Custom View for Android (SDK 22) in Kotlin 1.0.2

attrs.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <declare-styleable name="Themes">
        <attr name="circleMeterStyle" format="reference"/>
    </declare-styleable>

    <declare-styleable name="CircleMeter">
        <attr name="size" format="dimension" />
        <attr name="centerCircle" format="float" />
    </declare-styleable>

</resources>

CircleMeter.kt

package xxx.view

import android.content.Context
import android.graphics.*
import android.util.AttributeSet
import android.view.View
import xxx.R

class CircleMeter(context: Context, attrs: AttributeSet?, defStyleAttr: Int) :
        View(context, attrs, defStyleAttr) {

    constructor(context: Context, attrs: AttributeSet?) : this(context, attrs, R.attr.circleMeterStyle)

    constructor(context: Context) : this(context, null)

    companion object {
        private val DEFAULT_SIZE = 64f
        private val DEFAULT_CENTER_CIRCLE = 0.2f
    }

    private val paint = Paint()
    private val paintForArc = Paint()
    private val paintForCenterCircle = Paint()

    var size: Float = DEFAULT_SIZE
    var centerCircle: Float = DEFAULT_CENTER_CIRCLE // 0.0f..1.0f
    var range: IntRange = 0..100
    var value: Int = 100

    init {
        context.obtainStyledAttributes(attrs, R.styleable.CircleMeter, defStyleAttr, 0).let {
            size = it.getDimension(R.styleable.CircleMeter_size, DEFAULT_SIZE)
            centerCircle = it.getFloat(R.styleable.CircleMeter_centerCircle, DEFAULT_CENTER_CIRCLE)
            it.recycle()
        }
        paintForArc.let {
            it.color = context.resources.getColor(R.color.colorAccent)
        }
        paintForCenterCircle.let {
            it.color = Color.TRANSPARENT
            it.xfermode = PorterDuffXfermode(PorterDuff.Mode.CLEAR)
        }
        setLayerType(LAYER_TYPE_SOFTWARE, null) // Not painted black.
    }

    private val degreeOfArc: Float
        get() = 360f * (value - range.first) / (range.last - range.first)

    override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
        val widthMode = MeasureSpec.getMode(widthMeasureSpec)
        val widthSize = MeasureSpec.getSize(widthMeasureSpec)
        val heightMode = MeasureSpec.getMode(heightMeasureSpec)
        val heightSize = MeasureSpec.getSize(heightMeasureSpec)
        val width = when (widthMode) {
            MeasureSpec.AT_MOST -> Math.min(widthSize, size.toInt())
            MeasureSpec.UNSPECIFIED -> size.toInt()
            MeasureSpec.EXACTLY -> widthSize
            else -> throw AssertionError()
        }
        val height = when (heightMode) {
            MeasureSpec.AT_MOST -> Math.min(heightSize, size.toInt())
            MeasureSpec.UNSPECIFIED -> size.toInt()
            MeasureSpec.EXACTLY -> heightSize
            else -> throw AssertionError()
        }
        val size = Math.min(width, height)
        setMeasuredDimension(size, size)
    }

    override fun onDraw(canvas: Canvas) {
        /* if LayerType == LAYER_TYPE_HARDWARE then
        val bitmap = Bitmap.createBitmap(canvas.width, canvas.height, Bitmap.Config.ARGB_8888)
        Canvas(bitmap).let {
            it.drawArc(0f, 0f, width.toFloat(), height.toFloat(), -90f, degreeOfArc, true, paintForArc)
            it.drawCircle(width.toFloat()/2, height.toFloat()/2, height.toFloat()/2*centerCircle, paintForCenterCircle)
            canvas.drawBitmap(bitmap, 0f, 0f, paint)
        }
        */
        canvas.drawArc(0f, 0f, width.toFloat(), height.toFloat(), -90f, degreeOfArc, true, paintForArc)
        canvas.drawCircle(width.toFloat()/2, height.toFloat()/2, height.toFloat()/2*centerCircle, paintForCenterCircle)
    }
}

layout.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <xxx.CircleMeter
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:size="24dp"
        app:centerCircle="0.2" />

</LinearLayout>