Creating a safe subscript slice of a binding array - ios

Question

I have a safe subscript extension on Array to give me an ArraySlice from a Range like this:
extension Array {
subscript(safe range: Range<Index>) -> ArraySlice<Element>? {
if range.endIndex > endIndex {
if range.startIndex >= endIndex {
return nil
} else {
return self[range.startIndex..<endIndex]
}
} else {
return self[range]
}
}
}
I have a use case for needing a slice of a binding of an array such that I can do this:
let userSlice = $users[safe: 0..<20]
This should return Slice<Binding<[User]>>. How can I alter the above code to work with a Binding of an Array?

Answer 1

Here is a simplified demo of possible approach - at first, and main, is that binding requires wrapped value to be mutable, and at second binding, actually, has no sense on optional (about this see below alternate way).
Tested with Xcode 13.2 / iOS 15.2
extension Array {
subscript(safe range: Range<Index>) -> ArraySlice<Element>? {
get {
if range.endIndex > endIndex {
if range.startIndex >= endIndex {
return nil
} else {
return self[range.startIndex..<endIndex]
}
} else {
return self[range]
}
}
mutating set { // << add mutability !!
if let value = newValue {
_ = value.indices.map { self[$0] = value[$0] }
}
}
}
}
struct DemoView1: View {
struct Model: Identifiable {
let id: Int
var value: String
}
#State private var users = [
Model(id: 1, value: "1"), Model(id: 2, value: "2"), Model(id: 3, value: "3"),
Model(id: 4, value: "4"), Model(id: 5, value: "5")]
var body: some View {
VStack {
Text("Array slice")
List {
// Optional has not sense, so we must explicitly remove
// it that makes code more complicated and less readable
ForEach(Binding($users[safe: 1..<3]) ?? .constant(ArraySlice<Model>()), id: \.wrappedValue.id) {
TextField("", text: $0.value)
}
}
Divider()
Text("Full Array")
List {
ForEach(users) {
Text($0.value)
}
}
}
}
}
Alternate: due to binding has not sense for optionals and non-optional type is better controlled, I would recommend, as alternate, to consider possibility to avoid optional and use subscript with returned definite data, like
extension Array {
subscript(safe range: Range<Index>) -> ArraySlice<Element> {
get {
if range.endIndex > endIndex {
if range.startIndex >= endIndex {
// Here can be something known and defined
return ArraySlice<Element>() // << just empty !!
} else {
return self[range.startIndex..<endIndex]
}
} else {
return self[range]
}
}
mutating set {
_ = newValue.indices.map { self[$0] = newValue[$0] }
}
}
}
struct DemoView2: View {
struct Model: Identifiable {
let id: Int
var value: String
}
#State private var users = [
Model(id: 1, value: "1"), Model(id: 2, value: "2"), Model(id: 3, value: "3"),
Model(id: 4, value: "4"), Model(id: 5, value: "5")]
var body: some View {
VStack {
Text("Array slice")
List {
// Now code is simple and clear !!
ForEach($users[safe: 1..<3], id: \.wrappedValue.id) {
TextField("", text: $0.value)
}
}
Divider()
Text("Full Array")
List {
ForEach(users) {
Text($0.value)
}
}
}
}
}