基本语法
Ext.define(className, members, onClassCreated); to create a class
// className: The class name
// members: is an object that represents a collection of class members in key-value pairs
// onClassCreated: is an optional function callback that is invoked when all dependencies of the defined class are ready and the class itself is fully created. Due to the asynchronous nature of class creation, this callback can be useful in many situations. These will be discussed further in [Section IV](https://docs.sencha.com/extjs/6.0.1/guides/core_concepts/classes.html#Errors_Handling__amp__Debugging)
Ext.create() to create a new instance of a class
Ext.getDisplayName() to get the display name of any method. This is especially useful for throwing errors that have the class name and method name in their description:
throw new Error('['+ Ext.getDisplayName(arguments.callee) +'] Some message here');
