@"public_images" filename:filename completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
            UIImage *imageToShow = [[UIImage alloc] initWithData:data];
            dispatch_async(dispatch_get_main_queue(), ^{
                collectionImageView.image = imageToShow;
            });
        });
    }];

    ...
}

推荐答案

UIImage在第一次实际使用/绘制图像之前，不会读取和解码该图像.要强制在后台线程上执行此工作，必须在执行主线程-setImage:调用之前在后台线程上使用/绘制图像.许多人发现这违反直觉.我在另一个答案中对此进行了相当详细的解释.

UIImage doesn't read and decode the image until the first time it's actually used/drawn. To force this work to happen on a background thread, you have to use/draw the image on the background thread before doing the main thread -setImage: call. Many folks find this counter-intuitive. I explained this in considerable detail in another answer.

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