Snap.cs

using UnityEngine;
using System.Collections;

public class Looky : MonoBehaviour 
{
	/**
	 * The target we will snap to.
	 */
	public Transform target;
	
	/**
	 * The child socket transforms
	 */
	private Transform[] sockets;

	/**
	 * Indicates which socket we are using.
	 * @type {Number}
	 */
	public int index = 0;


	void Start()
	{
		sockets = GetComponentsInChildren<Transform>();
	}
	
	/**
	 * Rotates the cube so that the chosen socket is orientated toward the target 
	 * transform.
	 */
	void Update () 
	{
		if (sockets[index] != null) 
		{
			// this is the local direction to the socket
			Quaternion rot = Quaternion.FromToRotation(Vector3.forward, sockets[index].transform.localPosition);

			// look at the forward direction and also rotate with the extra socket rotation
			Quaternion lookRotation = Quaternion.LookRotation(-target.forward, transform.up) * rot;

			// slerp the transform rotation
			transform.rotation = lookRotation;

			Vector3 offset = transform.position - sockets[index].transform.position;

			// position!
        	transform.position = target.position - (target.forward * offset.magnitude);
		}
	}

	void OnDrawGizmos()
	{
		if (sockets != null && sockets.Length > 0) {
			Gizmos.color = Color.green;
			Gizmos.DrawLine(transform.position, sockets[index].transform.position);
		}

		if (target != null) {
			Gizmos.DrawLine(target.position, target.position - target.forward * 2f);
		}
	}
}