Arrays and Initialisation: Bug or "feature"? (and some interesting array trickery!)

Using Swift
,
9

Now you have the opposite problem. :upside_down_face: You are asking about arrays, but your problem has nothing to do with them specifically. Array is a value type in Swift, and your problem revolves around value type mutation.

struct ContentView: View {
  var mutable = 0
  var body: some View {
    mutable = 1 // Cannot assign to property: 'self' is immutable
    EmptyView()
  }
}

Take SwiftUI away for a moment. Your problem reduces to this:

struct ValueType {
  var mutable = 0
  var property: Void {
    mutable = 1 // Cannot assign to property: 'self' is immutable
    return // The return type is irrelevant to the problem.
  }
}

Property get accessors cannot mutate self. By default, get is nonmutating. All of these are the same. The first two are shorthand for the full form, nonmutating get.

struct ValueType {
  var property: Void { return }
}

struct ValueType {
  var property: Void {
    get { return }
  }
}

struct ValueType {
  var property: Void {
    nonmutating get { return }
  }
}

To compile, the get must be mutating, instead.

struct ValueType {
  var mutable = 0
  var property: Void {
    mutating get {
      mutable = 1
      return
    }
  }
}

But SwiftUI's View protocol requires that body be nonmutating.

  1. If you're going to use SwiftUI, you instead use property wrappers like @State and @Binding to mutate the variables that are associated with a View.
  2. body is not a place to perform imperative code. Imperative code is not compatible with ViewBuilder, which body uses. There are many correct options available, but which one is appropriate is dependent on use case.
struct ContentView: View {
  @State var int = 0
  var body: some View {
    int = 1  // 'buildExpression' is unavailable: this expression does not conform to 'View'
    EmptyView()
  }
}
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