When the value changes, the valueChange.evt event is automatically fired. The event has type="VALUE".
In the component, define a handler with name="change".
<aura:handler name="change" value="{!v.numItems}" action="{!c.itemsChange}"/>
The value attribute sets the component attribute that the change handler tracks.
The action attribute sets the client-side controller action to invoke when the attribute value changes.
A component can have multiple <aura:handler name="change"> tags to detect changes to different attributes.
In the controller, define the action for the handler.
({ itemsChange: function(cmp, evt) { console.log("numItems has changed"); console.log("old value: " + evt.getParam("oldValue")); console.log("current value: " + evt.getParam("value")); } })
The valueChange event gives you access to the previous value (oldValue) and the current value (value) in the handler action.
When a change occurs to a value that is represented by the change handler, the framework handles the firing of the event and rerendering of the component.