Custom view in layout doesn't show while using "match_parent"

Question

I have a constraint layout that consists of two TextViews and a custom BoxBackgroundView. I want the box background to scale in size with the text, but when I use "match_parent" on the background and "wrap_content" on the constraint layout, the background doesn't show. I'm guessing that it just has a height of zero, but I don't know how to fix that.

Layout file:

<?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:custom="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:paddingTop="10dp"
    android:paddingBottom="10dp">

    <me.fishy.testapp.app.views.BoxBackgroundView
        android:id="@+id/boxBackgroundView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:color="@color/box_background_green"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/titleBox"
        android:layout_width="256dp"
        android:layout_height="wrap_content"
        android:text="Filler title"
        android:textAlignment="center"
        android:textColor="@color/black"
        android:textSize="40sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        custom:layout_constraintStart_toStartOf="parent"
        custom:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/textBox"
        android:layout_width="256dp"
        android:layout_height="wrap_content"
        android:gravity="center_horizontal"
        android:text="Filler text"
        android:textAlignment="center"
        android:textColor="@color/black"
        android:textSize="25sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        custom:layout_constraintStart_toStartOf="parent"
        custom:layout_constraintTop_toBottomOf="@+id/titleBox" />


</androidx.constraintlayout.widget.ConstraintLayout>

BoxBackgroundView:

package me.fishy.testapp.app.views;

import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.RectF;
import android.graphics.drawable.Drawable;
import android.text.TextPaint;
import android.util.AttributeSet;
import android.view.View;

import me.fishy.testapp.R;

public class BoxBackgroundView extends View {
    private int mExampleColor = 0xFF0000;
    private RectF rectangle;
    private Paint rectColor;



    public BoxBackgroundView(Context context) {
        super(context);
        init(null, 0);
    }

    public BoxBackgroundView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init(attrs, 0);
    }

    public BoxBackgroundView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        init(attrs, defStyle);
    }

    private void init(AttributeSet attrs, int defStyle) {
        // Load attributes
        final TypedArray a = getContext().obtainStyledAttributes(
                attrs, R.styleable.BoxBackgroundView, defStyle, 0);

        mExampleColor = a.getColor(
                R.styleable.BoxBackgroundView_color,
                mExampleColor);

        rectColor = new Paint();
        rectColor.setColor(mExampleColor);
        a.recycle();
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        if (rectangle == null){
            rectangle = new RectF(0, 0, getWidth(), getHeight());
        }
        canvas.drawRoundRect(rectangle, 50f, 50.f, rectColor);
    }

    public int getExampleColor() {
        return mExampleColor;
    }

    public void setExampleColor(int exampleColor) {
        mExampleColor = exampleColor;
    }

}