How do I rotate object based off other object's rotation?

Questions & Answers
1

I have some code to rotate a game object based of the mouse click and drag. However, I have two game objects on top of one another in one instance. When I rotate the bottom object I need the top object to rotate at the same rate as the bottom object.

Here is the code that I’ve modified to handle the mouse based rotation:

using UnityEngine;
using System.Collections;

public class SpinWithMouse2 : MonoBehaviour {
  	public Camera Cam;
  	private Vector3 currentLoc;
  	private float dotUp;
  	private float dotRight;
  
  	private Vector3 Right;
  	private Vector3 Up;
  
  	void Start () 
	{
    	//Set initial up and right directions for dot product calc
    	Right = transform.right;
    	Up = transform.up;
  	}
  
	public void OnPress () 
	{
	    Ray ray = Cam.ScreenPointToRay(UICamera.currentTouch.pos);
	    RaycastHit hit;
	      
	    if (Physics.Raycast(ray, out hit, Mathf.Infinity, 1 << 0)) 
		{
	      	currentLoc = transform.InverseTransformPoint(hit.point);
	      	currentLoc.z = 0;
	      	currentLoc = transform.TransformPoint(currentLoc);
	    
	      	//initialize dot products
	      	dotUp = Vector3.Dot(Up.normalized, (currentLoc - transform.position).normalized);
	      	dotRight = Vector3.Dot(Right.normalized, (currentLoc - transform.position).normalized);
	      
	    }
	}
  
	public void OnDrag () 
	{
    
    	Ray ray = Cam.ScreenPointToRay(UICamera.currentTouch.pos);
    	RaycastHit hit;
      
    	if (Physics.Raycast(ray, out hit, Mathf.Infinity, 1 << 0)) 
		{
    
      		Vector3 newLoc = transform.InverseTransformPoint(hit.point);
      		newLoc.z = 0;
      		newLoc = transform.TransformPoint(newLoc);
    
      		//Calculate the dot product between initial state and current mouse location
      		float newDotUp = Vector3.Dot(Up.normalized, (newLoc - transform.position).normalized);
      		float newDotRight = Vector3.Dot(Right.normalized, (newLoc - transform.position).normalized);
    
     		//calculate angle between previous mouse location and current mouse location
      		float ang = Vector3.Angle(currentLoc - transform.position, newLoc - transform.position);
    
      		//Determine which direction to rotate.  Based off of previous and current dot products.
      		if (newDotUp >= 0) 
			{
        		if (dotUp >= 0) 
				{
          			if (newDotRight > dotRight) 
					{
            			transform.Rotate(0, 0, -ang);
          			} else
            			transform.Rotate(0, 0, ang);
        		} else 
				{
          			if (newDotRight > dotRight) 
					{
            			transform.Rotate(0, 0, ang);
          			} else
            			transform.Rotate(0, 0, -ang);
        		}
      		} else 
			{
        		if (dotUp < 0) 
				{
          			if (newDotRight > dotRight) 
					{
            			transform.Rotate(0, 0, ang);
         			} else
            			transform.Rotate(0, 0, -ang);
        		} else 
				{
          			if (newDotRight > dotRight) 
					{
            			transform.Rotate(0, 0, -ang);
          			} else
            			transform.Rotate(0, 0, ang);
        		}
      		}
    
      		//Reset previous information
      		currentLoc = newLoc;
      		dotUp = newDotUp;
      		dotRight = newDotRight;
    	}
	}
}
2

Why not just parent the top to the bottom object? Or you can just set the rotation of the top to the rotation of the bottom.