References in the Inspector

Part of the Unity Scripting Basics section of Coddy's Unity journey. Lesson 29 of 67.

A field can hold another object. Give it a type such as Transform or GameObject, and the Inspector shows an empty slot that you fill by dragging an object from the Hierarchy onto it. This camera script follows whatever object was dragged into player, staying 3 units above it and 6 behind:

public Transform player;

void Update()
{
    transform.position = player.position + new Vector3(0, 3, -6);
}

A slot left empty shows None, and the first line that uses it throws, on every frame: UnassignedReferenceException: The variable player of CameraFollow has not been assigned. The next line of the message says what to do: You probably need to assign the player variable of the CameraFollow script in the inspector. It names the field and the script, so you know which slot to fill.

Some scripts should work with the slot empty too. Unity makes an empty slot compare equal to null, so such a script checks before using it. With if (player != null) around the line in Update(), a camera with no player stays where it is instead of throwing.

Challenge

Medium

Each guard walks to its own post. In the scene, Guard1's Inspector puts the Post1 object in a field named post. Guard2's post is not built yet, so its post slot is empty.

Give the Guard script a Transform field named post.
A guard walks to its post's position at speed units per second and stops on it, even if the post is moved later.
A guard whose post slot is empty stays where it is, without an error.

Press Play, select each guard in the Hierarchy to see its Post slot, then Check.

Try it yourself

using UnityEngine;

public class Guard : MonoBehaviour
{
    // post: the Transform this guard walks to, set in the Inspector
    public float speed = 2f;

    void Update()
    {
        // Walk to the post at speed. A guard with an empty post slot stays put.
    }
}