Routing with a generic function

Question

So I am trying to create a router. I intended to route directly to a view in the body such as..

struct MyView: View {
  let page: Page
   var body: some View {
     router(page: page)
    }
 }

In the body I tried to declare a function...

func router(page: Page) -> View

which gave me the error:

Protocol 'View' can only be used as a generic constraint because it has Self or associated type requirements

so I then made it

func router<T: View>(page: Page) -> T

then in the body I used a switch statement:

 func router<T: View>(page: Page) -> T {

switch page {
  case .page1: 
     return Page1()
        ....

 }
}

then Xcode recommended that I cast all returned views as! T

which ultimately led me to the generic warning against generics as been below:

Text

I search SO for this problem and there are many variants but none led to me understand why this doesn't work.

Does this answer your question? [How to pass one SwiftUI View as a variable to another View struct](https://stackoverflow.com/questions/56938805/how-to-pass-one-swiftui-view-as-a-variable-to-another-view-struct) — lorem ipsum, Jan 28 '21 at 02:03
@loremipsum It looks like it might but its a deep dive and I am literally burnt out for the night, thanks for the reference, will read when I can come to my senses. — Sergio Bost, Jan 28 '21 at 02:06

jnpdx · Accepted Answer · 2021-01-28T02:18:52.370

In this case, you're overcomplicating things a bit. Your router function can just return some View like your other body functions can:

enum AirFormulas {
    case formula1
    case formula2
}

struct RouterView : View {
    let formula : AirFormulas
    var body: some View {
        router(formula: formula)
    }
}

@ViewBuilder func router(formula: AirFormulas) -> some View {
    switch formula {
    case .formula1:
        Text("Hi")
    case .formula2:
        Image(systemName: "plus")
    }
}

Note a couple of things:

I used @ViewBuilder so that I could return a variety of View types from the function. It also means I'm not going to explicitly call return. More reading: https://swiftwithmajid.com/2019/12/18/the-power-of-viewbuilder-in-swiftui/
Another (uglier) way to do this is to wrap all of the returned values with AnyView and return AnyView instead of some View

Finally, why you were getting the error: In the case of your original code, T couldn't be inferred because there was nothing to infer it from. You pass an AirFormulas, which has no information about T. And the return value can be a variety of View types, but that's what we use some View to achieve.

Thanks, that led to quick understanding and gave me food for thought for the future on understanding more about ViewBuilders — Sergio Bost, Jan 28 '21 at 02:21

Routing with a generic function

1 Answers1