XWiki is not just your average wiki engine; it exposes a flexible data model that can be leveraged at the presentation level. Combined with its powerful presentation level scripting capabilities, XWiki's data model can help you build simple to complex Web applications with little or no need to access the XWiki core. In other words, you can build custom applications through the XWiki web interface without having to compile, package, and deploy software components.

This area explains XWiki's data model of Classes, Properties, and Objects for those users who want to build applications primarily at the presentation level. It will help you understand how you can achieve significant features by programming at this level alone.

For information about XWiki's underlying database schema (tables and fields), see: XWiki's Database Schema.

XWiki Classes, Objects, and Properties

If you are familiar with object oriented programming, you understand the concept of classes, properties, and objects. The presentation level data model that XWiki exposes is much the same.

Classes

An XWiki class is like a template that defines a unique type of object. It defines what properties an object can have. When you define a custom class, you are saying that your application may need to create one or many objects (instances) of that class. The class itself is like a cookie cutter and the objects are unique instances of that class. So, in this analogy, if the class is the cookie cutter, the objects are the cookies (and we are talking about real edible cookies in this analogy; not Web cookies).

A class is attached to a page. There can be at most one class per page. The class name is the name of the document it is attached to.

Properties

Properties are the characteristics that an object can have. In a class definition, the properties define the data fields that each unique instance of the class can have values for. Properties have displayers which control how the property value is displayed in view or edit mode. Some displayers use advanced pickers to help you input a valid property value.

XWiki currently supports the following kinds of properties (data types):

Field Type	Since Version	Description	Screenshots
String		Allows to store and display one line texts
TextArea		Allows to store and display large text fields (text and wysiwyg)
Number		Allows to store and display Numbers of type integer,long,float,double
Password		Allows to store password fields which can be encrypted or hashed
EMail	4.2M3	Allows to store email fields that can be obfuscated at display time
Boolean		Allows to store and display boolean values (yes/no or 1/0) which can be displayed as select or checkbox fields
Static List		Allows to store and display single-select or multi-select fields that can be displayed as select, checkbox, radio or suggests fields. The possible values of static list fields are configured in the field definition.
Database List		Allows to store and display single-select or multi-select fields that can be displayed as select, checkbox, radio or suggests fields. The possible values of database list fields are taken from the execution of a query on other XWiki data such as pages or objects. The query is typically expressed in the Hibernate Query Language (see XWiki Query Module) and it can use the Velocity script notation (as long as the XWiki class author has the required scripting permission) for including dynamic parameters such as the current page space, the current time, etc.
Database Tree List		Same as a Database List field, with the addition that the data can be displayed as a tree, using the Yahoo Javascript Tree library
Page	4.2M3	Same as a Database List field, but made to store XWiki page names.
Date		Allows to store and display date or datetime values. A date picker is used automatically for this field as shown below.
User List		Allows to store and display single-select or multi-select users. The field uses a user picker as shown below.
Group List		Allows to store and display single-select or multi-select groups. The field uses a user picker as shown below.
Computed Pseudofield	4.2M3	Allows to create a pseudo field for which the display can be configured using a custom displayer and the value retrieved by using a script. It allows to combine other fields together.
Time Zone	7.3M1	Allow to display and edit Time Zones.

Starting with version 4.3M2 you can define your own property types.

Objects

Objects are unique instances of a class with unique values defined for each of the properties that make up the class.

An object is attached to a specific page. Each page can have multiple objects.

Using objects, you insert structured information in the wiki, while in a traditional wiki you can only enter unstructured information (plain text). This is one of the reasons why XWiki is a second generation wiki.

In summary:

• Classes define an object type and the properties that are possible for an object of that type to have.
• Properties define the types of data that unique objects can have. The property types (datatypes) are defined for the class, but the actual values for the properties are defined for the objects when the objects are instantiated from the class.
• Objects are unique instances of a class with unique values for all of the properties that the class definition allows the objects to have.

Suggested Reading

• Armed with your knowledge of Classes and Objects you could try creating a small application, such as:
  • a FAQ application.
  • a Person Manager Application
• You could also start understanding how to display properties of Objects in wiki pages by using Scripting.
• You could also try using the App Within Minutes extension which simplifies how to get started developing an application with little required knowledge. After your application is generated, if you wish to go further and develop new features for your app, you'll need to use your knowledge about Classes, Objects and Scripting.
• More information on the Custom Display field and an example