|By Peter Bell||
|November 9, 2006 01:00 PM EST||
One of the first things that you encounter when moving to object-oriented (OO) programming are beans. Beans are simple representations of a business object (like a user or a product) that hide all of the information stored in the bean behind methods (functions) for getting and setting the information (called, unsurprisingly, getters and setters).
Typically, if you want to display a product view screen, you'll get the product information from the database, load the resulting query recordset into a bean and then, instead of displaying the variables from the query, call the methods from the bean. In the past your product view may have looked like this:
$#Price#<br /><br />
With a bean, the product view will change to the following:
$#Product.get("Price")#<br /><br />
The only difference is that you are calling a "get" method that hides (encapsulates) the way the title and the price are retrieved, but this small difference can have a big impact on the maintainability of your code. (The sharp eyed among you will have noticed that I'm using a generic getter. The sidebar explains why this might be a good idea for you to consider).
Object-oriented programming is all about writing more maintainable code. Many Object Oriented patterns take longer to code initially than a traditional procedural approach, but they are easier to maintain. Given that we tend to spend much more time maintaining than developing applications, this is usually a worthwhile trade-off.
The benefit of encapsulating business object attributes behind getters and setters is that if the logic required to calculate (or save) the attribute changes, you only have to change the code in one place. Let's say that the product price is calculated based on a sale price. In the past you might have put that logic right into the product view:
$<cfif SalePrice>#SalePrice#<cfelse>#Price#</cfif><br /><br />
The problem is, there may also be a product list screen, a product search results screen, and the cart and checkout may also need to know the price of a product, so if you changed the way you calculated the price, you would have to remember to make that change in five places. Even worse, there is the risk that you might forget to update one of the screens, so products displayed on the search screen would display a different price than when added to the basket.
With getters and setters encapsulating the logic, there would be no change at all to the display screens. They would still be:
$#Product.get("Price")#<br /><br />
The only difference is that you would change the getPrice() custom method within the product object to be something like:
<cffunction name="getPrice" returntype="numeric" access="public"
output="no" hint="I return the price to display and charge for the product">
<cfset var Local = structNew()>
<cfset Local.ReturnValue = variables.get("SalePrice")>
<cfset Local.ReturnValue = variables.Price>
So, encapsulation of getters and setters using beans is a key element of Object Oriented programming, and it is a best practice that most OO developers would use most of the time. So, what happens when we have more than one user or product or article?
The Performance Problem
In Java, this would be easy. You would take your recordset, use it to create a collection of objects (one for each record), and you would still have all of the benefits of encapsulated getting and setting of properties. Unfortunately, in ColdFusion, object creation is expensive (in terms of performance). Creating a large number of objects as part of every page load can substantially affect the performance of a ColdFusion script.
Because of this, a lot of ColdFusion developers currently use recordsets for displaying their lists, losing all of the benefits of encapsulation. If they want to change the way the price is calculated for a product list, they either need to push back the price calculation onto the database; write a script in their service object to loop through the query implementing any custom data transformations; or replicate their business logic all over their list screens.
Introducing the Iterating Business Object
The iterating business object (IBO) is a simple solution to the problem of encapsulating access to getters and setters for recordsets without sacrificing performance. It is implemented as a base class that your business objects can extend. As well as providing generic getters and setters, it can also load itself with a query, and it provides a basic set of iterating methods (first(), next(), and isLast()) for looping through the recordset.
The IBO allows you to get all of the benefits of encapsulated getting and setting of attributes - whether you are working with a single object or a collection of them. It also allows you to use exactly the same code for both your single objects and object collections. For example, it really doesn't matter whether you are viewing a single product on a product detail page or a list of products matching a search query. If you want to display the price for the product, you want to use the same business logic to calculate it in both cases. With an IBO, you always load the same object up with the results from your query, and you can maintain all of your getter and setter calculations in that one object. In addition, you are only instantiating a single object per page view, so the performance hit is nominal. You can see the code for a simple IBO in Listing 1.
Using the Iterating Business Object
When you are using the IBO to display a single record, it is exactly the same as using a bean:
$#Product.get("Price")#<br /><br />
If you want to display a list of records, it is almost as easy. You take the same basic display code and just wrap it with a loop based on the iterating methods of the object:
<cfloop condition="NOT #Product.isLast()#">
$#Product.get("Price")#<br /><br />
Improving the Object
Previously I showed an example of a custom getter for calculating the price of a product. If there was a sale price, it displayed that, otherwise it just accessed the variables scope to get the price. Well, that works for a single record, but if there are multiple records (as with the IBO), the actual code to access a particular record would be more like:
<cfset Local.ReturnValue = variables.Data[variables.IteratorCount].Price>
Where the variables.IteratorCount is a pointer to the current record within the recordset being displayed. This isn't "wrong", but over time all of the custom methods would have to include this code, and if I ever decided to change the structure of the data storage within the variables scope, all of those methods would have to be rewritten.
To encapsulate that change, I added one more pair of generic methods to the base IBO: access() and mutate(). Accessors and mutators are alternate terms for getters and setters, but these methods are private and are only to be used by the customer getters and setters to access the underlying data structure, so if I ever wished to change the structure, I would only have to change those two methods. Below is simplified code for the access() method (the full code is available in Listing 1):
<cffunction name="access" returntype="any" access="private" output="no"
hint="I encapsulate access to the attribute values within this object,
accepting an attribute name and returning the appropriate attribute value.">
<cfargument name="AttributeName" type="string" required="yes"
hint="I am the name of the attribute to return the value for">
<cfset var Local = structNew()>
<cfset Local.ReturnValue = variables.Data[variables.IteratorCount][arguments.AttributeName] >
With this in place, we can now simplify the data access in the custom methods to just:
<cfset Local.ReturnValue = variables.access("Price")>
I have used the IBO on a number of production projects over the last couple of months. One project in particular required almost daily changes to the (very complex) object model as we continually added new attributes and changed the rules for calculating existing attributes. I found the application extremely easy to maintain and it is now in production, powering over 20 different sites and performing very well under load.
If you don't have any calculations for getting or setting any of your object attributes and really don't expect any in the future, any kind of encapsulation is overkill, and you would be better just to display using recordsets and cfoutput - for one record or for many. If you do have calculated fields, give the IBO a try and see how it works for your projects.
|Peter Bell 01/02/07 07:19:46 PM EST|
Good catch! That was indeed a little messy and is something I plan to clean up later this month. if you're interested, I'll be reposting code on my Blog. I'm probably going to use isDone(), but will take a little time to look at some other iterators to see what syntax is most popular.
|Michael Long 11/26/06 07:49:10 PM EST|
Could just be me, but I'd expect a function named "isLast" to return true if I'm actually on the last record. Which would in turn lead to an off-by-one error, as the loop would terminate early.
Better, perhaps, would be isEOF, or isDone, either of which would be true if the current record count is greater than the actual record count.
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