import BackgroundTasks
import SwiftUI
import UIKit

@main
struct PSUScheduleApp: App {
    @State private var model = AppModel()
    @Environment(\.scenePhase) private var phase

    var body: some Scene {
        WindowGroup {
            RootView()
                .environment(model)
                // The theme is set on the window, not with preferredColorScheme: sheets that are
                // already open (the settings themselves) switch along with the screens behind them.
                .onChange(of: model.settings.theme, initial: true) { old, new in Theme.apply(new, animated: old != new) }
                .task { Background.schedule() }
        }
        .onChange(of: phase) { _, now in
            // Back in the app after a while: a quiet refresh, as the Android client does on start.
            if now == .active, model.settings.current != nil { Task { await model.refresh() } }
            if now == .background { Background.schedule() }
        }
        // Every few hours iOS wakes the app to refresh the timetable, widgets and reminders.
        .backgroundTask(.appRefresh(Background.identifier)) {
            await Background.refresh()
        }
    }
}

enum Theme {
    static func apply(_ mode: ThemeMode, animated: Bool) {
        let style: UIUserInterfaceStyle = switch mode {
        case .system: .unspecified
        case .light: .light
        case .dark: .dark
        }
        for scene in UIApplication.shared.connectedScenes.compactMap({ $0 as? UIWindowScene }) {
            for window in scene.windows where window.overrideUserInterfaceStyle != style {
                if animated {
                    UIView.transition(with: window, duration: 0.3, options: .transitionCrossDissolve) {
                        window.overrideUserInterfaceStyle = style
                    }
                } else {
                    window.overrideUserInterfaceStyle = style
                }
            }
        }
    }
}

enum Background {
    static let identifier = "org.xtimms.psu.schedule.refresh"

    static func schedule() {
        let request = BGAppRefreshTaskRequest(identifier: identifier)
        request.earliestBeginDate = Date().addingTimeInterval(6 * 3600)
        try? BGTaskScheduler.shared.submit(request)
    }

    @MainActor
    static func refresh() async {
        schedule()
        let model = AppModel()
        let known = Set(model.allChanges.map(\.at))
        await model.refresh()
        // A change found by this run, for the open group: tell the student without waiting for
        // them to open the app. The overview's card shows the same record.
        if model.settings.changeAlerts, let record = model.changes, !record.seen, !known.contains(record.at) {
            await ChangeAlerts.show(record)
        }
    }
}