UIActivityItemSource在后台渲染图像

Question

My app takes about 10 seconds to render a full-resolution image with many filters applied. I want to allow the user to save it or share it using UIActivityViewController . I've subclassed UIActivityItemSource so it provides a thumbnail when the activity view opens.

How can I make it so my item source renders the image in the background? Specifically, I would like to allow the view controller to minimize and display a progress bar.

I already have the progress bar working, I just need to know how to do the following in the background:

func activityViewController(activityViewController: UIActivityViewController, itemForActivityType activityType: String) -> AnyObject? {

    // this step takes 10 seconds
    // is there any way to do it in the background?
    // So: the activity view can minimize
    // and the main thread is free to update my progress bar

    let image = renderFullImage()

    return image
}

Answer 1

The trick was to use a UIActivityItemProvider instead of UIActivityItemSource . You are supposed to override the item method, which is already called in the background.