I have this simple ThemedNavigationButton view that handles some stuff whilst creating a NavigationLink (The inner workings aren't important):

struct ThemedNavigationButton<Destination, L>: View where Destination: View, L: View {
    var destination: () -> Destination
    var label: () -> L
    
    var body: some View {
        ...
    }
}

I use L here and not Label because I need to use the SwiftUI Label next

which I use like this:

ThemedNavigationButton {
    NextView()
} label: {
    Label {
        Text("Some text")
    } icon: {
        Image(systemName: "check")
            .foregroundColor(theme.tint)
    }
}

I want to create a simpler initialiser when it is used in this manner, so I came up with this:

extension ThemedNavigationButton where L == Label<Text, Image> {
    
    init(text: String, systemImage: String, destination: @escaping () -> Destination) {
        self.destination = destination
        self.label = {
            Label {
                Text(text   text)
            } icon: {
                Image(systemName: systemImage)
            }
        }
    }
}

which works great like this:

ThemedNavigationButton(text: "Some text", systemImage: "check") { NextView() }

The problem I have, is as soon as I add the image tint colour to the new initialiser I get the error:

Cannot convert value of type 'some View' to closure result type 'Image'

I'm guessing because my Image is no longer an Image. But what is it and how do I declare it. I can't use some View which is what the compiler is telling me it is.

CodePudding user response：

Generics specialisation requires concrete types, so here is a possible approach to resolve this situation - introduce custom wrapper/proxy type and use it in extension.

Tested with Xcode 13.2

struct MyLabel: View {     // new wrapper type
    let text: String
    let systemImage: String
    var tintColor = Color.green
    var body: some View {
        Label {
             Text(text   text)
        } icon: {
             Image(systemName: systemImage)
                .foregroundColor(tintColor)
        }
    }
}

extension ThemedNavigationButton where L == MyLabel {   // << here !!
    init(text: String, systemImage: String, destination: @escaping () -> Destination) {
        self.destination = destination
        self.label = {
            MyLabel(text: text, systemImage: systemImage)
        }
    }
}

CodePudding user response：

To use the following notation:

ThemedNavigationButton(text: "Some text", systemImage: "check") { NextView() }

you can create a View with just one generic type for Destination, because the Label will receive basic String types.

You can set ThemedNavigationButton as follows:

// Only one generic type needed
struct ThemedNavigationButton<Destination: View>: View {
    
    // Constants for the label (make them appear before "destination")
    let text: String
    let systemImage: String
    
    // Destination view
    var destination: () -> Destination

    var body: some View {
        
        // Show the views the way you want
        VStack {
            destination()
            
            // Use the label this way
            Label {
                Text(text)
            } icon: {
                Image(systemName: systemImage)
            }
        }
    }
}

Customise the body the way you need. You can use it calling:

ThemedNavigationButton(text: "Some text", systemImage: "check") { NextView() }