attendi/Packages/Features/Sources/Recording/RecordingViewModel.swift

import Observation
import SwiftUI

extension RecordingView {

    /// The observable model that drives ``RecordingView``.
    ///
    /// The model implements the recording flow as a ``State`` machine: it exposes the visibility and icon of the view's controls for the current state, counts the
    /// elapsed recording time, and processes the recorded input once it is sent. The audio capture and its transcription are delegated to the
    /// ``RecordingService`` and ``TranscribingService`` attached at initialization; when either of them fails, the model falls back to the not-recording state.
    @MainActor
    @Observable
    final class Model {

        // MARK: Properties

        /// The current state of the recording flow.
        var state: State = .notRecording

        /// The number of seconds spent recording, excluding any time spent paused.
        private(set) var elapsedSeconds: Int = 0
        
        /// The transcription of the last processed recording, or `nil` when none has been processed yet.
        private(set) var textTranscription: String?

        /// The service that captures the audio from a microphone.
        @ObservationIgnored
        private let recorder: any RecordingService

        /// The task that increments ``elapsedSeconds`` every second while recording.
        @ObservationIgnored
        private var taskTimer: Task<Void, Never>?

        /// The service that transcribes the recorded audio into text.
        @ObservationIgnored
        private let transcriber: any TranscribingService

        // MARK: Initializers

        /// Creates a model attached to the given recording and transcribing services.
        ///
        /// - Parameters:
        ///   - recorder: The service that captures the audio from a microphone. Defaults to ``SimulatedRecordingService``.
        ///   - transcriber: The service that transcribes the recorded audio into text. Defaults to ``SimulatedTranscribingService``.
        init(
            recorder: any RecordingService = SimulatedRecordingService(),
            transcriber: any TranscribingService = SimulatedTranscribingService()
        ) {
            self.recorder = recorder
            self.transcriber = transcriber
        }

        // MARK: Computed

        /// The image resource for the main button: a pause icon while recording, a record icon otherwise.
        var iconMain: ImageResource {
            switch state {
            case .notRecording, .paused, .processing: .Icon.record
            case .recording: .Icon.pause
            }
        }

        /// The image resource for the send button while paused, or `nil` when the button should show a progress indicator instead.
        var iconSend: ImageResource? {
            switch state {
            case .paused: .Icon.send
            default: nil
            }
        }

        /// Whether the recorded input is currently being processed.
        var isProcessing: Bool {
            state == .processing
        }

        /// Whether the main button should be disabled.
        var shouldDisableMain: Bool {
            state == .processing
        }

        /// Whether the send button should be visible.
        var shouldShowSend: Bool {
            state != .notRecording
                && state != .recording
        }

        /// Whether the timer label should be visible.
        var shouldShowTimer: Bool {
            state != .notRecording
        }

        /// The elapsed recording time, formatted as `mm:ss` for the timer label.
        var textTimer: String {
            Duration
                .seconds(elapsedSeconds)
                .formatted(.time(pattern: .minuteSecond(padMinuteToLength: 2)))
        }

        // MARK: Methods

        /// Handles a press of the main button.
        ///
        /// Starts a recording when idle, pauses an ongoing recording, or resumes
        /// a paused one. Does nothing while the input is being processed.
        func pressedMain() {
            guard state != .processing else {
                return
            }

            switch state {
            case .notRecording:
                state = .recording
            case .recording:
                state = .paused
            case .paused:
                state = .recording
            case .processing:
                break
            }
        }

        /// Handles a press of the send button.
        ///
        /// Moves a paused recording into processing. Does nothing in any other state.
        func pressedSend() {
            guard state == .paused else {
                return
            }

            switch state {
            case .paused:
                state = .processing
            default:
                break
            }
        }

        /// Reacts to a change of ``state``, expected to be called from the view whenever it observes one.
        ///
        /// Starts the timer and the audio capture when a recording begins or resumes, stops the timer in every other state — pausing the capture
        /// while paused — and kicks off ``processInput()`` when the input is sent.
        ///
        /// - Parameter shouldRestartTimer: Whether ``elapsedSeconds`` should be reset to zero before the timer starts, which is the case for
        /// a new recording as opposed to one resuming from a pause.
        func updatedState(
            shouldRestartTimer: Bool
        ) {
            switch state {
            case .recording:
                startTimer(shouldRestartTimer)
                startRecorder(shouldRestartTimer)
            case .paused:
                stopTimer()

                Task {
                    try? await recorder.pause()
                }
            case .processing:
                stopTimer()

                Task {
                    await processInput()
                }
            case .notRecording:
                stopTimer()
            }
        }

    }
}

// MARK: - Helpers

private extension RecordingView.Model {

    // MARK: Methods

    /// Processes the recorded input: it stops the audio capture, transcribes the captured audio, and publishes the result in ``textTranscription``;
    /// when finished — or when either service fails — it resets ``elapsedSeconds`` and returns the model to the not-recording state.
    func processInput() async {
        guard state == .processing else {
            return
        }

        do {
            let audio = try await recorder.stop()

            textTranscription = try await transcriber.transcribe(audio)
        } catch {
            textTranscription = nil
        }

        elapsedSeconds = 0
        state = .notRecording
    }

    /// Starts the audio capture through the attached ``RecordingService``, falling back to the not-recording state when the service fails.
    ///
    /// - Parameter isNewRecording: Whether a new recording should be started, as opposed to a paused one being resumed.
    func startRecorder(
        _ isNewRecording: Bool
    ) {
        Task {
            do {
                isNewRecording
                    ? try await recorder.start()
                    : try await recorder.resume()
            } catch {
                stopTimer()

                state = .notRecording
            }
        }
    }

    /// Starts the timer task, which increments ``elapsedSeconds`` once per second until it is cancelled. Any previously running timer task is cancelled first.
    ///
    /// - Parameter shouldRestartTimer: Whether ``elapsedSeconds`` and ``textTranscription`` should be cleared before the timer starts,
    /// which is the case for a new recording as opposed to one resuming from a pause.
    func startTimer(
        _ shouldRestartTimer: Bool
    ) {
        if shouldRestartTimer {
            elapsedSeconds = 0
            textTranscription = nil
        }

        taskTimer?.cancel()
        taskTimer = Task { [weak self] in
            while !Task.isCancelled {
                try? await Task.sleep(for: .seconds(1))

                guard let self, !Task.isCancelled else {
                    return
                }

                self.elapsedSeconds += 1
            }
        }
    }

    /// Stops the timer task, if any, keeping ``elapsedSeconds`` at its current value.
    func stopTimer() {
        taskTimer?.cancel()
        taskTimer = nil
    }

}

// MARK: - States

extension RecordingView.Model {
    /// The states of the recording flow.
    enum State {
        /// No recording is in progress.
        case notRecording
        /// A recording is in progress and the timer is running.
        case recording
        /// The recording is paused, and can be either resumed or sent.
        case paused
        /// The recorded input is being processed.
        case processing
    }
}