.. include:: ../_header.rst

Implementing behaviors with the Phaser events
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Many |UserComponents|_ you will create will implement certain behavior of an object. This behavior should be initialized and later updated at every step of the game loop.

Game engines like `Unity <https://unity.com>`_, that support a similar design pattern, `use a Start and Update methods <https://docs.unity3d.com/Manual/CreatingAndUsingScripts.html>`_ to implement the behavior of the component.

The Phaser_ framework does not provide a similar approach, components are not part of the Phaser_ API. However, Phaser_ provides a lot of events that you can handle and implement the gameplay. For example, you can simulate the Unity scripting using the |UserComponents|_ and the Phaser_ events. You can create an **EventComponent** class that handles the events:

.. code::

    class EventComponent {

        /**
        * @param {Phaser.GameObjects.GameObject} gameObject
        */
        constructor(gameObject) {        
            
            this.scene = gameObject.scene;        
            
            // first time the scene is updated, call the `start` method
            this.scene.events.once(Phaser.Scenes.Events.UPDATE, this.start, this);
            // each time the scene is updated, call the `update` method
            this.scene.events.on(Phaser.Scenes.Events.UPDATE, this.update, this);
            // if the object is destroyed, call the `destroy` method
            gameObject.on(Phaser.GameObjects.Events.DESTROY, this.destroy, this);        
        }

        start() {
            // to be overridden in derived classes
        }

        update() {
            // to be overridden in derived classes
        }

        destroy() {
            // the object is destroyed, so we remove all the event handlers
            this.scene.events.off(Phaser.Scenes.Events.UPDATE, this.start, this);
            this.scene.events.off(Phaser.Scenes.Events.UPDATE, this.update, this);
        }        
    }

Then, you can set the **EventComponent** class as super class to all your components:

.. image:: ../images/scene-editor-user-components-super-class-07302020.webp
    :alt: Setting a super class to components.

Now, you can override the **start**, **update**, and **destroy** methods in the component classes.

.. code::

    class HorizontalMove extends EventComponent {

        ...

        update() {

            this.gameObject.x += this.deltaX;
        }
    }

The ``EventComponent`` class is just an example of what you can do. It has a weak point: not all components need to listen to all the events. It is a waste of resources adding scene listeners in all the cases.

Since v3.13.0, the editor can "write" for you a base class that could be a better solution. However,  you can change its code and adapt it to your specific domain. Check the next section for more details.