ConditionalEnvironmentOverlay

ConditionalEnvironmentOverlay.swift

private struct ConditionalEnvironmentOverlay<Value: Equatable, OverlayContent: View>: ViewModifier {
  @Environment(\.self) private var environmentValues
  
  private let keyPath: KeyPath<EnvironmentValues, Value>
  private let value: Value
  private let overlayContent: OverlayContent
  
  init(
    condition: KeyPath<EnvironmentValues, Value>,
    equals value: Value,
    content: OverlayContent
  ) {
    self.keyPath = condition
    self.value = value
    self.overlayContent = content
  }
  
  func body(content: Content) -> some View {
    content
      .overlay {
        if environmentValues[keyPath: keyPath] == value {
          overlayContent
        }
      }
  }
}

extension View {
  func overlay<T: Equatable>(
    when: KeyPath<EnvironmentValues, T>,
    equals: T,
    @ViewBuilder content: () -> some View
  ) -> some View {
    modifier(
      ConditionalEnvironmentOverlay(
        condition: when,
        equals: equals,
        content: content()
      )
    )
  }
}

struct SampleView: View {
  var body: some View {
    Text("Hello World!")
      // → Usage:
      .overlay(when: \.colorScheme, equals: .dark) {
        Color.red
      }
  }
}

// Beware of performances before iOS 17! (106310433)
// https://developer.apple.com/documentation/ios-ipados-release-notes/ios-ipados-17-release-notes