新版 SwiftUI 的 ViewBuilder 可以輸入 if let，switch & 宣告變數常數

新版的 SwiftUI 現在可以在 ViewBuilder 裡輸入 if let，switch & 宣告變數常數了。

if let

struct ProfileView: View {
    
    var userName: String
    var imageName: String?
    
    var body: some View {
        VStack {
            Text(userName)
            if let imageName = imageName {
                Image(imageName)
                    .resizable()
                    .scaledToFit()
            }
        }
      
    }
}
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ProfileView(userName: "peter", imageName: "peter")
    }
}

switch

enum Gender {
    case boy, girl
}
struct ProfileView: View {
    
    var userName: String
    var gender: Gender
    
    var body: some View {
        VStack {
            Text(userName)
            switch gender {
            case .boy:
                Text("👦")
            case .girl:
                Text("👧")
            }
        }
    }
}
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ProfileView(userName: "peter", gender: .boy)
    }
}

宣告變數常數

struct ContentView: View {
    
    var body: some View {
        HStack {
            let number1 = Int.random(in: 1...6)
            let number2 = Int.random(in: 1...6)
            let number3 = Int.random(in: 1...6)
Image(systemName: "\(number1).circle")
                .resizable()
            Image(systemName: "\(number2).circle")
                .resizable()
            Image(systemName: "\(number3).circle")
                .resizable()
        }
        .scaledToFit()
    }
}

相反的，在舊版的 SwiftUI，在 ViewBuilder 宣告變數常數將產生錯誤，顯示 Closure containing a declaration cannot be used with function builder ‘ViewBuilder’。