Higher order reducer for TCA that enables better debugging

debugDiffing.swift

import ComposableArchitecture
import Difference
import Foundation

/// A container for storing action filters.
///
/// The logic behind having this rather than a normal closure is that it allows us to namespace and gather action filters together in a consistent manner.
/// - Note: You should be adding extensions in your modules and exposing common filters you might want to use to focus your debugging work, e.g.
/// ```swift
/// extension ActionFilter where Action == AppAction {
///    static var windowActions: Self {
///        Self(isIncluded: {
///            switch $0 {
///            case .windows:
///                return true
///            default:
///                return false
///            }
///        })
///    }
/// }
/// ```
public struct ActionFilter<Action: Equatable> {
    let isIncluded: (Action) -> Bool

    public init(isIncluded: @escaping (Action) -> Bool) {
        self.isIncluded = isIncluded
    }

    func callAsFunction(_ action: Action) -> Bool {
        isIncluded(action)
    }

    /// Include all actions
    public static var all: Self {
        .init(isIncluded: { _ in true })
    }

    /// negates the filter
    public static func not(_ filter: Self) -> Self {
        .init(isIncluded: { !filter($0) })
    }

    /// Allows all actions except those specified
    public static func allExcept(_ actions: Self...) -> Self {
        allExcept(actions)
    }

    /// Allows all actions except those specified
    public static func allExcept(_ actions: [Self]) -> Self {
        .init(isIncluded: { action in
            !actions.contains(where: { $0(action) })
        })
    }

    /// Allows any of the specified actions
    public static func anyOf(_ actions: Self...) -> Self {
        .anyOf(actions)
    }

    /// Allows any of the specified actions
    public static func anyOf(_ actions: [Self]) -> Self {
        .init(isIncluded: { action in
            actions.contains(where: { $0(action) })
        })
    }
}

extension Reducer where State: Equatable, Action: Equatable {
    /// Adds diffing instrumentation to the TCA
    /// - Parameters:
    ///   - actionFormat: whether to use prettyPrint or just name labels
    ///   - allowedActions: Lets you specify actions that you care about. Defaults to allowing all actions.
    ///   - toDebugEnvironment: Lets you construct custom environment for printing
    /// - Note: Don't use this at root reducer level because it's going to be slow due to all reflection / equality checks
    /// - Returns: Wrapped reducer
    public func debugDiffing(
        actionFormat: ActionFormat = .prettyPrint,
        allowedActions: ActionFilter<Action> = .all,
        environment toDebugEnvironment: @escaping (Environment) -> DebugDiffingEnvironment = { _ in
            DebugDiffingEnvironment()
        }
    ) -> Self {
        .init { state, action, env -> Effect<Action, Never> in
            let oldState = state
            let effects = self.run(&state, action, env)
            let newState = state

            guard allowedActions(action) else {
                return effects
            }
            let debugEnvironment = toDebugEnvironment(env)

            return .merge(
                .fireAndForget {
                    debugEnvironment.queue.async {
                        let actionOutput =
                            actionFormat == .prettyPrint
                                ? debugOutput(action).indent(by: 2)
                                : debugCaseOutput(action).indent(by: 2)

                        var stateOutput: String = "🖨️ (No state changes)"
                        if oldState != newState {
                            stateOutput = diff(oldState, newState, indentationType: .pipe, skipPrintingOnDiffCount: true, nameLabels: .comparing).joined(separator: ", ")
                        }

                        debugEnvironment.printer(
                            """
                            🎬 Received action:
                            \(actionOutput)
                            🖨️ State:
                            \(stateOutput)
                            """
                        )
                    }
                },
                effects
            )
        }
    }

    // This is internal code from TCA
    // swiftlint:disable all

    func debugCaseOutput(_ value: Any) -> String {
        func debugCaseOutputHelp(_ value: Any) -> String {
            let mirror = Mirror(reflecting: value)
            switch mirror.displayStyle {
            case .enum:
                guard let child = mirror.children.first else {
                    let childOutput = "\(value)"
                    return childOutput == "\(type(of: value))" ? "" : ".\(childOutput)"
                }
                let childOutput = debugCaseOutputHelp(child.value)
                return ".\(child.label ?? "")\(childOutput.isEmpty ? "" : "(\(childOutput))")"
            case .tuple:
                return mirror.children.map { label, value in
                    let childOutput = debugCaseOutputHelp(value)
                    return
                        "\(label.map { isUnlabeledArgument($0) ? "_:" : "\($0):" } ?? "")\(childOutput.isEmpty ? "" : " \(childOutput)")"
                }
                .joined(separator: ", ")
            default:
                return ""
            }
        }

        return "\(type(of: value))\(debugCaseOutputHelp(value))"
    }

    private func isUnlabeledArgument(_ label: String) -> Bool {
        label.contains(where: { $0 != "." && !$0.isNumber })
    }

    func debugOutput(_ value: Any, indent: Int = 0) -> String {
        var visitedItems: Set<ObjectIdentifier> = []

        func debugOutputHelp(_ value: Any, indent: Int = 0) -> String {
            let mirror = Mirror(reflecting: value)
            switch (value, mirror.displayStyle) {
            case let (value as CustomDebugOutputConvertible, _):
                return value.debugOutput.indent(by: indent)
            case (_, .collection?):
                return """
                [
                \(mirror.children.map { "\(debugOutput($0.value, indent: 2)),\n" }.joined())]
                """
                .indent(by: indent)

            case (_, .dictionary?):
                let pairs = mirror.children.map { _, value -> String in
                    let pair = value as! (key: AnyHashable, value: Any)
                    return
                        "\("\(debugOutputHelp(pair.key.base)): \(debugOutputHelp(pair.value)),".indent(by: 2))\n"
                }
                return """
                [
                \(pairs.sorted().joined())]
                """
                .indent(by: indent)

            case (_, .set?):
                return """
                Set([
                \(mirror.children.map { "\(debugOutputHelp($0.value, indent: 2)),\n" }.sorted().joined())])
                """
                .indent(by: indent)

            case (_, .optional?):
                return mirror.children.isEmpty
                    ? "nil".indent(by: indent)
                    : debugOutputHelp(mirror.children.first!.value, indent: indent)

            case (_, .enum?) where !mirror.children.isEmpty:
                let child = mirror.children.first!
                let childMirror = Mirror(reflecting: child.value)
                let elements =
                    childMirror.displayStyle != .tuple
                        ? debugOutputHelp(child.value, indent: 2)
                        : childMirror.children.map { child -> String in
                            let label = child.label!
                            return "\(label.hasPrefix(".") ? "" : "\(label): ")\(debugOutputHelp(child.value))"
                        }
                        .joined(separator: ",\n")
                        .indent(by: 2)
                return """
                \(mirror.subjectType).\(child.label!)(
                \(elements)
                )
                """
                .indent(by: indent)

            case (_, .enum?):
                return """
                \(mirror.subjectType).\(value)
                """
                .indent(by: indent)

            case (_, .struct?) where !mirror.children.isEmpty:
                let elements = mirror.children
                    .map { "\($0.label.map { "\($0): " } ?? "")\(debugOutputHelp($0.value))".indent(by: 2) }
                    .joined(separator: ",\n")
                return """
                \(mirror.subjectType)(
                \(elements)
                )
                """
                .indent(by: indent)

            case let (value as AnyObject, .class?)
                where !mirror.children.isEmpty && !visitedItems.contains(ObjectIdentifier(value)):
                visitedItems.insert(ObjectIdentifier(value))
                let elements = mirror.children
                    .map { "\($0.label.map { "\($0): " } ?? "")\(debugOutputHelp($0.value))".indent(by: 2) }
                    .joined(separator: ",\n")
                return """
                \(mirror.subjectType)(
                \(elements)
                )
                """
                .indent(by: indent)

            case let (value as AnyObject, .class?)
                where !mirror.children.isEmpty && visitedItems.contains(ObjectIdentifier(value)):
                return "\(mirror.subjectType)(↩︎)"

            case let (value as CustomStringConvertible, .class?):
                return value.description
                    .replacingOccurrences(
                        of: #"^<([^:]+): 0x[^>]+>$"#, with: "$1()", options: .regularExpression
                    )
                    .indent(by: indent)

            case let (value as CustomDebugStringConvertible, _):
                return value.debugDescription
                    .replacingOccurrences(
                        of: #"^<([^:]+): 0x[^>]+>$"#, with: "$1()", options: .regularExpression
                    )
                    .indent(by: indent)

            case let (value as CustomStringConvertible, _):
                return value.description
                    .indent(by: indent)

            case (_, .struct?), (_, .class?):
                return "\(mirror.subjectType)()"
                    .indent(by: indent)

            case (_, .tuple?) where mirror.children.isEmpty:
                return "()"
                    .indent(by: indent)

            case (_, .tuple?):
                let elements = mirror.children.map { child -> String in
                    let label = child.label!
                    return "\(label.hasPrefix(".") ? "" : "\(label): ")\(debugOutputHelp(child.value))"
                        .indent(by: 2)
                }
                return """
                (
                \(elements.joined(separator: ",\n"))
                )
                """
                .indent(by: indent)

            case (_, nil):
                return "\(value)"
                    .indent(by: indent)

            @unknown default:
                return "\(value)"
                    .indent(by: indent)
            }
        }

        return debugOutputHelp(value, indent: indent)
    }
}

extension String {
    func indent(by indent: Int) -> String {
        let indentation = String(repeating: " ", count: indent)
        return indentation + replacingOccurrences(of: "\n", with: "\n\(indentation)")
    }
}

public struct DebugDiffingEnvironment {
    public var printer: (String) -> Void
    public var queue: DispatchQueue

    public init(
        printer: @escaping (String) -> Void = { print($0) },
        queue: DispatchQueue
    ) {
        self.printer = printer
        self.queue = queue
    }

    public init(
        printer: @escaping (String) -> Void = { print($0) }
    ) {
        self.init(printer: printer, queue: _queue)
    }
}

private let _queue = DispatchQueue(
    label: "com.merowing.info.DebugDiffingEnvironment",
    qos: .background
)

Did you try to present this in pointfree repo? Maybe this idea could be incorporated into the project.

CustomDebugOutputConvertible was removed