life-cycle-callbacks.md

Life cycle callback

Cocos Creator provides the life cycle callback function for component script. As long as the user defines a specific callback function, Creator will execute the script in a specific period, users do not need to manually call them.

Currently, the major life-cycle callback functions provided for users are:

• onLoad
• start
• update
• lateUpdate
• onDestroy
• onEnable
• onDisable

onLoad

In the initialization phase of the component script, we provide the onLoad callback function. onLoad callback will be triggered when the node is first activated, such as when the scene is loaded, or when the node is activated. The onLoad phase guarantees you can get other nodes from the scene and the resource data associated with the node. onLoad is always called before any start functions, this allows you to order initialization of scripts. Normally, we will do some operation associated with initialization in the onLoad phase. For example:

cc.Class({
  extends: cc.Component,

  properties: {
    bulletSprite: cc.SpriteFrame,
    gun: cc.Node,
  },

  onLoad: function () {
    this._bulletRect = this.bulletSprite.getRect();
    this.gun = cc.find('hand/weapon', this.node);
  },
});

start

The start callback function will be triggered before the first activation of the component, which is before executing update for the first time. start is usually used to initialize data that needs frequent modification, which may have changed during update.

cc.Class({
  extends: cc.Component,

  start: function () {
    this._timer = 0.0;
  },

  update: function (dt) {
    this._timer += dt;
    if ( this._timer >= 10.0 ) {
      console.log('I am done!');
      this.enabled = false;
    }
  },
});

update

One of the key points for game development is to update an object's behavior, status and orientation before rendering every frame. These update operations are normally put in the update callback.

cc.Class({
  extends: cc.Component,

  update: function (dt) {
    this.node.setPosition( 0.0, 40.0 * dt );
  }
});

lateUpdate

update will execute before all the animations' update, but if we're going to do some extra work after the effects (such as animation, particle, physics, etc.) are updated or want to perform other operations after update of all the components are done, then we'll need the lateUpdate callback.

cc.Class({
  extends: cc.Component,

  lateUpdate: function (dt) {
    this.node.rotation = 20;
  }
});

onEnable

When the enabled property of the component turns from false to true, or the active property of the node turns from false to true, it will trigger onEnable callback. If the node is created for the first time, and enabled is true, then it will be called after onLoad but before start.

onDisable

When the enabled property of the component turns from true to false, or the active property of the node turns from true to false, it will trigger onEnable callback, it will activate the onDisable callback.

onDestroy

When the component or node calls destroy(), it will call the onDestroy callback. Then they will be collected when this frame is done. When you declare both onLoad and onDestroy, they will always be called in pairs, which means that from component's initialization to destruction, they will either all be called or none will be called.

Tips

The execution order of lifecycle functions over a component's complete lifetime from initialization to activation and final destruction is: onLoad -> onEnable -> start -> update -> lateUpdate -> onDisable -> onDestroy.

Where onLoad and start are often used for a component's initialization and will only be executed once when the node become activeInHierarchy. In addition to the content mentioned above and the different execution order, there are the following differences:

	onLoad	start
When the node is activated	Calls immediately	Deferred Call
Only called when the component is enabled?	No	Yes

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Continue on to read about Creating and Destroying Nodes.