How to have a dynamic List of Views using SwiftUI

问题:

I can do a static List like

List {
   View1()
   View2()
}

But how do i make a dynamic list of elements from an array? I tried the following but got error: Closure containing control flow statement cannot be used with function builder 'ViewBuilder'

    let elements: [Any] = [View1.self, View2.self]

    List {
       ForEach(0..<elements.count) { index in
          if let _ = elements[index] as? View1 {
             View1()
          } else {
             View2()
          }
    }
}

Is there any work around for this? What I am trying to accomplish is a List contaning dynamic set of elements that are not statically entered.

回答1:

Looks like the answer was related to wrapping my view inside of AnyView

struct ContentView : View {
    var myTypes: [Any] = [View1.self, View2.self]
    var body: some View {
        List {
            ForEach(0..<myTypes.count) { index in
                self.buildView(types: self.myTypes, index: index)
            }
        }
    }

    func buildView(types: [Any], index: Int) -> AnyView {
        switch types[index].self {
           case is View1.Type: return AnyView( View1() )
           case is View1.Type: return AnyView( View2() )
           default: return AnyView(EmptyView())
        }
    }

With this, i can now get view-data from a server and compose them. Also, they are only instanced when needed.

回答2:

You can use dynamic list of subviews, but you need to be careful with the types and the instantiation. For reference, this is a demo a dynamic 'hamburger' here, github/swiftui_hamburger.

// Pages View to select current page
/// This could be refactored into the top level
struct Pages: View {
    @Binding var currentPage: Int
    var pageArray: [AnyView]

    var body: AnyView {
        return pageArray[currentPage]
    }
}

// Top Level View
/// Create two sub-views which, critially, need to be cast to AnyView() structs
/// Pages View then dynamically presents the subviews, based on currentPage state
struct ContentView: View {
    @State var currentPage: Int = 0

    let page0 = AnyView(
        NavigationView {
            VStack {
                Text("Page Menu").color(.black)

                List(["1", "2", "3", "4", "5"].identified(by: \.self)) { row in
                    Text(row)
                }.navigationBarTitle(Text("A Page"), displayMode: .large)
            }
        }
    )

    let page1 = AnyView(
        NavigationView {
            VStack {
                Text("Another Page Menu").color(.black)

                List(["A", "B", "C", "D", "E"].identified(by: \.self)) { row in
                    Text(row)
                }.navigationBarTitle(Text("A Second Page"), displayMode: .large)
            }
        }
    )

    var body: some View {
        let pageArray: [AnyView] = [page0, page1]

        return Pages(currentPage: self.$currentPage, pageArray: pageArray)

    }
}

回答3:

if/let flow control statement cannot be used in a @ViewBuilder block.

Flow control statements inside those special blocks are translated to structs.

e.g.

if (someBool) {
    View1()
} else {
    View2()
}

is translated to a ConditionalValue<View1, View2>.

Not all flow control statements are available inside those blocks, i.e. switch, but this may change in the future.

More about this in the function builder evolution proposal.

---

In your specific example you can rewrite the code as follows:

struct ContentView : View {

    let elements: [Any] = [View1.self, View2.self]

    var body: some View {
        List {
            ForEach(0..<elements.count) { index in
                if self.elements[index] is View1 {
                    View1()
                } else {
                    View2()
                }
            }
        }
    }
}

回答4:

Is it possible to return different Views based on needs?

In short: Sort of

As it's fully described in swift.org, It is IMPOSSIIBLE to have multiple Types returning as opaque type

If a function with an opaque return type returns from multiple places, all of the possible return values must have the same type. For a generic function, that return type can use the function’s generic type parameters, but it must still be a single type.

So how List can do that when statically passed some different views?

List is not returning different types, it returns EmptyView filled with some content view. The builder is able to build a wrapper around any type of view you pass to it, but when you use more and more views, it's not even going to compile at all! (try to pass more than 10 views for example and see what happens)

As you can see, List contents are some kind of ListCoreCellHost containing a subset of views that proves it's just a container of what it represents.

What if I have a lot of data, (like contacts) and want to fill a list for that?

You can conform to Identifiable or use identified(by:) function as described here.

What if any contact could have a different view?

As you call them contact, it means they are same thing! You should consider OOP to make them same and use inheritance advantages. But unlike UIKit, the SwiftUI is based on structs. They can not inherit each other.

So what is the solution?

You MUST wrap all kind of views you want to display into the single View type. The documentation for EmptyView is not enough to take advantage of that (for now). BUT!!! luckily, you can use UIKit

How can I take advantage of UIKit for this

• Implement View1 and View2 on top of UIKit.
• Define a ContainerView with of UIKit.
• Implement the ContainerView the way that takes argument and represent View1 or View2 and size to fit.
• Conform to UIViewRepresentable and implement it's requirements.
• Make your SwiftUI List to show a list of ContainerView So now it's a single type that can represent multiple views

回答5:

I found a little easier way than the answers above.

Create your custom view.

Make sure that your view is Identifiable

(It tells SwiftUI it can distinguish between views inside the ForEach by looking at their id property)

For example, lets say you are just adding images to a HStack, you could create a custom SwiftUI View like:

struct MyImageView: View, Identifiable {
    // Conform to Identifiable:
    var id = UUID()
    // Name of the image:
    var imageName: String

    var body: some View {
        Image(imageName)
            .resizable()
            .frame(width: 50, height: 50)
    }
}

Then in your HStack:

// Images:
HStack(spacing: 10) {
    ForEach(images, id: \.self) { imageName in
        MyImageView(imageName: imageName)
    }
    Spacer()
}

回答6:

You can do this by polymorphism:

struct View1: View {
    var body: some View {
        Text("View1")
    }
}

struct View2: View {
    var body: some View {
        Text("View2")
    }
}

class ViewBase: Identifiable {
    func showView() -> AnyView {
        AnyView(EmptyView())
    }
}

class AnyView1: ViewBase {
    override func showView() -> AnyView {
        AnyView(View1())
    }
}

class AnyView2: ViewBase {
    override func showView() -> AnyView {
        AnyView(View2())
    }
}

struct ContentView: View {
    let views: [ViewBase] = [
        AnyView1(),
        AnyView2()
    ]

    var body: some View {
        List(self.views) { view in
            view.showView()
        }
    }
}

回答7:

I wanted to use different views in the same list as well and therefore implemented an advanced list view, see here.