In Ext JS 4, we introduce a dedicated config property that gets processed by the powerful Ext.Class pre-processors before the class is created. Features include:
Configurations are completely encapsulated from other class members
Getter and setter, methods for every config property are automatically generated into the class' prototype during class creation if the class does not have these methods already defined.
An apply method is also generated for every config property. The auto-generated setter method calls the apply method internally before setting the value. Override the apply method for a config property if you need to run custom logic before setting the value. If apply does not return a value then the setter will not set the value. For an example see applyTitle below.