.if(let:then:)

🔸 定義

extension View {
    /// usage: `view.if(let: value) { (view, value) in ... }`
    @ViewBuilder func if<Value, Content: View>(
        `let` value: Value?, 
        @ViewBuilder then modifySelfWithValue: (Self, Value) -> Content 
    ) -> some View {
        if let value = value { modifySelfWithValue(self, value) } 
        else { self }
    }
}

• 用法：

//  type:  `Value?`         `Value`
//             ↓               ↓
view.if(let: value) { (view, value) in
    // modify the view with value (⭐️ @ViewBuilder closure)
}

💈 範例

struct ContentView: View {
    
    @State private var isNil = false
    var value: Int? { isNil ? nil : 2 }
    
    var body: some View {
        VStack {
            
            Color.red.overlay(Text("1"))
            
                // ⭐️ view.if(let:then:)
                .if(let: value) { (view, value) in
                    // construct `thisView` with `value`
                    VStack {
                        view
                        Color.orange.overlay(Text("\(value)"))
                    }
                } // view modified by `ifLet`
                .border(Color.blue, width: 3)
                
            // toggles
            Toggle(isOn: $isNil) { Text("value is nil") }
                .padding()
                .overlay(Rectangle().stroke(Color.gray, style: .init(dash: [6])))
            
        } // VStack (container)
            .padding()
            .border(Color.gray, width: 3).frame(height: 300)
    }
}

💈 範例

📗 參考

• Optional SwiftUI View - Swift by Sundell

• Optional @ViewBuilder closure - StackOverflow

📘 手冊

• Swift ⟩ Language Guide ⟩ Generic Functions

👥 相關

• ViewBuilder

• .if(_:then:)