从cgpoint开始仅在一个方向上绘制线

如何解决从cgpoint开始仅在一个方向上绘制线

我下面的swift代码目标是能够画一条直线。触摸uiview时，用户只能在初始点上方绘制90度。方向已经确定。因此，用户可以在所触摸的点上方画线。您可以在左侧行下方的gif代码中执行以下操作。右边的线是我要达到的目标。

https://github.com/silverstripe/silverstripe-blog

import UIKit



class ViewController: UIViewController{
    
    
    
  
    
    var draw = Canvas()
    
    
    override func viewDidLoad() {
        super.viewDidLoad()
        [draw].forEach {
            view.addSubview($0)
            $0.translatesAutoresizingMaskIntoConstraints = false}
    }
    
    
    
    override func viewDidLayoutSubviews() {
        
        draw.backgroundColor = .clear
        
        
        NSLayoutConstraint.activate ([
            
            
            
            draw.bottomAnchor.constraint(equalTo: view.bottomAnchor),draw.heightAnchor.constraint(equalTo: view.heightAnchor,multiplier: 0.77,constant: 0),draw.widthAnchor.constraint(equalTo: view.widthAnchor,multiplier: 1,draw.leadingAnchor.constraint(equalTo: view.leadingAnchor,constant : 0),])
        
        
        
        
    }
    
    
    
}



struct ColoredLine {
    var color = UIColor.black
    var points = [CGPoint]()
    var width = 5
}





class Canvas: UIView {
    
    
    var strokeColor = UIColor.red
    var strokeWidth = 5
    func undo() {
        _ = lines.popLast()
        setNeedsdisplay()
    }
    
    func clear() {
        lines.removeAll()
        setNeedsdisplay()
    }
    
    var lines = [ColoredLine]()
    
    override func draw(_ rect: CGRect) {
        super.draw(rect)
        
        guard let context = UIGraphicsGetCurrentContext() else { return }
        
        lines.forEach { (line) in
            for (i,p) in line.points.enumerated() {
                if i == 0 {
                    context.move(to: p)
                } else {
                    context.addLine(to: p)
                }
            }
            
            context.setstrokeColor(line.color.cgColor)
            context.setlinewidth(CGFloat(line.width))
            context.strokePath()
            context.beginPath()
        }
        
        
    }
    
    override func touchesBegan(_ touches: Set<UITouch>,with event: UIEvent?) {
        var coloredLine = ColoredLine()
        coloredLine.color = strokeColor
        coloredLine.width = strokeWidth
        lines.append(coloredLine)
        
    }
    
    override func touchesMoved(_ touches: Set<UITouch>,with event: UIEvent?) {
        guard let point = touches.first?.location(in: self) else { return }
        guard var lastLine = lines.popLast() else { return }
        lastLine.points.append(point)
        lines.append(lastLine)
        setNeedsdisplay()
    }
    
}

解决方法

可以制造坐标，点由x和y组成。

保持y，并完成。

• 保持一个触摸事件的第一点

    var firstPt: CGPoint?
  
    //  get the first point
    override func touchesBegan(_ touches: Set<UITouch>,with event: UIEvent?) {
        guard let first = touches.first?.location(in: self) else { return }
  
        // store first as property 
        firstPt = first
    }
  

• 制造剩余点，新的获取点的x不相关

    override func touchesMoved(_ touches: Set<UITouch>,with event: UIEvent?) {
        guard let point = touches.first?.location(in: self),let first = firstPt else { return }
  
        let pointNeeded = CGPoint(x:  first.x,y:  point.y)
        //  ...,do as usual
    }
  

以及touchesEnd和touchesCancell中的firstPt = nil