Template for an Android custom view. The example shows a vertical dotted line.

android-kotlin-custom-view-template

/**
 * VerticalDottedLineView
 * Add to attrs.xml:
 *  <declare-styleable name="VerticalDottedLineView">
 *      <attr name="dots_color" format="color" />
 *  </declare-styleable>
 */
class VerticalDottedLineView : View {

    private var dotColor: Int = 0
    private var dotDiameterPx: Float = 0.0f
    private var dotMarginPx: Float = 0.0f
    private var numberOfDots: Int = 0
    private lateinit var dotPaint: Paint

    constructor(context: Context) : super(context) {
        init(context, null)
    }

    constructor(context: Context, attrs: AttributeSet) : super(context, attrs) {
        init(context, attrs)
    }

    constructor(context: Context, attrs: AttributeSet, defStyleAttr: Int) : super(context, attrs, defStyleAttr) {
        init(context, attrs)
    }

    private fun init(context: Context, attrs: AttributeSet?) {
        val attributes = context.obtainStyledAttributes(attrs, R.styleable.VerticalDottedLineView)
        dotColor = attributes.getColor(
            R.styleable.VerticalDottedLineView_dots_color,
            ContextCompat.getColor(context, R.color.black)
        )

        attributes.recycle()

        dotDiameterPx = DOT_DIAMETER_DP.dpToPx()
        dotMarginPx = DOT_MARGIN_DP.dpToPx()
        dotPaint = Paint().apply {
            color = dotColor
            style = Paint.Style.FILL
            isAntiAlias = true
        }
    }

    override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec)
    }
    
    override fun onSizeChanged(w: Int, h: Int, oldw: Int, oldh: Int) {
        super.onSizeChanged(w, h, oldw, oldh)

        numberOfDots = ((h - (dotMarginPx * 2.0f)) / (dotDiameterPx + dotMarginPx)).toInt()
    }

    override fun onDraw(canvas: Canvas) {
        super.onDraw(canvas)

        for (i in 0 until numberOfDots) {
            val dotX = width / 2.0f
            val dotY = (height / numberOfDots) * i + dotMarginPx
            canvas.drawCircle(dotX, dotY, dotDiameterPx / 2.0f, dotPaint)
        }
    }

    private fun Float.dpToPx(): Float =
        TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, this, resources.displayMetrics)

    companion object {
        private var DOT_DIAMETER_DP = 3.0f
        private var DOT_MARGIN_DP = 6.0f
    }
}