DependenciesOfDependencies.swift

import SwiftUI
import Dependencies

struct Foo: DependencyKey {
  static var liveValue: Self {
    .init {
      "foo"
    }
  }
  
  var foo: () -> String
}
extension DependencyValues {
  var foo: Foo {
    get { self[Foo.self] }
    set { self[Foo.self] = newValue }
  }
}

extension DependencyValues {
  var bar: Bar {
    get { self[Bar.self] }
    set { self[Bar.self] = newValue }
  }
}

struct Bar: DependencyKey {
  static var liveValue: Self {
    @Dependency(\.foo) var foo
    return .init {
      return foo.foo()
    }
  }
  
  var bar: () -> String
}


struct DependenciesPlayground_Previews: View, PreviewProvider {
  static var previews: some View {
    VStack {
      self.init()
      withDependencies {
        $0.foo.foo = { "baz" }
      } operation: {
        Group {
          self.init()
          withDependencies {
            $0.bar = .liveValue
            $0.foo = .liveValue
          } operation: {
            Group {
              self.init()
              withDependencies {
                $0.bar = .liveValue
              } operation: {
                self.init()
              }
            }
          }
        }
      }
    }
  }
  @Dependency(\.foo) private var foo
  @Dependency(\.bar) private var bar
  var body: some View {
    VStack {
      Text(foo.foo())
      Text(bar.bar())
      Text("---")
    }
  }
}