XEO Concepts

This chapter introduces the basic concepts necessary to use the XEO framework an XEO's approach to development.

XEO's approach to development

XEO was designed from scratch with the purpose of modelling business (real-world) entities using an object oriented approach, as the team felt that that was the most natural way of developing a web-application. Being able to create an entity, its data model and web pages for listing all instances of that entity (with powerful options such as ordering and grouping), editing single instances and searching a particular instance, while still being able to use customized logic to implement behavior, without dealing with all the low-level complexities was truly the ultimate goal.

Developing starts by defining a set of entities (XEO Object Models) and their properties, using XEO Studio's Object Model editor (basically a graphic editor for a specific XML language). Those entities are processed by the framework in a processes known as "XEO Build" which generates the data model, and support classes to interact with the entities. Using the Object Model as a base, web viewers for listing, edit and search instances can be "scaffolded" (ala Ruby on Rails) by XEO Studio.

As seen above, using XEO is all about modelling business entities as XEO Object Models, which will be used by the framework to create the data model, logic and web layer for each of those entities, which can then be further customized to fit the business logic of the application. Let's begin with the basics of a XEO Object Model.

XEO Object Model

An Object Model is the XEO representation of a business entity. Almost every business entity can be described in a number of "atributes" and relations with other entities (be it relations of extension, 1-1 relation, or 1-N relation). For example, if we're modelling a entity that represents a "Book" in real life, that book would probably have attributes such as the book's title, the author's name, the edition number or category list (drama, romance, etc...) of the book.

Attributes

In a XEO Object Model, attributes modelling the properties of a business entity can be of different types. XEO provides attributes that represent Textual values ( AttributeText), Numeric Values ( AttributeNumber), Date and Time values ( AttributeDate and AttributeDateTime), Sequential Values ( AttributeSequence), Long textual values ( AttributeLongText) and Boolean values ( AttributeBoolean) as well as attributes to make a 1:1 relation (single relation - AttributeObject) and to make a 1:N relation (a relation to a collection of objects - AttributeObjectCollection)

Each of those types of attribute have specific properties which can be set to customize their behavior, which we'll see further in the documentation. These properties vary with the type of attribute, naturally. For example, a Numeric Value can have a property maxValue and minValue (which specify maximum and maximum values for the attribute) while a collection attribute can have the maximum number of objects to which the first can relate to (or declare the type of objects to which the first can relate to)

Custom Behavior (Logic)

One of the defining features of XEO is the possibility to associate business logic directly to the Object Models, and also with their attributes. By allowing business logic to be "embedded" in the Object Model, instances of a given model are now almost like "living entities" which respond to events and have customized actions all without complex code managing events handling and etc. Custom Business Logic is defined using the Java Language and interaction with the current instance object (or other objects) can be done by using XEO's Java API. Such logic can influence the viewer layer, because logic can determine whether a given object/attribute is visible, required, etc...

Let's take a look at the possibilities for defining business logic in an Object Model.

Attributes

Attributes themselves can have logic associated. For example, a given attribute may have a default value that is calculated, or its value may need be validated when its value is changed (and that validation, may required more or less complicated operations).

Attributes can have logic to define the following

Validation (code that will run when the parent instance object is saved, to check it the current value of the attribute is valid)
Default Value (code that will run when the parent instance object is first saved and the value of the attribute was not set, default value can be made to depend on the values of other attribute)
Requirement (code that determines whether or not the value of this attribute must be set before the parent object can be saved)
OnChange (code that will run whenever the value of this attribute is changed)
Disabled When (code that will determine when the attribute is disabled (i.e. its value cannot be changed))
Hidden When (code that will determine when the attribute is hidden - this logic is directly associated with the viewers layer
Formula (code that will run to determine the value of the attribute, allows to do calculations and such)

Events

Every instance of an Object Model can react (i.e. execute Java code) to certain actions in its life cycle. Each action generates two events (an onBeforeAction event and an onAfterAction event) in which you can write your code, the list of actions (and events) is as follows:

create ( onBeforeCreate and onAfterCreate)
load ( onBeforeLoad and onAfterCreate)
save ( onBeforeSave and onAfterSave)
destroy ( onBeforeDestroy and onAfterDestroy)

Each onBeforeEvent returns a boolean value. If the returned value is true, the action (load, create, save, destroy) associated with the event is carried on, otherwise the action is not executed (this allows, for example, to control when an object is to be deleted, if all required conditions for that to happen are met)

In the following figure is depicted the XEO Events mechanims and when they are triggered given a certain action performed on an instance object.

XEO Object Events

Attribute Events

Attributes may also react to events triggered by actions made upon the attributes and there are three groups of events available. The first group of events (regular events) is applicable to all attributes, except collection attribute (AttributeObjectCollection), the second group is only applicable to collection attributes (collection events) and the last group contains the events that are common to all attributes (common events). The List is as follows:

Regular Events:

OnBeforeGetValue (Triggered before a value is loaded into the attribute)
OnAfterGetValue (Triggered after the value is loaded into the attribute)

Collection Events:

OnBeforeAdd (Triggered before an object is added to the collection)
OnBeforeLoadBridge (Triggered before the collection is loaded)
OnBeforeChangeOrder (Triggered before an object in a collection is moved inside the collection, when it changes the order of the object)
OnBeforeRemove (Triggered before an object is removed from the collection)
OnAfterAdd (Triggered after an object is added to the collection)
OnAfterLoadBridge (Triggered after the collection is loaded)
OnAfterChangeOrder (Triggered after an object is moved inside the collection, when it changes the order of object)
OnAfterRemove (Triggered after an element is removed from the bridge)