Sample of auto-sizable Text composable

AutoSizableText.kt

@Composable
fun AutoSizableText(
    text: String,
    modifier: Modifier = Modifier,
    minFontSize: TextUnit = 12.sp,
    maxFontSize: TextUnit = 112.sp,
    granularityInPx: Int = 1,
) {
    val autoSizer = rememberAutoSizer(minFontSize, maxFontSize, granularityInPx)
    val style = LocalTextStyle.current

    BoxWithConstraints(modifier = modifier) {
        val fontSize = remember(text, autoSizer, style, constraints) {
            autoSizer.autoSize(
                text = text,
                style = style,
                constraints = constraints,
                overflow = TextOverflow.Ellipsis,
            )
        }

        Text(
            text = text,
            fontSize = fontSize,
            overflow = TextOverflow.Ellipsis,
            style = style,
        )
    }
}

AutoSizer.kt

class AutoSizer(
    minFontSize: TextUnit,
    maxFontSize: TextUnit,
    granularityInPx: Int,
    density: Density,
    private val textMeasurer: TextMeasurer,
) {

    // 候補となるフォントサイズのリスト
    // 二分探索できるように順に並べる
    private val fontSizes: List<TextUnit> = generateSequence(minFontSize) { previous ->
        with(density) {
            (previous.toPx() + granularityInPx).toSp()
        }
    }
        .takeWhile { it < maxFontSize }
        .plus(maxFontSize)
        .toList()

    fun autoSize(
        text: String,
        style: TextStyle,
        constraints: Constraints,
        overflow: TextOverflow,
    ): TextUnit {
        // 二分探索で最適なフォントサイズを取得する
        val index = fontSizes.binarySearch { targetFontSize ->
            val result = textMeasurer.measure(
                text = text,
                style = style.copy(fontSize = targetFontSize),
                constraints = constraints,
                overflow = overflow,
            )

            // はみ出していれば正、そうでなければ負を返すと、
            // 「最適なフォントサイズの『inverted insertion point』」が得られる
            // see: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/binary-search.html
            if (result.hasVisualOverflow) 1 else -1
        }
        assert(index < 0)

        val insertionPoint = -(index + 1)
        // 最適なフォントサイズの挿入位置は、はみ出すフォントサイズのうち最小のものの位置に等しいので、
        // その一つ前を返す
        val resultIndex = (insertionPoint - 1).coerceAtLeast(0)
        return fontSizes[resultIndex]
    }
}

@Composable
fun rememberAutoSizer(
    minFontSize: TextUnit,
    maxFontSize: TextUnit,
    granularityInPx: Int,
): AutoSizer {
    val density = LocalDensity.current
    val textMeasurer = rememberTextMeasurer()

    return remember(minFontSize, maxFontSize, granularityInPx, density, textMeasurer) {
        AutoSizer(minFontSize, maxFontSize, granularityInPx, density, textMeasurer)
    }
}

SimpleAutoSizableText.kt

@Composable
fun SimpleAutoSizableText(
    text: String,
    modifier: Modifier = Modifier,
    minFontSize: TextUnit = 12.sp,
    maxFontSize: TextUnit = 112.sp,
    granularityInPx: Int = 1,
) {
    val density = LocalDensity.current
    var fontSize by remember(text, minFontSize, maxFontSize, granularityInPx, density) {
        mutableStateOf(maxFontSize)
    }

    Text(
        text = text,
        fontSize = fontSize,
        overflow = TextOverflow.Ellipsis,
        modifier = modifier,
        onTextLayout = { textLayoutResult ->
            if (textLayoutResult.hasVisualOverflow && fontSize > minFontSize) {
                fontSize = with(density) {
                    (fontSize.toPx() - granularityInPx).toSp()
                }
            }
        }
    )
}