calendula/app/src/main/java/de/jeanlucmakiola/calendula/ui/RootScreen.kt

package de.jeanlucmakiola.calendula.ui

import androidx.compose.animation.AnimatedContent
import androidx.compose.animation.fadeIn
import androidx.compose.animation.fadeOut
import androidx.compose.animation.togetherWith
import androidx.compose.runtime.Composable
import androidx.compose.runtime.DisposableEffect
import androidx.compose.runtime.LaunchedEffect
import androidx.compose.runtime.SideEffect
import androidx.compose.runtime.mutableIntStateOf
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.setValue
import androidx.compose.runtime.getValue
import androidx.compose.ui.Modifier
import androidx.compose.ui.platform.LocalContext
import androidx.hilt.navigation.compose.hiltViewModel
import androidx.lifecycle.compose.LocalLifecycleOwner
import androidx.lifecycle.compose.collectAsStateWithLifecycle
import androidx.lifecycle.Lifecycle
import androidx.lifecycle.LifecycleEventObserver
import de.jeanlucmakiola.calendula.data.contacts.SpecialDatesScheduler
import de.jeanlucmakiola.calendula.data.contacts.hasContactsPermission
import de.jeanlucmakiola.calendula.ui.common.canReadCalendar
import de.jeanlucmakiola.calendula.ui.common.rememberCalendarFadeSpec
import de.jeanlucmakiola.calendula.ui.onboarding.OnboardingStep
import de.jeanlucmakiola.calendula.ui.onboarding.OnboardingSteps
import de.jeanlucmakiola.calendula.ui.onboarding.OnboardingViewModel
import de.jeanlucmakiola.calendula.ui.onboarding.onboardingSlideTransition
import de.jeanlucmakiola.calendula.ui.onboarding.rememberOnboardingSlideSpec
import de.jeanlucmakiola.floret.identity.rememberReduceMotion

/** What the root is showing: a wizard step, the app, or neither yet. */
private sealed interface RootTarget {
    /** DataStore has not emitted yet — render nothing rather than the wrong screen. */
    data object Loading : RootTarget
    data object App : RootTarget
    data class Step(val step: OnboardingStep) : RootTarget
}

@Composable
fun RootScreen(
    requests: ExternalRequests,
    modifier: Modifier = Modifier,
) {
    val context = LocalContext.current
    var hasPermission by remember {
        mutableStateOf(context.canReadCalendar())
    }

    // A launch scan already covers the app coming up with the permission, so
    // only a grant made during this session owes a re-scan.
    val grantedAtLaunch = remember { hasPermission }

    val lifecycle = LocalLifecycleOwner.current.lifecycle
    DisposableEffect(lifecycle) {
        val obs = LifecycleEventObserver { _, event ->
            if (event == Lifecycle.Event.ON_RESUME) {
                hasPermission = context.canReadCalendar()
                // Refresh the contact special-dates mirror on foreground (the
                // worker is debounced and no-ops when the feature is off). Gated
                // on the permission so users without the opt-in never enqueue it.
                if (context.hasContactsPermission()) {
                    SpecialDatesScheduler.runNow(context, foreground = true)
                }
            }
        }
        lifecycle.addObserver(obs)
        onDispose { lifecycle.removeObserver(obs) }
    }

    val onboarding: OnboardingViewModel = hiltViewModel()
    val plan by onboarding.plan.collectAsStateWithLifecycle()
    // The grant is reported however it was taken, including via Android's
    // app-settings screen (caught by the ON_RESUME above); arming the wizard
    // hangs off that, not off the in-app callback.
    LaunchedEffect(hasPermission) {
        onboarding.setHasPermission(hasPermission)
        if (hasPermission && !grantedAtLaunch) onboarding.onPermissionGranted()
    }

    // Arms the one-time explainer for the switch to the device's own calendar
    // visibility (#75), which the wizard then carries as a step of its own.
    LaunchedEffect(hasPermission) {
        if (hasPermission) onboarding.reconcileCalendarVisibility()
    }

    val current = plan
    val target: RootTarget = when {
        current == null -> RootTarget.Loading
        current.current == null -> RootTarget.App
        else -> RootTarget.Step(current.current)
    }

    // Which way the flow is moving, so a step-back slides back. The app sorts
    // after every step, so finishing the wizard reads as one more move forward.
    val ordinal = when (target) {
        RootTarget.Loading -> Int.MIN_VALUE
        RootTarget.App -> Int.MAX_VALUE
        is RootTarget.Step -> current?.steps?.indexOf(target.step) ?: 0
    }
    var lastOrdinal by remember { mutableIntStateOf(ordinal) }
    val slideDir = if (ordinal < lastOrdinal) -1 else 1
    SideEffect { lastOrdinal = ordinal }

    val slideSpec = rememberOnboardingSlideSpec()
    val fadeSpec = rememberCalendarFadeSpec()
    val reduceMotion = rememberReduceMotion()
    AnimatedContent(
        targetState = target,
        transitionSpec = {
            // Coming off the blank first frame is not a step change.
            if (initialState == RootTarget.Loading) {
                fadeIn(fadeSpec).togetherWith(fadeOut(fadeSpec))
            } else {
                onboardingSlideTransition(
                    slideDir = slideDir,
                    spec = slideSpec,
                    fadeSpec = fadeSpec,
                    reduceMotion = reduceMotion,
                )
            }
        },
        label = "onboardingGate",
    ) { shown ->
        when (shown) {
            RootTarget.Loading -> Unit
            RootTarget.App -> CalendarHost(
                requests = requests,
                modifier = modifier,
            )
            // Both halves of a transition read their position out of the live
            // plan; the one the outgoing step was drawn with is already gone.
            is RootTarget.Step -> current?.let { plan ->
                OnboardingSteps(
                    step = shown.step,
                    plan = plan,
                    viewModel = onboarding,
                    onPermissionGranted = { hasPermission = true },
                    modifier = modifier,
                )
            }
        }
    }
}