How to pass one SwiftUI View as a variable to anot

问题:

I'm implementing a very custom NavigationLink called MenuItem and would like to reuse it across the project. It's a struct that conforms to View and implements var body : some View which contains a NavigationLink. I need to somehow store the view that shall be presented by NavigationLink in the body of MenuItem but have yet failed to do so.

I have defined destinationView in MenuItem's body as some View and tried two initializers:

This seemed too easy:

struct MenuItem: View {
    private var destinationView: some View

    init(destinationView: View) {
        self.destinationView = destinationView
    }

    var body : some View {
        // Here I'm passing destinationView to NavigationLink...
    }
}

--> Error: Protocol 'View' can only be used as a generic constraint because it has Self or associated type requirements.

2nd try:

struct MenuItem: View {
    private var destinationView: some View

    init<V>(destinationView: V) where V: View {
        self.destinationView = destinationView
    }

    var body : some View {
        // Here I'm passing destinationView to NavigationLink...
    }
}

--> Error: Cannot assign value of type 'V' to type 'some View'.

Final try:

struct MenuItem: View {
    private var destinationView: some View

    init<V>(destinationView: V) where V: View {
        self.destinationView = destinationView as View
    }

    var body : some View {
        // Here I'm passing destinationView to NavigationLink...
    }
}

--> Error: Cannot assign value of type 'View' to type 'some View'.

I hope someone can help me. There must be a way if NavigationLink can accept some View as an argument. Thanks ;D

回答1:

The way Apple does it is using function builders, there is a predefined one called ViewBuilder, make the last argument, or only argument, of you init method for MenuItem this

...., @ViewBuilder builder: @escaping () -> Content)

assign it to a property defined something like

  let                 viewBuilder: () -> Content

and then where you want to output the Views just call the function like

HStack {
    viewBuilder()
}

then you can do

MenuItem {
   Image("myImage")
   Text("My Text")
}

This will let you pass up to 10 view and use if conditions etc. though if you want it to be more restrictive you will have to define your own function builder. I haven't done that so you will have to google that.

回答2:

You should make the generic parameter part of MenuItem:

struct MenuItem<Content: View>: View {
    private var destinationView: Content

    init(destinationView: Content) {
        self.destinationView = destinationView
    }

    var body : some View {
        // ...
    }
}

回答3:

You can create your custom view like this:

struct ENavigationView<Content: View>: View {

    let viewBuilder: () -> Content

    var body: some View {
        NavigationView {
            VStack {
                viewBuilder()
                    .navigationBarTitle("My App")
            }
        }
    }

}

struct ENavigationView_Previews: PreviewProvider {
    static var previews: some View {
        ENavigationView {
            Text("Preview")
        }
    }
}

Using:

struct ContentView: View {

    var body: some View {
        ENavigationView {
            Text("My Text")
        }
    }

}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

回答4:

You can pass a NavigationLink (or any other view widget) as a variable to a subview as follows:

import SwiftUI

struct ParentView: View {
    var body: some View {
        NavigationView{

            VStack(spacing: 8){

                ChildView(destinationView: Text("View1"), title: "1st")
                ChildView(destinationView: Text("View2"), title: "2nd")
                ChildView(destinationView: ThirdView(), title: "3rd")
                Spacer()
            }
            .padding(.all)
            .navigationBarTitle("NavigationLinks")
        }
    }
}

struct ChildView<Content: View>: View {
    var destinationView: Content
    var title: String

    init(destinationView: Content,  title: String) {
        self.destinationView = destinationView
        self.title = title
    }

    var body: some View {
        NavigationLink(destination: destinationView){
            Text("This item opens the \(title) view").foregroundColor(Color.black)
        }
    }
}

struct ThirdView: View {
    var body: some View {
        VStack(spacing: 8){

            ChildView(destinationView: Text("View1"), title: "1st")
            ChildView(destinationView: Text("View2"), title: "2nd")
            ChildView(destinationView: ThirdView(), title: "3rd")
            Spacer()
        }
        .padding(.all)
        .navigationBarTitle("NavigationLinks")
    }
}

回答5:

I really struggled to make mine work for an extension of View. Full details about how to call it are seen here.

The extension for View (using generics) - remember to import SwiftUI:

extension View {

    /// Navigate to a new view.
    /// - Parameters:
    ///   - view: View to navigate to.
    ///   - binding: Only navigates when this condition is `true`.
    func navigate<SomeView: View>(to view: SomeView, when binding: Binding<Bool>) -> some View {
        modifier(NavigateModifier(destination: view, binding: binding))
    }
}


// MARK: - NavigateModifier
fileprivate struct NavigateModifier<SomeView: View>: ViewModifier {

    // MARK: Private properties
    fileprivate let destination: SomeView
    @Binding fileprivate var binding: Bool


    // MARK: - View body
    fileprivate func body(content: Content) -> some View {
        NavigationView {
            ZStack {
                content
                    .navigationBarTitle("")
                    .navigationBarHidden(true)
                NavigationLink(destination: destination
                    .navigationBarTitle("")
                    .navigationBarHidden(true),
                               isActive: $binding) {
                    EmptyView()
                }
            }
        }
    }
}