要保持一个ImageView在屏幕边界范围内，您可以使用以下Swift代码：

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var imageView: UIImageView!
    var imageWidth: CGFloat = 0
    var imageHeight: CGFloat = 0
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 获取图片的原始尺寸
        imageWidth = imageView.frame.size.width
        imageHeight = imageView.frame.size.height
        
        // 添加拖动手势识别器
        let panGesture = UIPanGestureRecognizer(target: self, action: #selector(handlePan(_:)))
        imageView.addGestureRecognizer(panGesture)
        
        // 开启交互
        imageView.isUserInteractionEnabled = true
    }
    
    @objc func handlePan(_ recognizer: UIPanGestureRecognizer) {
        let translation = recognizer.translation(in: self.view)
        
        if let view = recognizer.view {
            let centerX = view.center.x + translation.x
            let centerY = view.center.y + translation.y
            
            // 获取屏幕的尺寸
            let screenWidth = view.superview!.frame.size.width
            let screenHeight = view.superview!.frame.size.height
            
            // 计算ImageView的边界范围
            let minX = imageWidth / 2
            let maxX = screenWidth - imageWidth / 2
            let minY = imageHeight / 2
            let maxY = screenHeight - imageHeight / 2
            
            // 将ImageView的中心点限制在边界范围内
            let constrainedX = max(minX, min(maxX, centerX))
            let constrainedY = max(minY, min(maxY, centerY))
            
            view.center = CGPoint(x: constrainedX, y: constrainedY)
        }
        
        recognizer.setTranslation(CGPoint.zero, in: self.view)
    }
}

这段代码首先获取了ImageView的原始尺寸，并添加了一个拖动手势识别器。当用户拖动ImageView时，通过计算ImageView的中心点和屏幕的尺寸，将ImageView的中心点限制在屏幕边界范围内。最后，通过设置recognizer.setTranslation(CGPoint.zero, in: self.view)将拖动的偏移量重置为零。