SKAction followPath 无法按预期使用两个圆圈（八形）

Question

我正在尝试创建一个形状为 8 或无穷大的 CGPath。我确实认为路径创建应该返回所需的结果，但我仍然会在这里发布：

let path = CGMutablePath()

path.addEllipse(in: CGRect(x: -height/4, y: -height/2, width: height/2, height: height/2))
path.addEllipse(in: CGRect(x: -height/4, y: 0.0, width: height/2, height: height/2))

path.closeSubpath()

return path

现在我有另一个节点应该遵循这条路径，我认为它可以像这样使用 SKAction 来工作：

highlighter.run(SKAction.repeatForever(SKAction.follow(shape.path!, asOffset: false, orientToPath: true, duration: 5.0)))

highlighter 是我的节点的名称，shape 是我分配路径的 SKShapeNode 的名称。然而，该节点仅绘制其中一个圆圈，而不是所需的形状。我一直在谷歌搜索这个问题，但找不到任何类似的案例，现在已经坐了一天多，所以我可能看不到明显的东西。任何见解将不胜感激！

Answer 1

如果你绘制你生成的图 8，你会看到你的精灵只跟随底部的圆圈，因为这是你绘制路径的方式。

您需要从顶部/底部到中间绘制顶部或底部圆圈的一半，然后添加从中间开始和结束的完整的另一个圆圈，最后再将另一半圆圈返回到您开始的位置。

这是一个例子，它还绘制了边界矩形和要遵循的图 8 路径：

class GameScene: SKScene {

    //    var sprite = SKSpriteNode()
    var figureEightPath = CGMutablePath()

    override func didMove(to view: SKView) {

        let boundingRect = CGRect(x: -200, y: -400, width: 400, height: 800)
        addChild(drawSKShapeNode(fromRect: rect, withWidth: 2, inColour: SKColor.green, called: "rect"))

        figureEightPath = drawFigureEight(in: boundingRect)
        addChild(drawSKShapeNode(fromPath: figureEightPath, withWidth: 2, inColour: SKColor.red, called: "path"))

        let sprite = SKSpriteNode(color: SKColor.yellow, size: CGSize(width: 50, height: 50))
        addChild(sprite)

        let moveAction = SKAction.follow(figureEightPath, asOffset: false, orientToPath: true, speed: 200)
        sprite.run(SKAction.repeatForever(moveAction))

    }

    func drawSKShapeNode(fromPath path: CGMutablePath,
                         withWidth width: CGFloat,
                         inColour colour: SKColor,
                         called name: String) -> SKShapeNode {

        let shape = SKShapeNode(path: path, centered: true)
        shape.lineWidth = width
        shape.strokeColor = colour
        shape.name = name
        return shape
    }

    func drawSKShapeNode(fromRect rect: CGRect,
                         withWidth width: CGFloat,
                         inColour colour: SKColor,
                         called name: String) -> SKShapeNode {

        let shape = SKShapeNode(rect: rect)
        shape.lineWidth = width
        shape.strokeColor = colour
        shape.name = name
        return shape
    }

    func drawFigureEight(in rect: CGRect) -> CGMutablePath {
        let bottomCentre = CGPoint(x: rect.midX, y: rect.height/4+rect.minY)
        let topCentre = CGPoint(x: rect.midX, y:(rect.height/4)*3+rect.minY)
        let radius = rect.height/4

        let bottom = CGPoint(x: rect.midX, y: rect.minY)

        let path = CGMutablePath()
        path.move(to: bottom)
//Draw a semi-circle from the bottom counter-clockwise to the middle
        path.addArc(center: bottomCentre, radius: radius, startAngle: CGFloat(Double.pi*1.5), endAngle: CGFloat(Double.pi/2), clockwise: false)
//Draw to top circle in a clockwise direction
        path.addArc(center: topCentre, radius: radius, startAngle: CGFloat(Double.pi*1.5), endAngle: CGFloat(-Double.pi/2), clockwise: true)
//Draw the rest of the bottom circle by drawing a semi-circle from the middle to the bottom counter-clockwise.
        path.addArc(center: bottomCentre, radius: radius, startAngle: CGFloat(Double.pi/2), endAngle: CGFloat(-Double.pi/2), clockwise: false)

        return path
    }

    override func update(_ currentTime: TimeInterval) {
        // Called before each frame is rendered
    }
}

有几个辅助函数 - drawSKShapeNode - 您不需要，但它们对调试很有用，因为它们通过根据给定的内容创建 SKShapeNode 来“绘制”CGPath 或 CGRect。