iOS13分享表：分享UIImage时如何设置预览缩略图

在iOS13中，我们可以通过使用`UIActivityViewController`来实现分享功能。当我们分享一个`UIImage`对象时，系统会自动为我们生成一个预览缩略图，以便在分享面板中显示。但是有时候，我们可能希望自定义这个预览缩略图，以使其更符合我们的需求。那么，在这篇文章中，我们将学习如何在iOS13中设置自定义的预览缩略图。

swift
let image = UIImage(named: "exampleImage")
let activityViewController = UIActivityViewController(activityItems: [image], applicationActivities: nil)
present(activityViewController, animated: true, completion: nil)

swift
class CustomActivityItemSource: NSObject, UIActivityItemSource {
    
    let image: UIImage
    
    init(image: UIImage) {
        self.image = image
    }
    
    func activityViewControllerPlaceholderItem(_ activityViewController: UIActivityViewController) -> Any {
        return image
    }
    
    func activityViewController(_ activityViewController: UIActivityViewController, itemForActivityType activityType: UIActivity.ActivityType?) -> Any? {
        return image
    }
    
    func activityViewController(_ activityViewController: UIActivityViewController, thumbnailImageForActivityType activityType: UIActivity.ActivityType?, suggestedSize size: CGSize) -> UIImage? {
        return // 返回自定义的预览缩略图
    }
}

swift
let image = UIImage(named: "exampleImage")
let activityItemSource = CustomActivityItemSource(image: image!)
let activityViewController = UIActivityViewController(activityItems: [activityItemSource], applicationActivities: nil)
present(activityViewController, animated: true, completion: nil)