How to use / alternatives to constructors with AddComponent?

Questions & Answers
1

I have a script that looks like this:

using UnityEngine;
using System.Collections;

//Moves object in a fixed direction and speed.
public class FixedVelMover : Mover {

	public float speed, direction; // direction is in degrees, starting from right, counterclockwise
	private Rigidbody2D rigidbody;

	// Use this for initialization
	void Start () {
		rigidbody = GetComponent<Rigidbody2D> ();
		float directionRad = direction * Mathf.Deg2Rad;
		rigidbody.velocity = new Vector2(Mathf.Cos(directionRad), Mathf.Sin(directionRad)) * speed;
	}

	public FixedVelMover (float speed, float direction) {
		this.speed = speed;
		this.direction = direction;
	}
}

But when I try to use the constructor with something like

FixedVelMover mover = new FixedVelMover (1f, 270);

I get an error message saying that I can’t create a MonoBehaviour using the ‘new’ keyword. It suggests AddComponent, but if I do that, I can’t choose the speed and direction when initializing, and setting the speed and direction at runtime does nothing.

2

Components (MonoBehaviours) do not let you have access to the constructors, you would need to approach the situation differently.

A MonoBehaviour is meant to be attached to a GameObject so you will need to have an object available to attach it to, a simple example code will be as follows:

    public void createMover()
    {
        // Create a new object
        GameObject obj = new GameObject();
        // Attach a new instance of your component to the object and store it in 'mover'
        FixedVelMover mover = obj.AddComponent<FixedVelMover>();
        // Call a method which can initialize the data values on the object
        mover.initialize(speed, direction);
    }

You would need to have an initialize method but It’s not that much harder to do this than it is to use a normal constructor. With your current setup however, you might run into an issue where your object runs the Start method before your Initialize method so it may behave unexpectedly.

I really do just suggest that instead of using a constructor behaviour, you could create a prefab and use the editor to adjust these values instead.