SImple view to add scale bar in map container layout

ScaleView.kt

import android.content.Context
import android.graphics.Canvas
import android.graphics.Color
import android.graphics.Paint
import android.graphics.Paint.Style.FILL
import android.graphics.Paint.Style.STROKE
import android.graphics.Path
import android.graphics.Rect
import android.util.AttributeSet
import android.view.View
import kotlin.math.abs
import kotlin.math.cos
import kotlin.math.pow

class ScaleView @JvmOverloads constructor(context: Context?,
                                          attrs: AttributeSet? = null,
                                          defStyleAttr: Int = 0
) : View(context, attrs, defStyleAttr) {

    private val density = resources.displayMetrics.density

    private val color = Color.parseColor("#333333")
    private val scaleData: ScaleData
    private val textSize: Float
    private val strokeWidth: Float
    private val textPaint = Paint()
    private val strokePaint = Paint()
    private val strokePath = Path()
    private val textHeight: Float
    private val linePosition: Float

    init {
        scaleData = ScaleData(density)
        textSize = 12 * density
        strokeWidth = 1.5f * density

        textPaint.isAntiAlias = true
        textPaint.color = color
        textPaint.style = FILL
        textPaint.textSize = textSize

        strokePaint.isAntiAlias = true
        strokePaint.color = color
        strokePaint.style = STROKE
        strokePaint.strokeWidth = strokeWidth

        val textRect = Rect()
        val possibleText = "1234567890 nmi"
        textPaint.getTextBounds(possibleText, 0, possibleText.length, textRect)
        textHeight = textRect.height().toFloat()
        linePosition = textHeight + textHeight / 2
    }

    fun update(zoom: Float = -1f, latitude: Double = -100.0) {
        scaleData.update(zoom, latitude)
        invalidate()
    }

    val maxHeight: Int
        get() = (textPaint.textSize * 3 + textPaint.strokeWidth).toInt()

    override fun onSizeChanged(w: Int, h: Int, oldw: Int, oldh: Int) {
        super.onSizeChanged(w, h, oldw, oldh)
        scaleData.maxWidth = w
        update()
    }

    override fun onDraw(canvas: Canvas) {
        if (scaleData.lineLength > 0) {
            canvas.drawText(scaleData.toString(), 0f, textHeight, textPaint)
            strokePath.rewind()
            strokePath.moveTo(0f, linePosition)
            strokePath.lineTo(scaleData.lineLength, linePosition)
            canvas.drawPath(strokePath, strokePaint)
        }
    }

    /**
     * All calculations are based on the http://wiki.openstreetmap.org/wiki/Slippy_map_tilenames#Resolution_and_Scale
     *
     * @field TILE_SIZE : 513592.62 feet or 156543.03 meters
     */
    internal data class ScaleData(private val density: Float) {
        internal var maxWidth = 0
        private var lastZoom = -1f
        private var lastLatitude = -100.0
        private var distance = 0f
        var lineLength: Float = 0f

        fun update(zoom: Float, latitude: Double) {
            lastZoom = zoom
            lastLatitude = latitude
            distance = 0f

            if (lastZoom < 0 || abs(lastLatitude) > 90) return

            val resolution =
                TILE_SIZE / density * cos(lastLatitude * Math.PI / 180) / 2.0.pow(lastZoom.toDouble())
            var dIndex = DISTANCES.size
            var screenDistance = maxWidth + 1.0
            while (screenDistance > maxWidth && dIndex > 0) {
                distance = DISTANCES[--dIndex]
                screenDistance = abs(distance / resolution)
            }

            lineLength = screenDistance.toFloat()
        }

        override fun toString() = if (distance < UNIT_SIZE) {
            "${distance.toInt()} m"
        } else {
            "${(distance / UNIT_SIZE).toInt()} nmi"
        }

        companion object {
            private const val TILE_SIZE = 156543.03 // 513592.62;
            private const val UNIT_SIZE = 1852f // 6076.12f;
            private val DISTANCES = floatArrayOf(
                1f, 2f, 5f, 10f, 20f, 50f, 100f, 200f, 500f, 1000f, 2000f,
                UNIT_SIZE, 2 * UNIT_SIZE, 5 * UNIT_SIZE, 10 * UNIT_SIZE, 20 * UNIT_SIZE, 50 * UNIT_SIZE,
                100 * UNIT_SIZE, 200 * UNIT_SIZE, 500 * UNIT_SIZE, 1000 * UNIT_SIZE, 2000 * UNIT_SIZE
            )
        }
    }
}