UIKit中如何构建一个支持拖拽上传文件的用户界面

要构建一个支持拖拽上传文件的用户界面，可以使用UIDragInteraction和UIDropInteraction这两个类来实现。下面是一个简单的示例代码：

import UIKit

class DragDropViewController: UIViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let imageView = UIImageView(frame: CGRect(x: 0, y: 0, width: 200, height: 200))
        imageView.backgroundColor = .lightGray
        imageView.isUserInteractionEnabled = true
        view.addSubview(imageView)
        
        let dragInteraction = UIDragInteraction(delegate: self)
        imageView.addInteraction(dragInteraction)
        
        let dropInteraction = UIDropInteraction(delegate: self)
        view.addInteraction(dropInteraction)
    }
}

extension DragDropViewController: UIDragInteractionDelegate {
    
    func dragInteraction(_ interaction: UIDragInteraction, itemsForBeginning session: UIDragSession) -> [UIDragItem] {
        guard let image = (interaction.view as? UIImageView)?.image else { return [] }
        let itemProvider = NSItemProvider(object: image)
        let dragItem = UIDragItem(itemProvider: itemProvider)
        return [dragItem]
    }
}

extension DragDropViewController: UIDropInteractionDelegate {
    
    func dropInteraction(_ interaction: UIDropInteraction, performDrop session: UIDropSession) {
        for item in session.items {
            if item.canLoadObject(ofClass: UIImage.self) {
                item.loadObject(ofClass: UIImage.self) { (image, error) in
                    if let image = image as? UIImage {
                        DispatchQueue.main.async {
                            (interaction.view as? UIImageView)?.image = image
                        }
                    }
                }
            }
        }
    }
}

在这个示例中，我们创建了一个UIImageView并为其添加了UIDragInteraction和UIDropInteraction。在UIDragInteractionDelegate的实现中，我们指定了拖动操作的UIDragItem为UIImageView的图像。而在UIDropInteractionDelegate的实现中，我们在放置操作时将拖动的图像设置为UIImageView的图像。这样就实现了一个支持拖拽上传文件的用户界面。