On-change handlers for Composable Architecture Reducers

Reducer+OnChange.swift

// Original version from the authors of ComposableArchitecture here: 
// https://github.com/pointfreeco/isowords/blob/fcff52ccf176f40db55951d5897def7cd9b879cf/Sources/TcaHelpers/OnChange.swift
//
// The following adds a method for filtering changes with a predicate.
// This is written using reactiveswift-composable-architecture, but can be adapted very quickly for Combine.

import ComposableArchitecture
import ReactiveSwift

public extension Reducer {
    func onChange<LocalState>(
        of toLocalState: @escaping (State) -> LocalState,
        by predicate: @escaping (_ from: LocalState, _ to: LocalState) -> Bool = { $0 != $1 },
        perform additionalEffects: @escaping (LocalState, inout State, Action, Environment)
            -> Effect<
                Action, Never
            >
    ) -> Self where LocalState: Equatable {
        .init { state, action, environment in
            var previousLocalState = toLocalState(state)
            defer { previousLocalState = localState }

            let effects = self.run(&state, action, environment)

            let localState = toLocalState(state)

            var mergedEffects: SignalProducer<Action, Never> {
                .merge(
                    effects,
                    additionalEffects(localState, &state, action, environment)
                )
            }

            return predicate(previousLocalState, localState) ?
                mergedEffects :
                effects
        }
    }

    func onChange<LocalState>(
        of toLocalState: @escaping (State) -> LocalState,
        perform additionalEffects: @escaping (
            LocalState,
            LocalState,
            inout State,
            Action,
            Environment
        )
            -> Effect<Action, Never>
    ) -> Self where LocalState: Equatable {
        .init { state, action, environment in
            var previousLocalState = toLocalState(state)
            defer { previousLocalState = localState }

            let effects = self.run(&state, action, environment)

            let localState = toLocalState(state)

            return previousLocalState != localState
                ? .merge(
                    effects, additionalEffects(
                        previousLocalState,
                        localState,
                        &state,
                        action,
                        environment
                    )
                )
                : effects
        }
    }
}