The heart of the Plug-in Integrator Pattern is the configurator (the configuration file is shown in Figure 1), which contains the definition of the “Container” that needs to load components as a “plug-in component” with individual “initialization parameters” and a “message hookup” declaration of each plug-in components as shown in Figure 1.

Any component that needs to be plugged-in to the container should implement the plug-in interface exposed by the “Container”. Each plug-in component has its own implementation for “incoming message” and “outgoing message,”  but expose these messages with proper callback functions and message definitions so at runtime “message hookups” can be performed as described in the configurator. Moreover, the “incoming message” and “outgoing message” documentation should be done in the API level, which is a good programming practice anyway! After the configuration file is written, it depends on these public APIs to initialize and integrate the components.

Especially in Flex, making the container plug-in a generic interface like mx.Core.IUIComponent provides an open solution where any Flex container can be made to implement this pattern with ease to load and manage a hoard of different components at runtime.

Only one thing to note here is that the container has to be plug-in integration-enabled to make this pattern work and this can only be achieved when the integrator hookup is implemented to the container at compile time.

• Provides runtime integration and dependency injection to maintain loosely coupled software components.
• Provides an easy way to build new Flex applications by integrating already released tested components to cut down software development costs.
• Doesn't really tie down developers with several interfaces to be implemented to use the pattern. For a container going with mx.core.IUIComponent interface, there is absolutely no overhead when using this pattern.
• With this infrastructure it’s possible to build a fully functional enterprise system by integrating Flex components in the runtime by injecting dependencies.
• This pattern is an architectural framework where the individual components can be written based on other frameworks but can be used as a plug-in component afterward.
• This pattern exposes the components to have maximum flexibility and reusability and works as a true add-on to any project. It also tries to advocate good programming practices to follow general OOP patterns and event-driven programming.
• The real reward for using this pattern comes while programming huge enterprise projects where there are several different groups involved in implementing different UI functionalities; all their components can then be integrated using this pattern to build a robust solution.
• As SOA provides the basic infrastructure to businesses to tie down the business processes, this pattern provides a way to take individual UI components from disparate sources to tie them together and also maintain the UI workflow.
• It’s very easy to set up in Flex, but provides the ultimate platform to come up with intra-widget communications to maximize a user experience that is apt for any UI technology.

Key Concepts of the Plug-in Integrator
Here are the five key roles for a plug-in integrator:

The Plug-in Interface
The Plug-in Interface is the interface exposed by the container to populate its child list as plug-in components. This interface can be any application-specific interface that can be used by the container to load components as plug-ins. For simplicity it can be any mx.core.IUIComponent (the base Flex UI interface), so that any Flex-based UI component can be plugged in to the container. While going with a generic interface, it’s possible to plug in a lot of components but it becomes difficult to initialize and maintain them. To address this issue, it’s possible to expose some of the internal container variables as hookup points and list them in the configurator, enabling the plug-in components to be initialized through the integrator by function executions at runtime. 

<xs:complexType name="containerConfigurationType">
    <xs:sequence>
        <xs:element ref="keepStaticChildren" minOccurs="1"/>
<xs:element name="pluginComponent" type="pluginComponentType"
     maxOccurs="unbounded"/>
    </xs:sequence>
    <xs:attribute name="clazz" type="xs:string" use="required"/>
    <xs:attribute name="pluginInterface" type="xs:string" use="required"/>
</xs:complexType>

Complex type “containerConfigurationType” defines the basic structure to hook up plug-in integrator functionality to any container. “clazz” attribute is the unique identifier of the container and the “plugInInterface” refers to the plug-in interface being exposed by the container to plug-in components. “keepStaticChildren” tells the container to keep the already-added components or just to add components from the configurator definitions.

<xs:complexType name="pluginComponentType">
<xs:sequence>
    <xs:element name="property" type="propertyType" maxOccurs="unbounded"/>
            <xs:element ref="moduleURL"/>
            <xs:element ref="asClass"/>
            <xs:element name="initParam" type="initParamType" maxOccurs="unbounded"/>
            <xs:element name="msgMap" type="msgMapType" maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="id" type="xs:string" use="required"/>
<xs:attribute name="name" type="xs:string" use="required"/>
</xs:complexType>

The “pluginComponentType” complex type definition is self-explanatory. The two important attributes here are “moduleURL” and “asClass”. To load the component as an external module(runtime loading), moduleURL is used, but to load a component statically, asClass attribute is used. The moduleURL is given precedence over the asClass to enable the loading of new versions of components in the production environment.

1. Using the classical interface-based approach where at compile time the components are compiled with the interface being exposed by the container and the hookup to container is automatic
2. If the interface is generic enough, then the actual hookup between the container and the plug-in component may be injected in the runtime where the public variables or functions in the container can be hooked up to the plugged-in component’s variable or functions using the configurator definitions.

The first case is more obvious and more extensively used but the second way is more robust and integration friendly. The “initparamType” is used mostly to hook up a container to plug-in components using the second method. This hookup can be performed in two ways: as function hookup or a variable hookup. In a function hookup any public function in the plug-in component is hooked up and bound to a container variable, while in the variable hookup it’s a variable in the plug-in component that is bound to a container variable. To ActionScript programmers, data binding is very familiar, where as for Java or any other language, data binding means depending on the state change of the source field and the destination of data binding automatically gets a property change event to reflect the change.

Implementing the container on a very specific plug-in interface works but limits the plug-in components, which can be easily plugged in to the container, but a more generic plug-in interface enables a wide range of components to be plugged in and readily hooked up and bound in the runtime, enabling easy integration. 

<xsd:sequence>
    <xsd:element name="srcExtentionPoint" type="srcExtentionPointType"/>
    <xsd:element name="destHandler" type="destHandlerType"/>
</xsd:sequence>
<xsd:attribute name="id" type="xsd:string" use="required"/>
<xsd:attribute name="handler" use="required">
    <xsd:simpleType>
        <xsd:restriction base="xsd:NMTOKEN">
            <xsd:enumeration value="event"/>
            <xsd:enumeration value="function"/>
        </xsd:restriction>
    </xsd:simpleType>
</xsd:attribute>
</xsd:complexType>

“msgMapType” is a very important complex type that defines the mechanism to exchange messages between the individual plug-in components. This provides an easy way to integrate plug-in components to enhance and develop new functionality. Any message map element of type msgMapType has a “srcExtentionPointType” and a desination handler of type “destHandlerType”. With this approach it is possible to map a source message or callback function to a destination message or directly to any public function in the destination plug-in component. This provides a loosely coupled event-driven way to hook up plug-in components in the runtime to build functional applications.

The other important complex types defined are various “functionTypes”, “objectType” and “objectRefType”. All these are self-explanatory and can be found in Annexure-1.

Here is the list of core duties of the integrator:

1. Loads the configuration details from the configurator
2. Depending on the configuration, initializes the containers for plug-in integration
3. Loads plug-in components statically (as inline compiled class) or as a separate binary executable (as a Flex module).
4. Plugs in the loaded plug-in component to the container
5. Initializes the plug-in component according to configuration details.
6. Hooks up messaging with other deployed plug-ins.

The integrator can be implemented in any platform and in any language such as ActionScript, JavaScript, Java, and C#.

Now let’s consider a simple example where a Flex viewer/editor needs to be written to view/edit an object.