Using @Binding with type casting in swiftui

Question

My data model:

class MyData: ObservableObject {
    @Published var aabbData = [AabbData]()
}

struct AbData: AabbData{
    var x = 0
}

protocol AabbData{

}

In my view code, i want to downcast aabbData's element to AbData type, and bind its x property to a control's value. So i try to code this:

@EnvironmentObject var data: MyData
let index = 0

//some other code

//method 1
Slider(value: ($data.aabbData[index] as! Binding<AbData>).x, in: -100...100, step: 1)
//error: Cast from 'Binding<AabbData>' to unrelated type 'Binding<AbData>' always fails

//method 2
Slider(value: Binding<Int>(
    get:{
        (data.aabbData[index] as! AbData).x
    },
    set:{
        (data.aabbData[index] as! AbData).x = $0
        //error: Cannot assign to immutable expression of type 'Int'
    }
), in: -100...100, step: 1)

The code is not working. How can i bind the downcast value's property to control's value?

Generic types of related types aren't themselves related in Swift. In other words, `Foo` is as different from `Foo` as any other type, even if `A: B` — New Dev, Aug 14 '20 at 15:11
If you're going force-cast then `aabbData` is always `[AbData]` then there is no sense in this artificial making it `[AabbData]` at all. And by the way in majority of cases forget about casting in SwiftUI, for easiness and safeness of project code. — Asperi, Aug 14 '20 at 15:11
It's simplified code, aabbData is a array contains multiple different struct type. — , Aug 14 '20 at 15:22

cobbal · Accepted Answer · 2020-08-14T15:34:54.440

1

The Binding type allow for both reading and writing. Since you can't safely read the value of an AabbData and assume it's an AbData this cannot work.

If you want to try anyway, then you'll have to create a new Binding (possibly with the Binding(get:set:) constructor) that does the casting there.

For example, noting that this forced cast is probably not advisable for all the usual reasons:

let binding = Binding<AbData>(get: {
    data.aabbData[index] as! AbData
}, set: {
    data.aabbData[index] = $0
})

edited Aug 14 '20 at 15:34

answered Aug 14 '20 at 15:08

cobbal

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thanks. I try to create a new binding, but fail to create a correct setter to make data binding working. Could you show some code? – Aug 14 '20 at 15:16
@swiftrustar sure – cobbal Aug 14 '20 at 15:35

Using @Binding with type casting in swiftui

1 Answers1