Handling Drag and Drop with Unity 3D

Drag and Drop is quite useful and we need to implement this quite often in our project. While working on one of my Unity Project, I required to implement the same.

There are some options but I found code described in this post quite simple to use with my project. In this post I will describe how I achieved the same.

So, Let's first create a C# script, I named it Drag.cs and added following code to it. This script will enable touch interaction to added GameObject and will move object along with touch. Also add BoxCollider to GameObject, so we can detect collision.

using UnityEngine;
using System.Collections;
 
public class Drag : MonoBehaviour {
    private Plane plane;
    private Vector3 v3Offset;


    void OnMouseDown() {
 plane.SetNormalAndPosition
             (Camera.main.transform.forward, transform.position);
 Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
 float dist;
 plane.Raycast (ray, out dist);
 v3Offset = transform.position - ray.GetPoint (dist);         
    }

    void OnMouseUp() {
    }

    void OnMouseDrag() {
 Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
 float dist;
 plane.Raycast (ray, out dist);
 Vector3 v3Pos = ray.GetPoint (dist);
 transform.position = v3Pos + v3Offset;  
    }
}

Now, suppose we have another GameObject and we want to detect when GameObject we just created with above script, is dropped on it. To detect drop easily let's first add RigidBody to this GameObject and BoxCollider. After adding BoxCollider also check isTrigger is true. After doing this we need to add C# script to it which will handle drop. Let's name it to Drop.cs and add below code to it.

Below script observe the Collision. When collision happens it checks which object is collided with it. If collided object is Drag object then do something.

using UnityEngine;
using System.Collections;

public class Drop : MonoBehaviour {


    void OnCollisionEnter2D (Collision2D other) {
    }
 
    void OnCollisionExit2D(Collision2D other){
    }
 
    void OnTriggerStay2D (Collider2D other) {
        if( other.gameObject.tag == "Drag" ) {
            // Object dropped, handle the drop
        } 
    }
} 

That's pretty much it, hope this helps.