我有一个简单的应用程序，它有一个ViewController，包含一个非常小的视图层次结构：DrawingView（UIView的子类）和三个ImageViews（它们都是DrawingView的子级）。
在DrawingView中，我重写了draw(_:)，以生成依赖于每个ImageView中心的图形（具体来说，一个顶点为图像中心的多边形）。同时，ImageView的位置由用户通过拖动手势控制。手势处理动作是ViewController的方法。
我希望DrawingView能够随着ImageView位置的变化而实时更新。为了实现这一点，我在手势处理程序中调用了DrawingView.setNeedsDisplay()。然而，这种方法只会离散地更新DrawingView，而且似乎直到下一个手势开始时才更新（无论调用出现在switch gesture.state语句中的何处）。
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我的问题：我应该在哪里/如何调用setNeedsDisplay以实现对DrawingView的平滑（实时）更新？或者有更好的方法吗？

以下是我的类定义：

class ViewController: UIViewController {
    
    @IBOutlet var drawingView: DrawingView!

    @IBOutlet var majorVertex1: UIImageView!
    @IBOutlet var majorVertex2: UIImageView!
    @IBOutlet var majorVertex3: UIImageView!
    
    var majorVertices: [UIImageView]!
    
    @IBOutlet var majorVertex1XConstraint: NSLayoutConstraint!
    @IBOutlet var majorVertex1YConstraint: NSLayoutConstraint!
    @IBOutlet var majorVertex2XConstraint: NSLayoutConstraint!
    @IBOutlet var majorVertex2YConstraint: NSLayoutConstraint!
    @IBOutlet var majorVertex3XConstraint: NSLayoutConstraint!
    @IBOutlet var majorVertex3YConstraint: NSLayoutConstraint!
    
    var majorVertexXConstraints: [NSLayoutConstraint]!
    var majorVertexYConstraints: [NSLayoutConstraint]!
    
    static var majorVertexXOffsets: [Double]?
    static var majorVertexYOffsets: [Double]?
        
    override func viewDidLoad() {
        
        super.viewDidLoad()
        
        majorVertices = [majorVertex1, majorVertex2, majorVertex3]
        majorVertexXConstraints = [majorVertex1XConstraint, majorVertex2XConstraint, majorVertex3XConstraint]
        majorVertexYConstraints = [majorVertex1YConstraint, majorVertex2YConstraint, majorVertex3YConstraint]
        ViewController.majorVertexXOffsets = majorVertexXConstraints.map {(constraint) -> Double in return constraint.constant}
        ViewController.majorVertexYOffsets = majorVertexYConstraints.map {(constraint) -> Double in return constraint.constant}
        
    }

    @IBAction func handlePan(_ gesture: UIPanGestureRecognizer) {
        
        guard let majorVertices = majorVertices,
              let gestureView = gesture.view
        else {return}
        
        guard let parentView = gestureView.superview,
              let gestureViewIndex = majorVertices.firstIndex(of: gestureView as! UIImageView)
        else {return}
        
        let translation = gesture.translation(in: parentView)
        
        switch gesture.state {
            
        case .began:
            ViewController.majorVertexXOffsets = majorVertexXConstraints.map {(constraint) -> Double in return constraint.constant}
            ViewController.majorVertexYOffsets = majorVertexYConstraints.map {(constraint) -> Double in return constraint.constant}
            break
            
        case .changed:
            majorVertexXConstraints[gestureViewIndex].constant = ViewController.majorVertexXOffsets![gestureViewIndex] + translation.x
            majorVertexYConstraints[gestureViewIndex].constant = ViewController.majorVertexYOffsets![gestureViewIndex] + translation.y
            drawingView.setNeedsDisplay()
            break
            
        case .ended, .cancelled:
            majorVertexXConstraints[gestureViewIndex].constant = gestureView.center.x - parentView.frame.size.width / 2.0
            majorVertexYConstraints[gestureViewIndex].constant = gestureView.center.y - parentView.frame.size.height / 2.0
            break

        default:
            break
            
        }
        
    }

}

class DrawingView: UIView {
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        setupView()
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setupView()
    }
    
    func setupView() {
        backgroundColor = .clear
    }
        
    override func draw(_ rect: CGRect) {
        super.draw(rect)
        drawTriangle(rect)
    }
    
    internal func drawTriangle(_ rect: CGRect) {
        
        guard let context = UIGraphicsGetCurrentContext(),
              let majorVertexXOffsets = ViewController.majorVertexXOffsets,
              let majorVertexYOffsets = ViewController.majorVertexYOffsets
        else {return}
        
        let majorVertexXCenters = majorVertexXOffsets.map {(x) -> Double in return x + rect.width / 2.0}
        let majorVertexYCenters = majorVertexYOffsets.map {(y) -> Double in return y + rect.height / 2.0}
                
        context.setStrokeColor(UIColor.lightGray.cgColor)
        context.setLineWidth(3)
        
        context.move(to: CGPoint(x: majorVertexXCenters[0], y: majorVertexYCenters[0]))
        context.addLine(to: CGPoint(x: majorVertexXCenters[1], y: majorVertexYCenters[1]))
        context.addLine(to: CGPoint(x: majorVertexXCenters[2], y: majorVertexYCenters[2]))
        context.addLine(to: CGPoint(x: majorVertexXCenters[0], y: majorVertexYCenters[0]))
        
        context.strokePath()
        
    }
    
}

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