recompose-focus.kt

package com.example.myapplication

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.foundation.lazy.LazyColumn
import androidx.compose.foundation.lazy.itemsIndexed
import androidx.compose.material.MaterialTheme
import androidx.compose.material.Surface
import androidx.compose.material.TextField
import androidx.compose.runtime.*
import androidx.compose.ui.Modifier
import androidx.compose.ui.composed
import androidx.compose.ui.draw.drawWithCache
import androidx.compose.ui.focus.onFocusChanged
import androidx.compose.ui.geometry.Offset
import androidx.compose.ui.geometry.Size
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.graphics.SolidColor
import androidx.compose.ui.graphics.drawscope.Fill
import androidx.compose.ui.graphics.drawscope.Stroke
import androidx.compose.ui.graphics.lerp
import androidx.compose.ui.platform.debugInspectorInfo
import androidx.compose.ui.unit.dp
import kotlinx.coroutines.delay
import kotlin.math.min

class MainActivity : ComponentActivity() {
  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContent {
      MaterialTheme {
        Surface(
          modifier = Modifier.fillMaxSize(),
          color = MaterialTheme.colors.background
        ) {
          Greeting()
        }
      }
    }
  }
}

@Composable
fun Greeting() {
  val state = remember {
    List(20) { it to "" }.toMutableStateMap()
  }

  LazyColumn(Modifier.fillMaxSize()) {
    itemsIndexed(state.values.toList()) { index, string ->
      TextField(
        value = string,
        onValueChange = { state[index] = it },
        modifier = Modifier
          .recomposeHighlighter()
          .onFocusChanged { }
      )
    }
  }
}

/**
 * A [Modifier] that draws a border around elements that are recomposing. The border increases in
 * size and interpolates from red to green as more recompositions occur before a timeout.
 */
@Stable
fun Modifier.recomposeHighlighter(): Modifier = this.then(recomposeModifier)

// Use a single instance + @Stable to ensure that recompositions can enable skipping optimizations
// Modifier.composed will still remember unique data per call site.
private val recomposeModifier =
  Modifier.composed(inspectorInfo = debugInspectorInfo { name = "recomposeHighlighter" }) {
    // The total number of compositions that have occurred. We're not using a State<> here be
    // able to read/write the value without invalidating (which would cause infinite
    // recomposition).
    val totalCompositions = remember { arrayOf(0L) }
    totalCompositions[0]++

    // The value of totalCompositions at the last timeout.
    val totalCompositionsAtLastTimeout = remember { mutableStateOf(0L) }

    // Start the timeout, and reset everytime there's a recomposition. (Using totalCompositions
    // as the key is really just to cause the timer to restart every composition).
    LaunchedEffect(totalCompositions[0]) {
      delay(3000)
      totalCompositionsAtLastTimeout.value = totalCompositions[0]
    }

    Modifier.drawWithCache {
      onDrawWithContent {
        // Draw actual content.
        drawContent()

        // Below is to draw the highlight, if necessary. A lot of the logic is copied from
        // Modifier.border
        val numCompositionsSinceTimeout =
          totalCompositions[0] - totalCompositionsAtLastTimeout.value

        val hasValidBorderParams = size.minDimension > 0f
        if (!hasValidBorderParams || numCompositionsSinceTimeout <= 0) {
          return@onDrawWithContent
        }

        val (color, strokeWidthPx) =
          when (numCompositionsSinceTimeout) {
            // We need at least one composition to draw, so draw the smallest border
            // color in blue.
            1L -> Color.Blue to 1f
            // 2 compositions is _probably_ okay.
            2L -> Color.Green to 2.dp.toPx()
            // 3 or more compositions before timeout may indicate an issue. lerp the
            // color from yellow to red, and continually increase the border size.
            else -> {
              lerp(
                Color.Yellow.copy(alpha = 0.8f),
                Color.Red.copy(alpha = 0.5f),
                min(1f, (numCompositionsSinceTimeout - 1).toFloat() / 100f)
              ) to numCompositionsSinceTimeout.toInt().dp.toPx()
            }
          }

        val halfStroke = strokeWidthPx / 2
        val topLeft = Offset(halfStroke, halfStroke)
        val borderSize = Size(size.width - strokeWidthPx, size.height - strokeWidthPx)

        val fillArea = (strokeWidthPx * 2) > size.minDimension
        val rectTopLeft = if (fillArea) Offset.Zero else topLeft
        val size = if (fillArea) size else borderSize
        val style = if (fillArea) Fill else Stroke(strokeWidthPx)

        drawRect(
          brush = SolidColor(color),
          topLeft = rectTopLeft,
          size = size,
          style = style
        )
      }
    }
  }