Cannot assign to property: 'lowerBound' is a 'let' constant — becomes 'let' in UIView

Question

ludzik OP

Created Sep ’20

Replies 1

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Views 1.9k

Participants 2

I have a struct:

Code Block public typealias CoordUnit = Double
public struct StyledAxis: StyledAxisProtocol {
		public var name: String
		public var bounds: ClosedRange<CoordUnit>
		public var distribution: Double?
		...
}

In SpaceAxisProtocol bounds are defined as:

Code Block     public protocol SpaceAxisProtocol: Equatable & Hashable {
        var name: String {get set}
        var bounds: ClosedRange<CoordUnit> {get set}
      	init(_ name: String, bounds: ClosedRange<CoordUnit>)
    }
    public protocol StyledAxisProtocol: SpaceAxisProtocol {...}

And I try to edit bounds in a View:

Code Block     struct StyledAxisView<SA:StyledAxisProtocol>: View {
        @Binding var axis: SA
        
        public var body : some View {
            VStack {
                // Axis name and distribution works
                HStack(alignment: .lastTextBaseline) {
                    TextField("", text: $axis.name)
                        .controlSize(.small)
                        .font(.headline)
                    
                    DistributionView(
                        value: $axis.distribution)
                        .controlSize(.mini)
                }
                // It causes compiler `Cannot assign to property: 'lowerBound' is a 'let' constant`. But it isn't. ?
                HStack {
                    ValueView(value: $axis.bounds.lowerBound)
                    ValueView(value: $axis.bounds.upperBound)
                }
                ...

When axis.bounds.lowerbound and axis.bounds.upperbound became let? How to edit them in View?

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Answer 1

OOPer OP

Sep ’20

When axis.bounds.lowerbound and axis.bounds.upperbound became let?

Please check the doc of ClosedRange. lowerBound and upperBound are let-constants.

How to edit them in View?

Replace the whole bounds. You can prepare this extension:

Code Block extension SpaceAxisProtocol {
    var lowerBound: CoordUnit {
        get {bounds.lowerBound}
        set {
            bounds = newValue...bounds.upperBound
        }
    }
    var upperBound: CoordUnit {
        get {bounds.upperBound}
        set {
            bounds = bounds.lowerBound...newValue
        }
    }
}

And use it like this:

Code Block struct StyledAxisView<SA:StyledAxisProtocol>: View {
    @Binding var axis: SA
    
    public var body : some View {
        VStack {
            //...
            HStack {
                ValueView(value: $axis.lowerBound) //<-
                ValueView(value: $axis.upperBound) //<-
            }
            //...
        }
    }
}

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