OnBecameVisible is called when the renderer became visible by any camera.

This message is sent to all scripts attached to the renderer. OnBecameVisible and OnBecameInvisible is useful to avoid computations that are only necessary when the object is visible.

	// Enables the behaviour when it is visible
	function OnBecameVisible () {
		enabled = true;
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void OnBecameVisible() {
        enabled = true;
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def OnBecameVisible() as void:
		enabled = true

OnBecameVisible can be a co-routine, simply use the yield statement in the function. When running in the editor, scene view cameras will also cause this function to be called.