zsmb.coHot off the keys



Pi Practice App in Compose

2021.04.18. 18h • Márton Braun

This article was originally planned for 🥧 Pi Day, and it clearly ran more than a month late. However, it’s finally complete!

Concept

This project is once again one of my older projects revived with Jetpack Compose. A couple years ago, I wrote an app that lets you practice and learn the digits of pi - always a fun thing to spend your commute on.

I’m rather rusty now and down to knowing about ~60 digits. How high can you score in the app? Tweet at me @zsmb13!

The app is still available in its original form on the Google Play Store for your pi practicing pleasure. Its sources are also available on GitHub.

The Pi app in the Play Store

If you take a look at the repository, you’ll find something rather interesting in its dependencies. It uses Anko Layouts to render some parts of its UI! This was an early, now deprecated and abandoned attempt by JetBrains to create declarative Android UI with Kotlin DSLs.

In this article, we’ll build the app in Compose, and also take a look at what some of the old Anko code looked like when we reach the relevant parts.

The completed app’s source code is available on GitHub, so you can browse it freely while reading the article.

Project setup, navigation

The app is rather simple, it consists of just two screens. The main screen contains the field displaying the current progress and the keypad to mash in the digits. The high scores screen displays the best runs you had at the digits of pi (a top 10 list of best attempts).

Navigation setup is simple in MainActivity:

setContent {
    PiPracticeComposeTheme {
        Surface(color = MaterialTheme.colors.background) {
            val navController = rememberNavController()
            NavHost(navController, startDestination = "home") {
                composable("home") {
                    Home(navController)
                }
                composable("highscores") {
                    HighScores(navController)
                }
            }
        }
    }
}

Main screen

The first building block of the main screen will be a composable that displays a button, which can notify us when it was clicked. We’ll design this with a bottom-up approach.

@Composable
private fun NumberButton(
    number: Int,
    onClick: (Char) -> Unit,
    modifier: Modifier = Modifier,
) {
    OutlinedButton(
        onClick = {
            @OptIn(ExperimentalStdlibApi::class)
            onClick(number.digitToChar())
        },
        modifier = modifier
            .aspectRatio(1f)
            .padding(4.dp),
        border = ButtonDefaults.outlinedBorder.copy(
            brush = SolidColor(MaterialTheme.colors.primary)
        )
    ) {
        Text(
            text = number.toString(),
            fontSize = 30.sp,
            color = MaterialTheme.colors.primary,
        )
    }
}

This NumberButton uses basic Compose concepts, and is based on a material OutlinedButton. It takes a modifier as an optional parameter, following the relevant section of the Jetpack Compose API Guidelines by Google (a must-read if you’re writing Compose code).

A NumberButton

Keypad

The next step up is assembling a grid of these on the screen, adding a progress indicator in the bottom row, and adding the current value of pi that’s been entered in a field on the top of the keypad:

@Composable
private fun KeyPad(
    input: String,
    scrollState: ScrollState, // 1
    onClick: (digit: Char) -> Unit, // 2
) {
    val progress = input.length - 2
    Column(
        modifier = Modifier
            .fillMaxSize()
            .padding(8.dp)
    ) {
        Text(
            text = input,
            fontSize = 32.sp,
            modifier = Modifier
                .verticalScroll(scrollState) // 1
                .weight(1f)
                .padding(4.dp),
        )
        Row(Modifier.fillMaxWidth()) {
            NumberButton(number = 1, onClick, Modifier.weight(1f))
            NumberButton(number = 2, onClick, Modifier.weight(1f))
            NumberButton(number = 3, onClick, Modifier.weight(1f))
        }
        Row(Modifier.fillMaxWidth()) {
            NumberButton(number = 4, onClick, Modifier.weight(1f))
            NumberButton(number = 5, onClick, Modifier.weight(1f))
            NumberButton(number = 6, onClick, Modifier.weight(1f))
        }
        Row(Modifier.fillMaxWidth()) {
            NumberButton(number = 7, onClick, Modifier.weight(1f))
            NumberButton(number = 8, onClick, Modifier.weight(1f))
            NumberButton(number = 9, onClick, Modifier.weight(1f))
        }
        Row(Modifier.fillMaxWidth(), verticalAlignment = Alignment.CenterVertically) {
            NumberButton(number = 0, onClick, Modifier.weight(1f))
            Text( // 3
                text = stringResource(id = R.string.current_progress, progress),
                modifier = Modifier.weight(2f),
                textAlign = TextAlign.Center,
                fontSize = 18.sp,
            )
        }
    }
}

This code is a bit wordy, but it mostly just creates all the rows of buttons we’ll need, and aligns them using weight modifiers. Let’s review some of the other interesting bits:

  1. We’ll make the top Text that displays the current value of pi scrollable, as we’d eventually run out of space while practicing. This is easily done by using the verticalScroll modifier. Note how the scroll state is hoisted to the containing function, which will let it control it externally.
  2. All number button clicks are passed to the same onClick event listener.
  3. We use the stringResource function to access Android string resources to make the few UI strings we have translatable.

This is what we built so far:

The completed keypad

Game logic

Now it’s time to add the main game logic.

@Composable
private fun PiPad() {
    val pi = stringResource(R.string.pi) // 1

    val scope = rememberCoroutineScope()

    // 2
    var input by remember { mutableStateOf("3.") }
    val scrollState = rememberScrollState()

    // 3
    var showDialog by remember { mutableStateOf(false) }
    var lastInput by remember { mutableStateOf('-') }

    fun reset() {
        input = "3."
        lastInput = '-'
        showDialog = false
    }

    KeyPad(
        input = input,
        scrollState = scrollState,
        onClick = { digit -> // 4
            val indexToTest = input.length
            val correctDigit = pi[indexToTest]

            lastInput = digit

            if (digit == correctDigit) {
                // 5
                input += digit.toString()
                scope.launch {
                    delay(100)
                    scrollState.animateScrollTo(scrollState.maxValue)
                }
            } else {
                // 6
                val decimalReached = input.length - 2
                ScoreKeeper.logScore(decimalReached)

                showDialog = true
            }
        }
    )

    if (showDialog) {
        val length = input.length

        EndOfGameDialog(onDismiss = { reset() }) {
            // 7
            Column {
                Text(stringResource(R.string.dialog_decimal_reached, length - 2))
                Text(stringResource(R.string.dialog_digit_info, lastInput, pi[length]))
                Text(stringResource(R.string.dialog_next_digits, pi.substring(length, length + 10)))
            }
        }
    }
}

This is quite a bit of code, but most of it is simple, and it handles all of the game logic for us!

  1. The true value of pi comes from string resources.
  2. The current value that the player has progressed to is always in the input variable. We also create the scroll state of the view that will display this value on the top of the screen here.
  3. When the game ends, we’ll show a dialog. This requires us to remember the last input digit, and to keep track of whether we need to show the dialog or not.
  4. We instantiate a KeyPad, and in its onClick listener, we implement all of the game logic.
  5. If the player inputs the correct digit, we’ll add it to the input, and make sure the view displaying it is scrolled to the bottom.
  6. If the player makes an error, we log the score they reached, and then display the end-of-game dialog.
  7. The dialog’s content is passed in with the content lambda convention.

ScoreKeeper is a relatively boring Kotlin object that reads/writes a file to keep track of the high scores. You can check out its implementation in the GitHub repository.

Displaying a dialog

The EndOfGameDialog function used above is a wrapper around AlertDialog, implemented like so:

@Composable
private fun EndOfGameDialog(
    onDismiss: () -> Unit,
    content: @Composable () -> Unit,
) {
    AlertDialog(
        onDismissRequest = onDismiss,
        title = {
            Text(stringResource(R.string.dialog_game_over_title))
        },
        text = content,
        confirmButton = {
            Button(onClick = onDismiss) {
                Text(stringResource(R.string.dialog_ok))
            }
        },
    )
}

The end-of-game dialog

This dialog was one of the parts implemented with Anko in the original app, which looked like this - quite familiar and easy to read!

alert {
    customView {
        verticalLayout {
            padding = dip(24)

            textView {
                text = getString(R.string.decimal_reached, indexReached)
                textSize = 16f
            }
            textView {
                text = getString(R.string.digit_info, digit, correctDigit)
                textSize = 16f
            }
            textView {
                val nextDigit = indexReached + 1
                text = getString(R.string.next_digits, digitsToShow,
                        pi.substring(nextDigit, nextDigit + digitsToShow))
                textSize = 16f
            }
        }

        positiveButton(getString(R.string.OK)) { resetState() }
        onCancelled { resetState() }
    }
}.show()

As the final touch for the main screen, we’ll add a toolbar on top to display the app name and provide navigation to the second screen.

@Composable
fun Home(navController: NavController) {
    Scaffold(topBar = {
        TopAppBar(
            title = { Text("Pi") },
            actions = {
                IconButton(onClick = { navController.navigate("highscores") }) {
                    Icon(
                        Icons.Default.Star,
                        stringResource(R.string.content_description_high_scores)
                    )
                }
            }
        )
    }) {
        PiPad()
    }
}

This now shows up on the top of our screen:

The main toolbar

High scores

The high scores screen is extremely simple, it will display just a toolbar and a list of values. Before we see the Compose implementation, here’s the original Anko code for this screen:

verticalLayout {
    padding = dip(16)
    verticalGravity = Gravity.CENTER

    tableLayout {
        tableRow {
            horizontalGravity = Gravity.CENTER
            cell("# ") // 2
            cell(getString(R.string.Score)) // 2
        }

        ScoreKeeper.scores.forEachIndexed { index, score -> // 1
            tableRow {
                horizontalGravity = Gravity.CENTER
                cell("${index + 1}.") // 2
                cell(score.toString()) // 2
            }
        }
    }
}
  1. The rows of the table are created with a simple forEachIndexed loop - a huge advantage and convenience of writing declarative layouts in Kotlin, having all the language features and constructs like the collection APIs available.
  2. Each row here was assembled by calls to the cell function, an extension I added on the receiver scope of the Anko tableRow function - the oddly named _TableRow type.
    private fun _TableRow.cell(text: String) {
        textView(text) {
            padding = dip(4)
            textSize = 20f
        }
    }
    

The Compose syntax for this screen will yet again be very similar to the Anko based approach. We’ll set up a toolbar much like on the main screen, and then add a header and some items underneath in a loop.

@Composable
fun HighScores(navController: NavController) {
    Scaffold(
        topBar = {
            TopAppBar(
                title = { Text(stringResource(R.string.high_scores_title)) },
                navigationIcon = {
                    IconButton(onClick = { navController.popBackStack() }) {
                        Icon(Icons.Default.ArrowBack, null)
                    }
                },
            )
        }
    ) {
        Column(
            modifier = Modifier
                .fillMaxSize()
                .padding(32.dp),
            horizontalAlignment = Alignment.CenterHorizontally,
        ) {
            Header(Modifier.width(120.dp))

            ScoreKeeper.scores.forEachIndexed { index, score ->
                ScoreItem(index + 1, score, Modifier.width(120.dp))
            }
        }
    }
}

The Header and the ScoreItem composables are extremely similar, they’ll just use slightly different text styling. Here’s just the latter of them:

@Composable
private fun ScoreItem(
    index: Int,
    score: Int,
    modifier: Modifier = Modifier,
) {
    Row(modifier) {
        Text(
            text = "${index}.",
            textAlign = TextAlign.Start,
            modifier = Modifier.weight(1f),
        )
        Text(
            text = score.toString(),
            textAlign = TextAlign.Center,
            modifier = Modifier.weight(2f),
        )
    }
}

Ta-da, now we can display the high scores in the app:

The High Scores screen

Wrap-up

That’s it for now! Thanks for following along with another Compose article. You can find the completed project on GitHub to play around with it and see the rest of the code that I omitted from the article.

If you liked this project, you’ll also like these other ones:



You might also like...

Prime Table Generator in Jetpack Compose

I've dusted off one of my oldest (and favourite) coding projects, and rewrote it in Jetpack Compose as a nice little practice exercise.

Wrap-up 2020

Let's send this one off so that we can get started on a better one. The "usual" recap of what I've done this year.

Announcing requireKTX

Introducing a new library to conveniently require values from common Android types.

Compose O'Clock

I started learning Jetpack Compose this week. Two days into that adventure, here's a quick look at how a neat clock design can be built up in Compose, step-by-step.