Unity: check if an object is visible for the main camera

example.cs

using UnityEngine;


public class Example : MonoBehaviour
{
    // Cache variables
    Camera cameraInstance;
    Transform currentTransform;
    
    // State
    bool wasVisible = false;
    
    private void Start()
    {
        this.cameraInstance = Camera.main;
        this.currentTransform = this.gameObject.GetComponent<Transform>();
    }
    
    private void Update()
    {
        bool isCurrentlyVisible = this.IsVisible();
        if(this.HasBecomeVisible(isCurrentlyVisible)) {
            Debug.Log("Now is visible");
        } else if (this.HasBecomeInvisible(isCurrentlyVisible)) {
            Debug.Log("Now is invisible");
        }
        
        this.wasVisible = isCurrentlyVisible;
    }
    
    // Idea from https://stackoverflow.com/a/46365015/2814721
    private bool IsVisible()
    {
        // We could use any Gameobject's transform here instead of this.currentTransform
        Vector3 position = this.cameraInstance.WorldToViewportPoint(this.currentTransform.position);
        return position.x > 0 && position.x < 1 && position.y > 0 && position.y < 1;
    }
    
    private bool HasBecomeVisible(bool isVisible)
    {
        return false == this.wasVisible && isVisible;
    }

    private bool HasBecomeInvisible(bool isVisible)
    {
        return true == this.wasVisible && false == isVisible;
    }
}