|By Samuel Neff||
|April 3, 2003 12:00 AM EST||
Components bring to ColdFusion MX developers much of the power and practices of object-oriented programming. The component implementation in ColdFusion MX introduces us to many of the principles of OOP, including encapsulation and inheritance. One aspect of these features is the ability to create new components that inherit or extend the functionality of other components.
In order to effectively extend functionality, developers often require the ability to override existing component functions as well as still call the original functions to incorporate their programming logic. Since ColdFusion MX doesn't currently provide this ability, we'll explore a technique to work around this limitation that enables us to call a parent component's overridden method.
With component-based development, related code can easily be grouped together in a consolidated entity, giving us the ability to encapsulate data and functionality. This consolidation can be further expanded with the ability to inherit functionality from a parent component, also called extension. When one component extends another, it can both reuse the functionality of the original component and add its own functionality.
Often, the new functionality added to a component will replace its original functionality. This is an advantage to developers who utilize the component in their code since the parent and child components will each have a similar call interface, or function names. By maintaining a consistent interface, developers can write code that can use either the parent or child component interchangeably without modifying the calling code.
For an example of this, see Listing 1, which sets up two components, GreetingBase.cfc and GreetingChild.cfc. GreetingChild extends the functionality of GreetingBase, and both provide a single function, sayGreeting, which returns a greeting to the caller. The caller can use either component and call the same function to retrieve the result appropriate for the component being used, a feature known in OOP as polymorphism.
Calling Overridden Parent Component Functions
We run into a limitation when we try to utilize the parent component functionality for a function that we've overridden, in this case sayGreeting. We want to add a function sayAllGreetings, which returns the results of both the parent and the child components' sayGreeting functions. While you can create an instance of the parent component and call the new instance's sayGreeting method, this would not return accurate results in cases where the call depends on data in the component instance. Unfortunately, there is no mechanism in ColdFusion MX to call an overridden parent method and have it run in the context of the calling component instance.
Many other OOP languages, such as Java and even ActionScript, allow calling a parent class's methods through the keyword Super. While ColdFusion MX does not provide this ability, there is an often-overlooked feature that we can use to achieve the desired effect.
ColdFusion stores function references and variables equally. A function can be called by referring to the function name followed by parentheses, as you are accustomed. Additionally, the function reference can also be utilized by simply referring to the function name. The most common utilization of this feature is to copy a function to another scope, and here we'll use this ability to make a parent component's functions available to its children.
Any code that exists within the cfcomponent tags and outside of any cffunction tags is executed when the component is instantiated. This code is most often used to set up or initialize the component and is called the Constructor in OOP terms.
<!--- constructor code goes here --->
In Listing 2 we added some code to our original components. GreetingBase.cfc now has two lines of code in the constructor.
<cfset super = structNew()>
<cfset super.sayGreeting = sayGreeting>
What we've done here is set up a structure called "super", which we'll use as a holder for our parent component methods. After creating the structure we use cfset to copy a reference to the function to our structure. It's important to note here that when this code is run, the component will exist only as the parent component, so any functionality that is overridden in the child component has not yet been incorporated.
With this structure created and our parent functions safely stored, we can utilize them in the child, GreetingChild.cfc, as follows:
<cfset var greetings="Parent says '" &
"', Child says '" &
sayGreeting() & "'">
In this code we put together a string in which we call our parent component's sayGreeting function, through our super structure, and also call the current sayGreeting function as we normally would.
Super Structure Limitations
Through a little creative utilization of obscure features, we've been able to implement a very important feature common to OOP. However, our implementation is not comprehensive and does have limitations.
This technique can only be used to implement one level of parent-component calls. Since each component in the inheritance chain does not have its own data location where we can store the parent component's methods, we can only store the methods from a single parent component.
Additionally, we must recognize that this is a workaround to provide for functionality missing from ColdFusion MX. Since this feature is standard for OOP and commonly requested from developers, Macromedia will probably implement this functionality natively in a future version of ColdFusion.
When adding support for parent-component calls, Macromedia is likely to use the keyword "super" in their own implementation since this is used in both Java and ActionScript. With this change, our code will stop working since "super" will become a keyword. While this may seem like a problem, it is actually an advantage.
Since we no longer need our workaround once parent component calls are implemented in ColdFusion, we want to ensure that we can easily remove this workaround when it becomes obsolete. To accomplish this, wrap all of the super workaround code, the parent-component constructor, inside consistent comments as shown in Listing 3.
With these comments, we can utilize the Regular Expression replace in either ColdFusion Studio or Dreamweaver to remove all of the super structures from our code. As long as the keyword super is used for this functionality natively, the rest of our code will continue to work.
Component-based development brings to ColdFusion MX many of the features of OOP. By using a structure and function references, we were able to bring a common feature of OOP to ColdFusion, albeit with limitations. In recognizing these limitations, we can write our code in a forward-thinking manner and facilitate future changes to the code when they come in conflict with possible new functionality.
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