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[译] SwiftUI 官方教程（六）

完整中文教程及代码请查看 github.com/WillieWangW…

动画 View 与转场

使用 SwiftUI 时，无论用作何处，我们都可以单独为 view 添加动画，或者对 view 的状态进行动画处理。 SwiftUI 为我们处理所有动画的组合、重叠和中断的复杂性。

在本文中，我们会给包含图表的 view 设置动画，跟踪用户在使用 Landmarks app 时行为。我们会看到通过使用 animation(_:) 方法为 view 设置动画是多么简单。

下载项目文件并按照以下步骤操作，也可以打开已完成的项目自行浏览代码。

预计完成时间：20 分钟
项目文件：下载

1. 给单个 View 添加动画

当我们在一个 view 上使用 animation(_:) 方法时， SwiftUI 会动态的修改这个 view 的可动画属性。一个 view 的颜色、透明度、旋转、大小以及其他属性都是可动画的。

1.1 在 HikeView.swift 中，打开实时预览来测试显示和隐藏图表。

确保在本文中过程中都打开了实时预览，这样就可以测试到每一步的结果。

1.2 添加 animation(.basic()) 方法来打开按钮的旋转动画。

HikeView.swift

import SwiftUI

struct HikeView: View {
    var hike: Hike
    @State private var showDetail = false

    var body: some View {
        VStack {
            HStack {
                HikeGraph(data: hike.observations, path: /.elevation)
                    .frame(width: 50, height: 30)

                VStack(alignment: .leading) {
                    Text(hike.name)
                        .font(.headline)
                    Text(hike.distanceText)
                }

                Spacer()

                Button(action: {
                    self.showDetail.toggle()
                }) {
                    Image(systemName: "chevron.right.circle")
                        .imageScale(.large)
                        .rotationEffect(.degrees(showDetail ? 90 : 0))
                        .padding()
                        .animation(.basic())
                }
            }

            if showDetail {
                HikeDetail(hike: hike)
            }
        }
    }
}
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1.3 添加一个在图表显示时让按钮变大的动画。

animation(_:) 会作用于 view 所包装的所有可动画的修改。

HikeView.swift

import SwiftUI

struct HikeView: View {
    var hike: Hike
    @State private var showDetail = false

    var body: some View {
        VStack {
            HStack {
                HikeGraph(data: hike.observations, path: /.elevation)
                    .frame(width: 50, height: 30)

                VStack(alignment: .leading) {
                    Text(hike.name)
                        .font(.headline)
                    Text(hike.distanceText)
                }

                Spacer()

                Button(action: {
                    self.showDetail.toggle()
                }) {
                    Image(systemName: "chevron.right.circle")
                        .imageScale(.large)
                        .rotationEffect(.degrees(showDetail ? 90 : 0))
                        .scaleEffect(showDetail ? 1.5 : 1)
                        .padding()
                        .animation(.basic())
                }
            }

            if showDetail {
                HikeDetail(hike: hike)
            }
        }
    }
}
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1.4 把动画类型从 .basic() 改成 .spring() 。

SwiftUI 包含带有预设或自定义缓动的基本动画，以及弹性和流体动画。我们可以调整动画的速度、在动画开始之前设置延迟，或指定动画的重复。

HikeView.swift

import SwiftUI

struct HikeView: View {
    var hike: Hike
    @State private var showDetail = false

    var body: some View {
        VStack {
            HStack {
                HikeGraph(data: hike.observations, path: /.elevation)
                    .frame(width: 50, height: 30)

                VStack(alignment: .leading) {
                    Text(hike.name)
                        .font(.headline)
                    Text(hike.distanceText)
                }

                Spacer()

                Button(action: {
                    self.showDetail.toggle()
                }) {
                    Image(systemName: "chevron.right.circle")
                        .imageScale(.large)
                        .rotationEffect(.degrees(showDetail ? 90 : 0))
                        .scaleEffect(showDetail ? 1.5 : 1)
                        .padding()
                        .animation(.spring())
                }
            }

            if showDetail {
                HikeDetail(hike: hike)
            }
        }
    }
}
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尝试在 scaleEffect 方法上方添加另一个动画方法来关闭旋转动画。

围绕 SwiftUI 尝试结合不同的动画效果，看看都有哪些效果。

HikeView.swift

import SwiftUI

struct HikeView: View {
    var hike: Hike
    @State private var showDetail = false

    var body: some View {
        VStack {
            HStack {
                HikeGraph(data: hike.observations, path: /.elevation)
                    .frame(width: 50, height: 30)

                VStack(alignment: .leading) {
                    Text(hike.name)
                        .font(.headline)
                    Text(hike.distanceText)
                }

                Spacer()

                Button(action: {
                    self.showDetail.toggle()
                }) {
                    Image(systemName: "chevron.right.circle")
                        .imageScale(.large)
                        .rotationEffect(.degrees(showDetail ? 90 : 0))
                        .animation(nil)
                        .scaleEffect(showDetail ? 1.5 : 1)
                        .padding()
                        .animation(.spring())
                }
            }

            if showDetail {
                HikeDetail(hike: hike)
            }
        }
    }
}
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1.6 在继续下一节前，删除两个 animation(_:) 方法。

HikeView.swift

import SwiftUI

struct HikeView: View {
    var hike: Hike
    @State private var showDetail = false

    var body: some View {
        VStack {
            HStack {
                HikeGraph(data: hike.observations, path: /.elevation)
                    .frame(width: 50, height: 30)

                VStack(alignment: .leading) {
                    Text(hike.name)
                        .font(.headline)
                    Text(hike.distanceText)
                }

                Spacer()

                Button(action: {
                    self.showDetail.toggle()
                }) {
                    Image(systemName: "chevron.right.circle")
                        .imageScale(.large)
                        .rotationEffect(.degrees(showDetail ? 90 : 0))

                        .scaleEffect(showDetail ? 1.5 : 1)
                        .padding()

                }
            }

            if showDetail {
                HikeDetail(hike: hike)
            }
        }
    }
}
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2. 将状态的改变动画化

现在我们已经学会如果给单个 view 添加动画，是时候给状态的值的改变添加动画了。

这一节，我们会给用户点击按钮并切换 showDetail 状态属性时发生的所有更改添加动画。

2.1 将 showDetail.toggle() 的调用包装到 withAnimation 函数中。

受 showDetail 属性影响的公开按钮和 HikeDetail view 现在就都有了动画过渡。

HikeView.swift

import SwiftUI

struct HikeView: View {
    var hike: Hike
    @State private var showDetail = false

    var body: some View {
        VStack {
            HStack {
                HikeGraph(data: hike.observations, path: /.elevation)
                    .frame(width: 50, height: 30)

                VStack(alignment: .leading) {
                    Text(hike.name)
                        .font(.headline)
                    Text(hike.distanceText)
                }

                Spacer()

                Button(action: {
                    withAnimation {
                        self.showDetail.toggle()
                    }
                }) {
                    Image(systemName: "chevron.right.circle")
                        .imageScale(.large)
                        .rotationEffect(.degrees(showDetail ? 90 : 0))
                        .scaleEffect(showDetail ? 1.5 : 1)
                        .padding()
                }
            }

            if showDetail {
                HikeDetail(hike: hike)
            }
        }
    }
}
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减缓动画，看看 SwiftUI 动画是如何可以中断的。

2.2 给 withAnimation 方法传递一个 4 秒的基础动画。

我们可以传递相同类型的动画给 animation(_:) 的 withAnimation 函数。

HikeView.swift

import SwiftUI

struct HikeView: View {
    var hike: Hike
    @State private var showDetail = false

    var body: some View {
        VStack {
            HStack {
                HikeGraph(data: hike.observations, path: /.elevation)
                    .frame(width: 50, height: 30)

                VStack(alignment: .leading) {
                    Text(hike.name)
                        .font(.headline)
                    Text(hike.distanceText)
                }

                Spacer()

                Button(action: {
                    withAnimation(.basic(duration: 4)) {
                        self.showDetail.toggle()
                    }
                }) {
                    Image(systemName: "chevron.right.circle")
                        .imageScale(.large)
                        .rotationEffect(.degrees(showDetail ? 90 : 0))
                        .scaleEffect(showDetail ? 1.5 : 1)
                        .padding()
                }
            }

            if showDetail {
                HikeDetail(hike: hike)
            }
        }
    }
}
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2.3 尝试在动画期间打开和关闭图表 view 。

2.4 在进入下一节前，从 withAnimation 函数中移除缓慢动画。

HikeView.swift

import SwiftUI

struct HikeView: View {
    var hike: Hike
    @State private var showDetail = false

    var body: some View {
        VStack {
            HStack {
                HikeGraph(data: hike.observations, path: /.elevation)
                    .frame(width: 50, height: 30)

                VStack(alignment: .leading) {
                    Text(hike.name)
                        .font(.headline)
                    Text(hike.distanceText)
                }

                Spacer()

                Button(action: {
                    withAnimation {
                        self.showDetail.toggle()
                    }
                }) {
                    Image(systemName: "chevron.right.circle")
                        .imageScale(.large)
                        .rotationEffect(.degrees(showDetail ? 90 : 0))
                        .scaleEffect(showDetail ? 1.5 : 1)
                        .padding()
                }
            }

            if showDetail {
                HikeDetail(hike: hike)
            }
        }
    }
}
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3. 自定义 View 的转场

默认情况下，view 通过淡入和淡出过渡到屏幕上和屏幕外。我们可以使用 transition(_:) 方法来自定义转场。

3.1 给满足条件时显示的 HikeView 添加一个 transition(_:) 方法。

现在图标会滑动显示和消失。

HikeView.swift

import SwiftUI

struct HikeView: View {
    var hike: Hike
    @State private var showDetail = false

    var body: some View {
        VStack {
            HStack {
                HikeGraph(data: hike.observations, path: /.elevation)
                    .frame(width: 50, height: 30)

                VStack(alignment: .leading) {
                    Text(hike.name)
                        .font(.headline)
                    Text(hike.distanceText)
                }

                Spacer()

                Button(action: {
                    withAnimation {
                        self.showDetail.toggle()
                    }
                }) {
                    Image(systemName: "chevron.right.circle")
                        .imageScale(.large)
                        .rotationEffect(.degrees(showDetail ? 90 : 0))
                        .scaleEffect(showDetail ? 1.5 : 1)
                        .padding()
                }
            }

            if showDetail {
                HikeDetail(hike: hike)
                    .transition(.slide)
            }
        }
    }
}
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3.2 将转场提取为 AnyTransition 的静态属性。

这可以在您展开自定义转场时保持代码清晰。对于自定义转场，我们可以使用与 SwiftUI 所用相同的 . 符号。

HikeView.swift

import SwiftUI

extension AnyTransition {
    static var moveAndFade: AnyTransition {
        AnyTransition.slide
    }
}

struct HikeView: View {
    var hike: Hike
    @State private var showDetail = false

    var body: some View {
        VStack {
            HStack {
                HikeGraph(data: hike.observations, path: /.elevation)
                    .frame(width: 50, height: 30)

                VStack(alignment: .leading) {
                    Text(hike.name)
                        .font(.headline)
                    Text(hike.distanceText)
                }

                Spacer()

                Button(action: {
                    withAnimation {
                        self.showDetail.toggle()
                    }
                }) {
                    Image(systemName: "chevron.right.circle")
                        .imageScale(.large)
                        .rotationEffect(.degrees(showDetail ? 90 : 0))
                        .scaleEffect(showDetail ? 1.5 : 1)
                        .padding()
                }
            }

            if showDetail {
                HikeDetail(hike: hike)
                    .transition(.moveAndFade)
            }
        }
    }
}
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3.3 换成使用 move(edge:) 转场，这样图表会从同一边滑入和滑出。

HikeView.swift

import SwiftUI

extension AnyTransition {
    static var moveAndFade: AnyTransition {
        AnyTransition.move(edge: .trailing)
    }
}

struct HikeView: View {
    var hike: Hike
    @State private var showDetail = false

    var body: some View {
        VStack {
            HStack {
                HikeGraph(data: hike.observations, path: /.elevation)
                    .frame(width: 50, height: 30)

                VStack(alignment: .leading) {
                    Text(hike.name)
                        .font(.headline)
                    Text(hike.distanceText)
                }

                Spacer()

                Button(action: {
                    withAnimation {
                        self.showDetail.toggle()
                    }
                }) {
                    Image(systemName: "chevron.right.circle")
                        .imageScale(.large)
                        .rotationEffect(.degrees(showDetail ? 90 : 0))
                        .scaleEffect(showDetail ? 1.5 : 1)
                        .padding()
                }
            }

            if showDetail {
                HikeDetail(hike: hike)
                    .transition(.moveAndFade)
            }
        }
    }
}
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3.4 使用 asymmetric(insertion:removal:) 方法来给 view 显示和消失时提供不同的转场。

HikeView.swift

import SwiftUI

extension AnyTransition {
    static var moveAndFade: AnyTransition {
        let insertion = AnyTransition.move(edge: .trailing)
            .combined(with: .opacity)
        let removal = AnyTransition.scale()
            .combined(with: .opacity)
        return .asymmetric(insertion: insertion, removal: removal)
    }
}

struct HikeView: View {
    var hike: Hike
    @State private var showDetail = false

    var body: some View {
        VStack {
            HStack {
                HikeGraph(data: hike.observations, path: /.elevation)
                    .frame(width: 50, height: 30)

                VStack(alignment: .leading) {
                    Text(hike.name)
                        .font(.headline)
                    Text(hike.distanceText)
                }

                Spacer()

                Button(action: {
                    withAnimation {
                        self.showDetail.toggle()
                    }
                }) {
                    Image(systemName: "chevron.right.circle")
                        .imageScale(.large)
                        .rotationEffect(.degrees(showDetail ? 90 : 0))
                        .scaleEffect(showDetail ? 1.5 : 1)
                        .padding()
                }
            }

            if showDetail {
                HikeDetail(hike: hike)
                    .transition(.moveAndFade)
            }
        }
    }
}
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4. 给复杂的效果组合动画

单击条形下方的按钮时，图形会在三组不同的数据之间切换。在本节中，我们将使用组合动画为构成图形的 Capsule 提供动态、波动的转场。

4.1 把 showDetail 的默认值改成 true ，并把 HikeView 的预览固定在 canvas 中，

这让我们在其他文件中制作动画时依然能在上下文中看到图表。

4.2 在 GraphCapsule.swift 中，添加一个新的计算动画属性，并将其应用于 Capsule 的 shape 。

GraphCapsule.swift

import SwiftUI

struct GraphCapsule: View {
    var index: Int
    var height: Length
    var range: Range<Double>
    var overallRange: Range<Double>

    var heightRatio: Length {
        max(Length(magnitude(of: range) / magnitude(of: overallRange)), 0.15)
    }

    var offsetRatio: Length {
        Length((range.lowerBound - overallRange.lowerBound) / magnitude(of: overallRange))
    }

    var animation: Animation {
        Animation.default
    }

    var body: some View {
        Capsule()
            .fill(Color.gray)
            .frame(height: height * heightRatio, alignment: .bottom)
            .offset(x: 0, y: height * -offsetRatio)
            .animation(animation)
        )
    }
}
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4.3 将动画改为弹性动画，使用初始速度让条形图跳跃。

GraphCapsule.swift

import SwiftUI

struct GraphCapsule: View {
    var index: Int
    var height: Length
    var range: Range<Double>
    var overallRange: Range<Double>

    var heightRatio: Length {
        max(Length(magnitude(of: range) / magnitude(of: overallRange)), 0.15)
    }

    var offsetRatio: Length {
        Length((range.lowerBound - overallRange.lowerBound) / magnitude(of: overallRange))
    }

    var animation: Animation {
        Animation.spring(initialVelocity: 5)
    }

    var body: some View {
        Capsule()
            .fill(Color.gray)
            .frame(height: height * heightRatio, alignment: .bottom)
            .offset(x: 0, y: height * -offsetRatio)
            .animation(animation)
        )
    }
}
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4.4 加快动画速度，缩短每个小节移动到新位置所需的时间。

GraphCapsule.swift

import SwiftUI

struct GraphCapsule: View {
    var index: Int
    var height: Length
    var range: Range<Double>
    var overallRange: Range<Double>

    var heightRatio: Length {
        max(Length(magnitude(of: range) / magnitude(of: overallRange)), 0.15)
    }

    var offsetRatio: Length {
        Length((range.lowerBound - overallRange.lowerBound) / magnitude(of: overallRange))
    }

    var animation: Animation {
        Animation.spring(initialVelocity: 5)
            .speed(2)
    }

    var body: some View {
        Capsule()
            .fill(Color.gray)
            .frame(height: height * heightRatio, alignment: .bottom)
            .offset(x: 0, y: height * -offsetRatio)
            .animation(animation)
        )
    }
}
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4.5 根据 Capsule 在图表上的位置为每个动画添加延迟。

GraphCapsule.swift

import SwiftUI

struct GraphCapsule: View {
    var index: Int
    var height: Length
    var range: Range<Double>
    var overallRange: Range<Double>

    var heightRatio: Length {
        max(Length(magnitude(of: range) / magnitude(of: overallRange)), 0.15)
    }

    var offsetRatio: Length {
        Length((range.lowerBound - overallRange.lowerBound) / magnitude(of: overallRange))
    }

    var animation: Animation {
        Animation.spring(initialVelocity: 5)
            .speed(2)
            .delay(0.03 * Double(index))
    }

    var body: some View {
        Capsule()
            .fill(Color.gray)
            .frame(height: height * heightRatio, alignment: .bottom)
            .offset(x: 0, y: height * -offsetRatio)
            .animation(animation)
        )
    }
}
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