Placing a Custom View inside a ConstraintLayout

As you said, your view is not correctly laid into the layout because its dimensions are not specified. When android:layout_width and/or android:layout_height of a view are defined wrap_content, the view itself should calculate its dimensions. In this way, the container layout knows the child's width and height. However, you can do this by overriding onMeasure like the following to achieve this.

CustomView.kt

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


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

    private var viewWidth = 0

    var text: String = ""
        set(value) {
            field = value
            calculateSize()
            invalidate()
        }

    var textSize = 20f
        set(value) {
            field = value
            paint.textSize = value
            calculateSize()
            invalidate()
        }

    private val paint = Paint(Paint.ANTI_ALIAS_FLAG).also {
        // Paint styles used for rendering are initialized here. This
        // is a performance optimization, since onDraw() is called
        // for every screen refresh.
        it.style = Paint.Style.FILL
        it.textAlign = Paint.Align.CENTER
        it.textSize = textSize
        it.typeface = Typeface.create("", Typeface.BOLD)
        it.color = Color.GREEN
    }

    init {
        calculateSize()
        if (isInEditMode) {
            invalidate()
        }
    }

    override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
        val width = paddingLeft + viewWidth + paddingRight
        val height = paddingTop + textSize.toInt() + paddingBottom
        setMeasuredDimension(width, height)
    }

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

    private fun calculateSize() {
        val bounds = Rect()
        paint.getTextBounds(text, 0, text.length, bounds)
        viewWidth = bounds.width()
    }

    private fun drawText(canvas: Canvas) {
        val x = paddingLeft + viewWidth / 2f
        var y = paddingTop + textSize / 2f
        paint.run {
            y -= ((descent() + ascent()) / 2)
            canvas.drawText(text, x, y, this)
        }
    }
}

MainActivity.kt

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        customView.text = "Hello World!"
        customView.textSize = 80f
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <com.example.learningcustomviewimplementation.CustomView
        android:id="@+id/customView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="#AEAEAE"
        android:padding="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

Result:

According to this https://developer.android.com/training/custom-views/custom-drawing#layouteevent. You need to override onSizeChanged() or onMeasure() to define how big your view's parent wants your view to be. In your code you just override onDraw(), this function is paint in your view a rect not define your view size.