✨selected button with underline

• SwiftUI ⟩ Data Flow ⟩ View Preferences ⟩ selected button with underline

本例使用 Anchor Preferences 與 Matched Geometry Effect 來達到同樣的效果。

⭐️ 重點

⭐️ 從此例可看出：

"source view" 才是 matched geometry effect 的「移動目標」，其他 non-source view (isSource:false) 都會移動到 source view 所在的位置 (frame)。

此例的 non-source view 都是 overlay，可以看出它們的「圖層關係」：

• 第一個 button (World) 有三個 overlay，所以幾乎看不到。

• 第二個 button (Alarm) 有兩個 overlay，所以可以看到一些，而且第一個 button 的 overlay 位在第二個 button 之下。

• 第三個 button (Bedtime) 只有一個 overlay，所以可以很清楚看出來。

💾 程式

⬆️ 需要： Text+, StackForEach

// 2022.02.24
// 2022.02.25 (r) + different colors/heights to reveal non-source views.

import SwiftUI
import PlaygroundSupport
import CustomViews

PlaygroundPage.current.setLiveView(ContentView())
    
// ----------------------------------------
//     🔸 Bounds (Anchor PreferenceKey)
// ----------------------------------------

private enum Bounds: PreferenceKey {
    public typealias Value = Anchor<CGRect>?
    public static var defaultValue: Value { nil }
    public static func reduce(value: inout Value, nextValue: () -> Value) {
        value = value ?? nextValue()   // ⭐️ first non-nil (if any)
    }
}

// 🌀 給設定與調用 PreferenceKey 的方法獨特的名字，配合程式碼的目的，
//    可提高程式碼的可讀性。
private extension View {
    
    /// 🔸 set selected bounds for views
    func setSelectedBounds(
        at index: Int, 
        selected: @escaping (Int) -> Bool
    ) -> some View 
    {
        anchorPreference(key: Bounds.self, value: .bounds) { 
            // ⭐️ tranform `anchor` to nil if not `selected`
            anchor in selected(index) ? anchor : nil 
        }
    }
    
    /// 🔸 underline the selected bounds
    func underlineSelectedBounds() -> some View {
        backgroundPreferenceValue(Bounds.self) { anchor in 
            GeometryReader { proxy in 
                let bounds = proxy[anchor!]    // ⭐️ force-unwrap is OK❗️ 
                Color.green 
                    .frame(width: bounds.width, height: 1)
                    .offset(x: bounds.minX, y: bounds.height)
            }
        }
    }
}

// -------------------
//     ContentView 
// -------------------

struct ContentView: View {
    
    // ⭐️ current index
    @State private var selected = 0
    
    // ⭐️ for matched geometry effect
    @Namespace private var ns
    
    var body: some View {
        VStack {
            Text("**Anchor Preference**")
                .font(.title3)
                .foregroundColor(.secondary)
            buttons1
                .padding()
                .border(.white.opacity(0.3))
            Text("**Matched Geometry Effect**")
                .font(.title3)
                .foregroundColor(.secondary)
            buttons2
                .padding()
                .border(.white.opacity(0.3))
        }
    }
}

extension ContentView {
    
    /// tabs
    var tabs: [Text] {
        [
            label("World", "globe.asia.australia", .blue),
            label("Alarm", "alarm", .pink),
            label("Bedtime", "bed.double.fill", .orange),
        ]
    }
    
    /// button for tab i
    func button(_ i: Int) -> some View {
        Button { 
            withAnimation {
                self.selected = i    // ⭐️ set current index on click
            }
        } label: { 
            self.tabs[i] 
        }
        .foregroundColor(.white)
    }
    
    /// custom label
    func label(_ text: String, _ symbol: String, _ color: Color) -> Text {
        Text(symbol: symbol).foregroundColor(color) + Text(text) 
    }
    
    // -------------------------
    //     anchor preference
    // -------------------------
    var buttons1: some View {
        HStackForEach(tabs.indices) { i in
            button(i)
                // ⭐️ set selected bounds
                .setSelectedBounds(at: i) { self.selected == $0 }
        }
        // ⭐️ underline "the selected bounds"
        .underlineSelectedBounds()
    }
    
    // -------------------------------
    //     matched geometry effect
    // -------------------------------
    var buttons2: some View {
        HStackForEach(tabs.indices) { i in
            button(i)
                // ⭐️ source view
                // ❓ "source view" is the "target" for matched geometry effect❓ 
                //    ❗️totally confused❗️
                .matchedGeometryEffect(id: i, in: ns)
                .overlay {
                    colors[i]
                        .opacity(0.9)
                        .frame(height: 10.0 * (3-i), alignment: .bottom)
                        .shadow(radius: 4)
                        // ⭐️ non-source view
                        .matchedGeometryEffect(id: selected, in: ns, isSource: false)
                }
        }
    }
    
    var colors: [Color] {
        [.red, .green, .blue]
    }
}

📗 參考

• Thinking in SwiftUI, Ch.5 - Layout, Anchors (p.100)

👥 相關

• example of Anchor Preferences.

• can be done by Matched Geometry Effect.

• dynamic underline - use ＠State var.

History

✏️

1. 2022.02.24

1

import SwiftUI
import PlaygroundSupport
import CustomViews

PlaygroundPage.current.setLiveView(ContentView())

/// 🔸 Bounds (Anchor PreferenceKey)
private enum Bounds: PreferenceKey {
    public typealias Value = Anchor<CGRect>?
    public static var defaultValue: Value { nil }
    public static func reduce(value: inout Value, nextValue: () -> Value) {
        value = value ?? nextValue()   // ⭐️ first non-nil (if any)
    }
}

struct ContentView: View {
    
    // ⭐️ current index
    @State private var selected = 0
    
    var body: some View {
        HStackForEach(tabs.indices) { i in
            Button { 
                self.selected = i    // ⭐️ set current index on click
            } label: { 
                self.tabs[i] 
            }
            .foregroundColor(.white)
            // ⭐️ set anchor preference (.bounds) for each button
            .anchorPreference(key: Bounds.self, value: .bounds) { 
                // ⭐️ tranform `anchor` to nil if not `selected`
                anchor in self.selected == i ? anchor : nil 
            }
        }
        // ⭐️ set background based on "the selected bounds"
        .backgroundPreferenceValue(Bounds.self) { anchor in 
            GeometryReader { proxy in 
                
                let bounds = proxy[anchor!]    // ⭐️ force unwrap❗️ 
                
                Color.indigo 
                    .frame(width: bounds.width, height: 4)
                    .offset(x: bounds.minX, y: bounds.height - 2) 
                    .animation(.default)
            }
        }
        .padding()
        .border(.white.opacity(0.3))
    }
}

extension ContentView {
    var tabs: [Text] {
        [ Text("World Clock"), Text("Alarm"), Text("Bedtime") ]
    } 
}