为用户创建的给定线路创建线路扩展

Question

我一直在开发一个应用程序，我必须捕捉用户的触摸并在屏幕上画一条线。到目前为止，这是有效的。

问题是我想为从行开始/结束到屏幕边界的那条线创建某种扩展。扩展名必须与主线对齐。

我这几天一直在努力实现这一目标，但没有取得积极成果。我的想法是使用某种线性方程来表示线条，然后在线条和屏幕边界上创建两个点。我遇到的第一个问题是垂直线。

y = m * x + b
slope = (y2 - y2)/(x2 - x1)
y_intercept = b = y - m * x

用户还可以在任何方向和方向上创建线条。

我尝试使用这些方程来找到任意点（x = 0，y = 0，x = 320，y = 480），但我遇到了一些奇怪的问题，如下图所示。

a) 线条确实超出了屏幕的限制。这导致应用程序几乎崩溃。

b) 我也无法确定如何将每个新点与当前接触点联系起来。

代码

import Foundation
import UIKit





public class TestView : UIView
{
    var lineLayer : CAShapeLayer?
    var lineExtensionALayer : CAShapeLayer?
    var lineExtensionBLayer : CAShapeLayer?

    var startPoint : CGPoint?
    var endPoint : CGPoint?

    override init(frame: CGRect){
        super.init(frame:frame)

        self.backgroundColor = UIColor.clearColor()

        self.lineLayer = self.createLine(color: UIColor.redColor())
        self.lineExtensionALayer = self.createLine(color: UIColor.greenColor())
        self.lineExtensionBLayer = self.createLine(color: UIColor.blueColor())
    }

    required public init(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    public override func touchesBegan(touches: Set<NSObject>, withEvent event: UIEvent) {
        let touch : UITouch = touches.first as! UITouch
        let position = touch.locationInView(self)

        self.startPoint = position
    }

    public override func touchesCancelled(touches: Set<NSObject>!, withEvent event: UIEvent!) {

    }

    public override func touchesEnded(touches: Set<NSObject>, withEvent event: UIEvent) {

    }

    public override func touchesMoved(touches: Set<NSObject>, withEvent event: UIEvent) {

        let touch : UITouch = touches.first as! UITouch
        let position = touch.locationInView(self)

        self.endPoint = position

        self.updateLine()
    }


    func createLine(color selectedColor : UIColor) -> CAShapeLayer
    {
        let line = CAShapeLayer()
        line.frame = self.bounds
        line.strokeColor = selectedColor.CGColor
        line.fillColor = nil
        line.lineWidth = 2

        self.layer.addSublayer(line)

        return line
    }

    func drawLine(line lineToDraw : CAShapeLayer,start pointA : CGPoint, end pointB : CGPoint)
    {
        let path = UIBezierPath()
        path.moveToPoint(pointA)
        path.addLineToPoint(pointB)

        lineToDraw.path = path.CGPath
    }

    func updateLine(){

        var line = LineFunction(point1: self.startPoint!, point2: self.endPoint!)

        // Draw main line.
        self.drawLine(line: self.lineLayer!, start: self.startPoint!, end: self.endPoint!)
    }
}

Answer 1

取自this answer，翻译成swift并稍作改动：

func getLines(xmin: CGFloat, ymin: CGFloat, xmax: CGFloat, ymax: CGFloat, x1: CGFloat, x2: CGFloat, y1: CGFloat, y2: CGFloat) -> (CGPoint, CGPoint) {
    if y1 == y2 {
        return (CGPoint(x: xmin, y: y1), CGPoint(x: xmax, y: y1))
    }
    if x1 == x2 {
        return (CGPoint(x: x1, y: ymin), CGPoint(x: x1, y: ymax))
    }

    let y_for_xmin = y1 + (y2 - y1) * (xmin - x1) / (x2 - x1)
    let y_for_xmax = y1 + (y2 - y1) * (xmax - x1) / (x2 - x1)

    let x_for_ymin = x1 + (x2 - x1) * (ymin - y1) / (y2 - y1)
    let x_for_ymax = x1 + (x2 - x1) * (ymax - y1) / (y2 - y1)

    if ymin <= y_for_xmin && y_for_xmin <= ymax {
        if xmin <= x_for_ymax && x_for_ymax <= xmax {
            return (CGPoint(x: xmin, y: y_for_xmin), CGPoint(x: x_for_ymax, y: ymax))
        }
        if xmin <= x_for_ymin && x_for_ymin <= xmax {
            return (CGPoint(x: xmin, y: y_for_xmin), CGPoint(x: x_for_ymin, y: ymin))
        }
        return (CGPoint(x: xmin, y: y_for_xmin), CGPoint(x: xmax, y: y_for_xmax))
    }

    if ymin <= y_for_xmax && y_for_xmax <= ymax {
        if xmin <= x_for_ymin && x_for_ymin <= xmax {
            return (CGPoint(x: x_for_ymin, y: ymin), CGPoint(x: xmax, y: y_for_xmax))
        }
        if xmin <= x_for_ymax && x_for_ymax <= xmax {
            return (CGPoint(x: x_for_ymax, y: ymax), CGPoint(x: xmax, y: y_for_xmax))
        }
        return (CGPoint(x: xmin, y: y_for_xmin), CGPoint(x: xmax, y: y_for_xmax))
    }

    return (CGPoint(x: x_for_ymin, y: ymin), CGPoint(x: x_for_ymax, y: ymax))
}

func updateLine(){
    let x1 = self.startPoint!.x
    let x2 = self.endPoint!.x
    let y1 = self.startPoint!.y
    let y2 = self.endPoint!.y

    let (start, end) = getLines(0, ymin: 0, xmax: bounds.width, ymax: bounds.height, x1: x1, x2: x2, y1: y1, y2: y2)

    print(start, appendNewline: false)
    print(" - ", appendNewline: false)
    print(end)

    // Draw main line.
    self.drawLine(line: self.lineLayer!, start: start, end: end)
}

不幸的是，它还没有完全发挥作用，因为在一半的情况下，用于返回正确扩展行的返回 if 构造没有返回任何有用的东西。我会在现在或明天尝试解决这个问题。

但它应该让你开始

编辑：如果起点和终点都在水平轴或垂直轴上，则似乎不起作用。如果它们在不同的轴上，它会起作用。

在我添加了另外三个返回语句后，代码现在可以正常运行了 :)

如果您查看记录的信息，您会看到绘制的线实际上完全延伸到边界，而不是更远。