如何使用 swiftUI 沿自定义路径移动视图/形状？

Question

似乎没有一种沿着自定义路径（尤其是弯曲路径）移动视图/形状的直观方法。我发现了几个 UIKit 库，它们允许视图在 Bézier 路径（DKChainableAnimationKit、TweenKit、Sica 等）上移动，但我不太习惯使用 UIKit 并不断遇到错误。

目前使用 swiftUI 我手动这样做：

import SwiftUI
struct ContentView: View {
    @State var moveX = true
    @State var moveY = true
    @State var moveX2 = true
    @State var moveY2 = true
    @State var rotate1 = true
    var body: some View {
        ZStack{
            Circle().frame(width:50, height:50)
                .offset(x: moveX ? 0:100, y: moveY ? 0:100)
                .animation(Animation.easeInOut(duration:1).delay(0))
                .rotationEffect(.degrees(rotate1 ? 0:350))
                .offset(x: moveX2 ? 0:-100, y: moveY2 ? 0:-200)
                .animation(Animation.easeInOut(duration:1).delay(1))

                .onAppear(){
                    self.moveX.toggle();
                    self.moveY.toggle();
                    self.moveX2.toggle();
                    self.moveY2.toggle();
                    self.rotate1.toggle();
                    //    self..toggle()
            }
        }
} }

它在某种程度上完成了工作，但灵活性受到严重限制，并且复合延迟很快就会变得一团糟。

如果有人知道我如何获得自定义视图/形状以沿着以下路径行进，将不胜感激。

  Path { path in
        path.move(to: CGPoint(x: 200, y: 100))
        path.addQuadCurve(to: CGPoint(x: 230, y: 200), control: CGPoint(x: -100, y: 300))
        path.addQuadCurve(to: CGPoint(x: 90, y: 400), control: CGPoint(x: 400, y: 130))
        path.addLine(to: CGPoint(x: 90, y: 600))
    }
    .stroke()

我设法找到的最接近的解决方案是在 SwiftUILab 上，但完整的教程似乎只适用于付费订阅者。

像这样的东西：

Answer 1

好吧，这并不简单，但我想帮助...

在下一个片段（macOS 应用程序）中，您可以看到可以根据需要进行调整的基本元素。

为简单起见，我选择简单的参数曲线，如果您喜欢使用更复杂的（复合）曲线，则必须解决如何将每个段的部分 t（参数）映射到整个曲线的复合 t（并且必须这样做）用于将部分沿轨道距离映射到复合轨道沿轨道距离）。

为什么会出现这样的并发症？

飞机位移（恒速）所需的沿航迹距离与参数曲线定义所依赖的曲线参数 t 之间存在非线性关系。

先来看看结果

接下来看看它是如何实现的。您需要研究此代码，并在必要时研究参数曲线的定义和行为方式。

//
//  ContentView.swift
//  tmp086
//
//  Created by Ivo Vacek on 11/03/2020.
//  Copyright © 2020 Ivo Vacek. All rights reserved.
//

import SwiftUI
import Accelerate

protocol ParametricCurve {
    var totalArcLength: CGFloat { get }
    func point(t: CGFloat)->CGPoint
    func derivate(t: CGFloat)->CGVector
    func secondDerivate(t: CGFloat)->CGVector
    func arcLength(t: CGFloat)->CGFloat
    func curvature(t: CGFloat)->CGFloat
}

extension ParametricCurve {
    func arcLength(t: CGFloat)->CGFloat {
        var tmin: CGFloat = .zero
        var tmax: CGFloat = .zero
        if t < .zero {
            tmin = t
        } else {
            tmax = t
        }
        let quadrature = Quadrature(integrator: .qags(maxIntervals: 8), absoluteTolerance: 5.0e-2, relativeTolerance: 1.0e-3)
        let result = quadrature.integrate(over: Double(tmin) ... Double(tmax)) { _t in
            let dp = derivate(t: CGFloat(_t))
            let ds = Double(hypot(dp.dx, dp.dy)) //* x
            return ds
        }
        switch result {
        case .success(let arcLength, _/*, let e*/):
            //print(arcLength, e)
            return t < .zero ? -CGFloat(arcLength) : CGFloat(arcLength)
        case .failure(let error):
            print("integration error:", error.errorDescription)
            return CGFloat.nan
        }
    }
    func curveParameter(arcLength: CGFloat)->CGFloat {
        let maxLength = totalArcLength == .zero ? self.arcLength(t: 1) : totalArcLength
        guard maxLength > 0 else { return 0 }
        var iteration = 0
        var guess: CGFloat = arcLength / maxLength

        let maxIterations = 10
        let maxErr: CGFloat = 0.1

        while (iteration < maxIterations) {
            let err = self.arcLength(t: guess) - arcLength
            if abs(err) < maxErr { break }
            let dp = derivate(t: guess)
            let m = hypot(dp.dx, dp.dy)
            guess -= err / m
            iteration += 1
        }

        return guess
    }
    func curvature(t: CGFloat)->CGFloat {
        /*
                    x'y" - y'x"
        ?(t)  = --------------------
                 (x'² + y'²)^(3/2)
         */
        let dp = derivate(t: t)
        let dp2 = secondDerivate(t: t)
        let dpSize = hypot(dp.dx, dp.dy)
        let denominator = dpSize * dpSize * dpSize
        let nominator = dp.dx * dp2.dy - dp.dy * dp2.dx

        return nominator / denominator
    }
}

struct Bezier3: ParametricCurve {

    let p0: CGPoint
    let p1: CGPoint
    let p2: CGPoint
    let p3: CGPoint

    let A: CGFloat
    let B: CGFloat
    let C: CGFloat
    let D: CGFloat
    let E: CGFloat
    let F: CGFloat
    let G: CGFloat
    let H: CGFloat


    public private(set) var totalArcLength: CGFloat = .zero

    init(from: CGPoint, to: CGPoint, control1: CGPoint, control2: CGPoint) {
        p0 = from
        p1 = control1
        p2 = control2
        p3 = to
        A = to.x - 3 * control2.x + 3 * control1.x - from.x
        B = 3 * control2.x - 6 * control1.x + 3 * from.x
        C = 3 * control1.x - 3 * from.x
        D = from.x
        E = to.y - 3 * control2.y + 3 * control1.y - from.y
        F = 3 * control2.y - 6 * control1.y + 3 * from.y
        G = 3 * control1.y - 3 * from.y
        H = from.y
        // mandatory !!!
        totalArcLength = arcLength(t: 1)
    }

    func point(t: CGFloat)->CGPoint {
        let x = A * t * t * t + B * t * t + C * t + D
        let y = E * t * t * t + F * t * t + G * t + H
        return CGPoint(x: x, y: y)
    }

    func derivate(t: CGFloat)->CGVector {
        let dx = 3 * A * t * t + 2 * B * t + C
        let dy = 3 * E * t * t + 2 * F * t + G
        return CGVector(dx: dx, dy: dy)
    }

    func secondDerivate(t: CGFloat)->CGVector {
        let dx = 6 * A * t + 2 * B
        let dy = 6 * E * t + 2 * F
        return CGVector(dx: dx, dy: dy)
    }

}

class AircraftModel: ObservableObject {
    let track: ParametricCurve
    let path: Path
    var aircraft: some View {
        let t = track.curveParameter(arcLength: alongTrackDistance)
        let p = track.point(t: t)
        let dp = track.derivate(t: t)
        let h = Angle(radians: atan2(Double(dp.dy), Double(dp.dx)))
        return Text("").font(.largeTitle).rotationEffect(h).position(p)
    }
    @Published var alongTrackDistance = CGFloat.zero
    init(from: CGPoint, to: CGPoint, control1: CGPoint, control2: CGPoint) {
        track = Bezier3(from: from, to: to, control1: control1, control2: control2)
        path = Path({ (path) in
            path.move(to: from)
            path.addCurve(to: to, control1: control1, control2: control2)
        })
    }
}

struct ContentView: View {
    @ObservedObject var aircraft = AircraftModel(from: .init(x: 0, y: 0), to: .init(x: 500, y: 600), control1: .init(x: 600, y: 100), control2: .init(x: -300, y: 400))

    var body: some View {
        VStack {
            ZStack {
                aircraft.path.stroke(style: StrokeStyle( lineWidth: 0.5))
                aircraft.aircraft
            }
            Slider(value: $aircraft.alongTrackDistance, in: (0.0 ... aircraft.track.totalArcLength)) {
                Text("along track distance")
            }.padding()
            Button(action: {
                // fly (to be implemented :-))
            }) {
                Text("Fly!")
            }.padding()
        }
    }
}


struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

如果您担心如何实现“动画”飞机运动，SwiftUI 动画不是解决方案。您必须以编程方式移动飞机。

你必须导入

import Combine

添加到模型

@Published var flying = false
var timer: Cancellable? = nil

func fly() {
    flying = true
    timer = Timer
        .publish(every: 0.02, on: RunLoop.main, in: RunLoop.Mode.default)
        .autoconnect()
        .sink(receiveValue: { (_) in
            self.alongTrackDistance += self.track.totalArcLength / 200.0
            if self.alongTrackDistance > self.track.totalArcLength {
                self.timer?.cancel()
                self.flying = false
            }
        })
}

并修改按钮

Button(action: {
    self.aircraft.fly()
}) {
    Text("Fly!")
}.disabled(aircraft.flying)
.padding()

最后我有

Answer 2

user3441734 的解决方案非常通用和优雅。读者将受益于对它ParametricCurve及其弧长和曲率的每一秒思考。这是我发现的唯一可以将移动物体（飞机）重新定位为在移动时指向前方的方法。

Asperi 还在Is it possible to animate view on a certain Path in SwiftUI中发布了一个有用的解决方案

这是一个事半功倍、事半功倍的解决方案。它确实使用了 SwiftUI 动画，这是喜忧参半的。（例如，您可以对动画曲线有更多选择，但动画完成后您不会收到公告或回调。）它的灵感来自 Asperi 在Problem animating with animatableData in SwiftUI中的答案。

import SwiftUI

// Use https://www.desmos.com/calculator/cahqdxeshd to design Beziers.

// Pick a simple example path.
fileprivate let W = UIScreen.main.bounds.width
fileprivate let H = UIScreen.main.bounds.height

fileprivate let p1 = CGPoint(x: 50, y: H - 50)
fileprivate let p2 = CGPoint(x: W - 50, y: 50)

fileprivate var samplePath : Path {
    let c1 = CGPoint(x: p1.x, y: (p1.y + p2.y)/2)
    let c2 = CGPoint(x: p2.x, y: (p1.y + p2.y)/2)

    var result = Path()
    result.move(to: p1)
    result.addCurve(to: p2, control1: c1, control2: c2)
    return result
}

// This View's position follows the Path.
struct SlidingSpot : View {
    let path    : Path
    let start   : CGPoint
    let duration: Double = 1

    @State var isMovingForward = false

    var tMax : CGFloat { isMovingForward ? 1 : 0 }  // Same expressions,
    var opac : Double  { isMovingForward ? 1 : 0 }  // different meanings.

    var body: some View {
        VStack {
            Circle()
            .frame(width: 30)

            // Asperi is correct that this Modifier must be separate. 
            .modifier(Moving(time: tMax, path: path, start: start))

            .animation(.easeInOut(duration: duration), value: tMax)
            .opacity(opac)

            Button {
                isMovingForward = true

                // Sneak back to p1. This is a code smell.
                DispatchQueue.main.asyncAfter(deadline: .now() + duration + 0.1) {
                    isMovingForward = false
                }
            } label: {
                Text("Go")
            }
        }
    }
}

// Minimal modifier. 
struct Moving: AnimatableModifier {
    var time : CGFloat  // Normalized from 0...1.
    let path : Path
    let start: CGPoint  // Could derive from path.

    var animatableData: CGFloat {
        get { time }
        set { time = newValue }
    }

    func body(content: Content) -> some View {
        content
        .position(
            path.trimmedPath(from: 0, to: time).currentPoint ?? start
        )
    }
}

struct ContentView: View {
    var body: some View {
        SlidingSpot(path: samplePath, start: p1)
    }
}

Answer 3

尝试这个：

但是：请小心：这不是在预览中运行，您必须在模拟器/设备上运行

struct MyShape: Shape {

    func path(in rect: CGRect) -> Path {
        let path =

        Path { path in
            path.move(to: CGPoint(x: 200, y: 100))
            path.addQuadCurve(to: CGPoint(x: 230, y: 200), control: CGPoint(x: -100, y: 300))
            path.addQuadCurve(to: CGPoint(x: 90, y: 400), control: CGPoint(x: 400, y: 130))
            path.addLine(to: CGPoint(x: 90, y: 600))
        }


        return path
    }
}

struct ContentView: View {

    @State var x: CGFloat = 0.0

    var body: some View {
        MyShape()
            .trim(from: 0, to: x)
            .stroke(lineWidth: 10)
            .frame(width: 200, height: 200)
            .onAppear() {
                withAnimation(Animation.easeInOut(duration: 3).delay(0.5)) {
                    self.x = 1
                }
        }
    }
}