What is the best practice for Effect to be executed only once onAppear?

[asa08]

The easiest way is to have State flagged, but it is not cool.

import ComposableArchitecture

enum TestCore {}

extension TestCore {
  struct State: Equatable {
    var haveSent: Bool = false
  }
}

extension TestCore {
  enum Action: Equatable {
    case onAppear
  }
}

extension TestCore {
  struct Environment {
    let analyticsLogRepository: AnalyticsLogRepository
  }
}

extension TestCore {
  static let reducer = AnyReducer<State, Action, Environment>.combine(
    .init { state, action, _ in

      // sendLog() is a function that returns AnyPublisher
      func sendOnAppearLog() -> Effect<Action, Never> {
        if state.haveSent { return .none }
        state.haveSent = true
        return environment.analyticsLogRepository
          .sendLog()
          .fireAndForget()
      }

      switch action {
        case .onAppear:
          return sendOnAppearLog()
      }
    }
  )
}

What if I want to run environment.analyticsLogRepository.sendLog() only once the first time?

Any help is appreciated, thanks in advance!

[corysullivan]

Another option is to create a View modifier that only fires on the first onAppear.

See https://www.swiftjectivec.com/swiftui-run-code-only-once-versus-onappear-or-task/?utm_campaign=iOS%2BDev%2BWeekly&utm_medium=web&utm_source=iOS%2BDev%2BWeekly%2BIssue%2B583

[asa08]

Thanks for the advice :)
Other than onAppear, what do you do if you want to execute only the first Effect from multiple calls to an Action?

[mbrandonw]

For that you can always track some extra state:

struct State {
  var hasSentTapAction = false
  …
}

And then in the reducer:

case .tap:
  guard !state.hasSentTapAction
  else { return .none }

  return .run { send in … }
}

Or, if you want something more implicit, you could look at what we do for effect cancellation and try to make your own custom effect that skips an effect if there is already one in flight.