MonoBehaviour.OnMouseDrag()

Description

OnMouseDrag is called when the user has clicked on a Collider and is still holding down the mouse.

OnMouseDrag is called every frame while the mouse is down.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    public Renderer rend;

    void Start()
    {
        rend = GetComponent<Renderer>();
    }

    void OnMouseDrag()
    {
        rend.material.color -= Color.white * Time.deltaTime;
    }
}

This function is not called on objects that belong to Ignore Raycast layer.

This function is called on Colliders marked as Trigger if and only if Physics.queriesHitTriggers is true.

OnMouseDrag can be a co-routine, simply use the yield statement in the function. This event is sent to all scripts attached to the Collider.