Using iOS 15+ API (@AccessibilityFocusState) without dropping support for earlier iOS versions

Question

Apple introduced the @FocusState and @AccessibilityFocusState and their respective APIs for iOS 15. Typically when I have an app that supports multiple versions and I need to use a new API, I would wrap the code with if #available (iOS x) {} or use @available. For managing focus state, I need to declare a var with the @AccessibilityFocusState property wrapper, and literally including the following code in a SwiftUI View will cause it to crash at runtime on an iOS 14 device, although the compiler has no complaints:

@available(iOS 15.0, tvOS 15.0, *)
@AccessibilityFocusState var focus: FocusLocation?

On tvOS, I can use the compiler directive #if os(tvOS) … #endif to this compile conditionally, but this isn't an option for iOS versions which are handled at runtime.

To be clear, I know that I can’t use this API for iOS 14 devices, but dropping support for iOS 14 is another issue entirely

Is there anyway to use this iOS 15+ API for iOS 15+ VoiceOver users, and still allow general iOS 14 users to run the rest of the app?

Answer 1

It turns out there is a good way to handle this: put @AccessibilityFocusState in a custom modifier.

@available(iOS 15, *)
struct FocusModifier: ViewModifier {
    @AccessibilityFocusState var focusTarget: AccessibilityFocusTarget?
    @Environment(\.lastAccessibilityFocus) @Binding var lastFocus
    // this is the value passed into the modifier that associates an enum value with this particular view
    var focusTargetValue: AccessibilityFocusTarget? 

    init(targetValue: AccessibilityFocusTarget) {
        focusTargetValue = targetValue
    }

    func body(content: Content) -> some View {
        content.accessibilityFocused($focusTarget, equals: focusTargetValue)
            .onChange(of: focusTarget) { focus in
                if focus == focusTargetValue {
                    lastFocus = focusTargetValue
                }
            }.onReceive(NotificationCenter.default.publisher(for: .accessibilityFocusAssign)) { notification in
                if let userInfo = notification.userInfo,
                   let target = userInfo[UIAccessibility.assignAccessibilityFocusUserInfoKey] as? AccessibilityFocusTarget,
                   target == focusTargetValue
                {
                    focusTarget = target
                }
            }
    }
}
public extension View {
    // Without @ViewBuilder, it will insist on inferring a single View type
    @ViewBuilder
    func a11yFocus(targetValue: AccessibilityFocusTarget) -> some View {
        if #available(iOS 15, *) {
            modifier(FocusModifier(targetValue: targetValue))
        } else {
            self
        }
    }
}

where AccessibilityFocusTarget is just an enum of programmatic focus candidates:

public enum AccessibilityFocusTarget: String, Equatable {
    case title
    case shareButton
    case favouriteButton
}

And I'm storing the last focused element as a Binding to AccessibilityFocusTarget in the environment:

public extension EnvironmentValues {

    private struct LastAccessibilityFocus: EnvironmentKey {
        static let defaultValue: Binding<AccessibilityFocusTarget?> = .constant(nil)
    }

    var lastAccessibilityFocus: Binding<AccessibilityFocusTarget?> {
        get { self[LastAccessibilityFocus.self] }
        set { self[LastAccessibilityFocus.self] = newValue
        }
    }
}

The .onReceive block lets us will take a notification with the AccessibilityFocusTarget value in userInfo and programmatically set focus to the View associated with that value via the modifier.

I've added a custom notification and userInfo key string:

extension Notification.Name {
    public static let accessibilityFocusAssign = Notification.Name("accessibilityFocusAssignNotification")
}

extension UIAccessibility {
    public static let assignAccessibilityFocusUserInfoKey = "assignAccessibilityFocusUserInfoKey"
}

Using this is simple. At the top of your SwiftUI View hierarchy, inject something into the binding in the environment:

struct TopView: View {
    @State var focus: AccessibilityFocusTarget?

    var body: some View {
        FirstPage()
            .environment(\.lastAccessibilityFocus, $focus)
            
    }
}

And for any Views within the hierarchy that might be candidates for programmatic focus, just use the modifier to associate it with a AccessibilityFocusTarget enum value:

Title()
  .a11yFocus(targetValue: .title)
ShareButton()
  .a11yFocus(targetValue: .shareButton)

Nothing else is need in any of those child views - all the heavy lifting is handled in the modifier!

To set focus, just fire off a notification using NotificationCenter, with the focus target in userInfo:

   public static func setAccessibilityFocus(_ target: AccessibilityFocusTarget, withDelayInSeconds delay: TimeInterval = 0) {
        DispatchQueue.main.asyncAfter(deadline: .now() + delay) {
            NotificationCenter.default.post(
                name: .accessibilityFocusAssign,
                object: nil,
                userInfo: [UIAccessibility.assignAccessibilityFocusUserInfoKey: target as Any]
            )
        }
    }