attendi/Apps/Attendi/View Models/ContentViewModel.swift

import Accessibility
import Foundation
import Observation
import OSLog
import Recording
import Speech

extension ContentView {

    /// The observable model that drives ``ContentView``.
    ///
    /// The model owns the recording and transcribing services attached to the recording feature, along with the ``locale`` the feature
    /// transcribes in, picked in the view's toolbar from the supported ``locales`` that ``load()`` fetches. It also holds the transcription
    /// currently presented in the view's modal sheet: ``received(_:)`` presents the transcription of a processed recording, and
    /// ``dismissed()`` clears it when the sheet closes.
    ///
    /// Every picked locale kicks off ``preinstall()``, a cancellable task that preinstalls the locale's speech model assets — cancelling
    /// any download still in flight for a previously picked locale — and reports the started, cancelled, and failed downloads through the
    /// transient, self-dismissing ``notifications``.
    @MainActor
    @Observable
    final class Model {

        // MARK: Properties

        /// The locale of the spoken language to transcribe.
        var locale: Locale
        
        /// The transcription currently presented in the modal sheet, or `nil` when none is shown.
        var transcription: Transcription?

        /// The locales the transcriber supports, sorted by their localized names, or empty while ``load()`` has not finished yet.
        private(set) var locales: [Locale]

        /// The in-app notifications currently presented, each dismissing itself a few seconds after it was posted.
        private(set) var notifications: [AppNotification] = []

        /// The service that captures the audio from the device's microphone.
        @ObservationIgnored
        let capturer: AudioCapturing

        /// The locale whose speech model assets are preinstalled or being preinstalled, or `nil` when none are.
        @ObservationIgnored
        private var localePreinstalling: Locale?

        /// The logger that records the failures of the speech model asset management.
        @ObservationIgnored
        private let logger = Logger(
            subsystem: Bundle.main.bundleIdentifier ?? "Attendi",
            category: "ContentView.Model"
        )

        /// The task that preinstalls the speech model assets of the last picked locale, or `nil` until the first locale is picked.
        @ObservationIgnored
        private var taskPreinstall: Task<Void, Never>?

        /// The service that transcribes the recorded audio into text on device.
        @ObservationIgnored
        let transcriber: AudioTranscribing
        
        // MARK: Initializers

        /// Creates a model set to the user's current locale, with no supported locales loaded yet, attached to the on-device recording
        /// and transcribing services.
        init() {
            self.locale = .current
            self.locales = []
            self.capturer = .init()
            self.transcriber = .init()
        }

        // MARK: Methods

        /// Loads the locales the transcriber supports into ``locales``, sorted by their localized names, and aligns ``locale`` with the
        /// supported equivalent of its current value — falling back to the supported equivalent of a default locale when none exists —
        /// so the locale picker starts with a valid selection.
        func load() async {
            locales = await SpeechTranscriber.supportedLocales
                .sorted {
                    name(for: $0).localizedStandardCompare(name(for: $1)) == .orderedAscending
                }

            locale = if let equivalent = await localeInTranscriber(equivalentTo: locale) {
                equivalent
            } else if let fallback = await localeInTranscriber(equivalentTo: .byDefault) {
                fallback
            } else {
                .current
            }

            preinstall()
        }

        /// Preinstalls the speech model assets for the current ``locale`` in a cancellable task, cancelling the preinstallation of any
        /// previously picked locale still in flight — which posts its cancellation to ``notifications``. Does nothing when the current
        /// locale is the one already preinstalled or being preinstalled.
        func preinstall() {
            guard localePreinstalling != locale else {
                return
            }

            taskPreinstall?.cancel()

            localePreinstalling = locale

            taskPreinstall = Task { [locale] in
                await preinstallAssets(for: locale)
            }
        }

        /// Returns the localized name of a locale for the locale picker.
        ///
        /// - Parameter locale: The locale to name.
        /// - Returns: The name of the locale in the user's current locale, or its identifier when no name is available.
        func name(
            for locale: Locale
        ) -> String {
            Locale
                .current
                .localizedString(
                    forIdentifier: locale.identifier
                )?.localizedCapitalized
            ?? locale.identifier
        }

        /// Handles the transcription of a processed recording, presenting it in the modal sheet.
        ///
        /// - Parameter transcription: The transcription of the processed recording.
        func received(
            _ transcription: Transcription
        ) {
            self.transcription = transcription
        }

        /// Handles the dismissal of the modal sheet.
        func dismissed() {
            transcription = nil
        }

    }

}

// MARK: - Helpers

private extension ContentView.Model {

    // MARK: Methods

    func localeInTranscriber(
        equivalentTo locale: Locale
    ) async -> Locale? {
        await SpeechTranscriber.supportedLocale(equivalentTo: locale)
    }

    /// Posts a notification for a download event of the given locale, announcing it to assistive technologies and scheduling its
    /// automatic dismissal a few seconds later — the only way a notification is dismissed.
    ///
    /// - Parameters:
    ///   - event: The download event to notify.
    ///   - locale: The locale whose speech model the event is about.
    func post(
        _ event: AppNotification.Event,
        for locale: Locale
    ) {
        let notification = AppNotification(
            event: event,
            localeName: name(for: locale)
        )

        notifications.append(notification)

        AccessibilityNotification.Announcement(
            String(localized: notification.textMessage)
        ).post()

        Task {
            try? await Task.sleep(for: Constant.Delay.dismissal)

            notifications.removeAll { $0.id == notification.id }
        }
    }

    /// Preinstalls the speech model assets for the supported equivalent of the given locale, so the first transcription in that locale
    /// does not have to download them mid-processing — posting a notification when a download starts, is cancelled, or fails; a
    /// download that finishes, or assets that are already installed, post none.
    ///
    /// The locale reservations of any other locales are released beforehand: the app transcribes a single locale at a time, and
    /// the system only permits a limited number of reservations — exceeding it would make the installation request throw.
    /// Failures are logged but not surfaced as errors: the transcribing service installs any missing assets itself as a fallback when
    /// a transcription starts.
    ///
    /// - Parameter target: The picked locale to preinstall the speech model assets for.
    func preinstallAssets(
        for target: Locale
    ) async {
        guard let locale = await localeInTranscriber(equivalentTo: target) else {
            return
        }

        for reserved in await AssetInventory.reservedLocales where reserved != locale {
            _ = await AssetInventory.release(reservedLocale: reserved)
        }

        let transcriber = SpeechTranscriber(
            locale: locale,
            preset: .transcription
        )

        do {
            guard let request = try await AssetInventory.assetInstallationRequest(
                supporting: [transcriber]
            ) else {
                return
            }

            post(.started, for: locale)

            try await withTaskCancellationHandler {
                try await request.downloadAndInstall()
            } onCancel: { [progress = request.progress] in
                progress.cancel()
            }
        } catch {
            if localePreinstalling == target {
                localePreinstalling = nil
            }

            if error is CancellationError || Task.isCancelled {
                post(.cancelled, for: locale)
            } else {
                post(.failed, for: locale)

                logger.error("The speech model assets for the \"\(locale.identifier, privacy: .public)\" locale failed to preinstall: \(String(describing: error), privacy: .public)")
            }
        }
    }

}

// MARK: - Constants

/// The constant values used across the model.
private enum Constant {
    /// The delay constants.
    enum Delay {
        /// The time a posted notification stays visible before its automatic dismissal.
        static let dismissal: Duration = .seconds(4)
    }
}

private extension Locale {
    /// The locale to fall back to when the transcriber supports no equivalent of the user's current locale.
    static let byDefault: Locale = .init(identifier: "en_US")
}