ai-app/app/androidApp/src/main/kotlin/com/example/aiapp/MainScreen.kt

package com.example.aiapp

import androidx.activity.compose.BackHandler
import androidx.compose.foundation.layout.Arrangement
import androidx.compose.foundation.layout.Column
import androidx.compose.foundation.layout.Row
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.foundation.layout.fillMaxWidth
import androidx.compose.foundation.layout.padding
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.PrimaryTabRow
import androidx.compose.material3.Tab
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable
import androidx.compose.runtime.LaunchedEffect
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableIntStateOf
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.setValue
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.unit.dp
import androidx.lifecycle.Lifecycle
import androidx.lifecycle.compose.LocalLifecycleOwner
import androidx.lifecycle.repeatOnLifecycle

/**
 * The app's root: one title, and four views of the backend behind it.
 *
 * These were four screens reached by four words in a row under the title, and the row was already
 * full -- the comment it replaced recorded that a fifth would have to go somewhere else. Tabs say
 * the same thing in less space and say one more thing besides: that these are places to be rather
 * than errands to run. Sessions, the machine's importable history, the models on it and the
 * machines themselves are all *the same backend*, looked at four ways, and none of them is a step
 * down from another. Settings still is a step down, which is why it stays a pushed screen and keeps
 * its own Back.
 */
private enum class MainTab(val label: String) {
    Sessions("Sessions"),
    Import("Import"),
    Models("Models"),
    Setups("Setups"),
}

@Composable
fun MainScreen(
    settings: ServerSettings,
    reloadToken: Int,
    onOpen: (SessionSummary) -> Unit,
    onSpawn: () -> Unit,
    onImported: (SessionSummary) -> Unit,
    onSettings: () -> Unit,
) {
    var tab by remember { mutableStateOf(MainTab.Sessions) }
    var refreshToken by remember { mutableIntStateOf(0) }

    // Coming back to the app asks again, on whichever tab is showing.
    //
    // What these four draw is a snapshot of a backend they are not connected to, so it is only as
    // fresh as the last answer -- and a *failed* answer is the one that outstays its welcome. A
    // phone that was away while the tunnel was down, or that fetched before the network came up,
    // came back to "Couldn't reach the server" sitting at the top of a list the server would now
    // answer for perfectly well, and nothing took it off until somebody pressed Refresh. A stale
    // failure is worse than a stale list: it is a claim about right now.
    //
    // Through the same token the Refresh button uses, so this is one instruction the tabs already
    // understand rather than a second path into each of them -- which is also what makes it cover
    // all four rather than the one the report came from.
    //
    // Not on the first entry: the tab composing already asks, and bumping here would make every
    // cold start fetch twice.
    val lifecycleOwner = LocalLifecycleOwner.current
    LaunchedEffect(lifecycleOwner) {
        var opening = true
        lifecycleOwner.repeatOnLifecycle(Lifecycle.State.STARTED) {
            if (!opening) refreshToken++
            opening = false
        }
    }

    // A tab the app put over the list has to step back to it rather than fall through to the
    // system default, which closes the app -- that reads as a crash to somebody who only meant to
    // get back to their sessions. Nested inside AppRoot's handler, so it wins while it is enabled.
    BackHandler(enabled = tab != MainTab.Sessions) { tab = MainTab.Sessions }

    Column(Modifier.fillMaxSize()) {
        Row(
            verticalAlignment = Alignment.CenterVertically,
            modifier = Modifier.fillMaxWidth().padding(start = 16.dp, end = 16.dp, top = 16.dp),
        ) {
            Text(
                "AI Sessions",
                style = MaterialTheme.typography.headlineSmall,
                modifier = Modifier.weight(1f),
            )
            // Glyphs rather than the words they replaced: neither ever changes, both are read
            // faster than they are spelled, and together they take the width that let the title
            // keep its own line. They sit on the title's row because they act on the whole
            // screen -- everything below this row is one tab's business, and a control belongs
            // with the thing it acts on.
            Row(horizontalArrangement = Arrangement.spacedBy(GLYPH_BUTTON_GAP)) {
                GlyphButton(REFRESH_GLYPH, "Refresh", { refreshToken++ })
                GlyphButton(SETTINGS_GLYPH, "Settings", onSettings)
            }
        }
        // Primary rather than the plain TabRow, which is deprecated in favour of the two that
        // say where they sit: these are the app's top-level destinations.
        PrimaryTabRow(selectedTabIndex = tab.ordinal) {
            MainTab.entries.forEach { entry ->
                Tab(
                    selected = tab == entry,
                    onClick = { tab = entry },
                    text = { Text(entry.label) },
                )
            }
        }

        // Refreshing means "ask again about what I am looking at", so the button feeds the tab
        // that is showing. The token from above means something else already changed what these
        // show; the two are the same instruction to the tab below, so they are summed rather than
        // tracked apart -- either one moving moves the sum, which is all a tab watches.
        val token = reloadToken + refreshToken
        when (tab) {
            MainTab.Sessions ->
                SessionListScreen(
                    settings = settings,
                    reloadToken = token,
                    onOpen = onOpen,
                    onSpawn = onSpawn,
                )
            MainTab.Import ->
                ImportScreen(settings = settings, reloadToken = token, onImported = onImported)
            MainTab.Models -> ModelsScreen(settings = settings, reloadToken = token)
            MainTab.Setups -> SetupsScreen(settings = settings, reloadToken = token)
        }
    }
}