I was going thorugh the posts from my old, now-defunct blog, seeing if there was anything I could bring over here â€” it is amazing how much can change in a year. There was an article I wrote about over use of cool techniques. In that article, I made mention to some new fangled technique called “Design Patterns”. At that point, I had no idea what they were and frankly couldn’t care.
Well, after being forced to look at them more closely for a uni assignment, I’m kind of a little hooked.
Design patterns are abstract solutions to common problems. Huh? Yeah, that is what I said. Many programmers strive towards code re-use. Design Patterns encourage thought re-use. Why re-invent the wheel? And because they are abstract (i.e. no code), they can be “ported” to different languages easily.
The Gang of Four introduced 23 of the things. They have put the challenge out for the discovery of more, and they haven’t been too successful as it is believed that almost all problems can be broken down to fit a composition of these rules.
To implement many of these design patterns correctly, you really need OOP features such as Abstraction and Interfacing. As I have pointed out many times, PHP4 doesn’t have these. PHP5 does. However! if you think about it, you can still use design patterns in PHP4 â€” you just need to be a little bit careful.
I won’t go through ever design pattern just now, but, I’ll outline one, and maybe add to them in the future. [By the way â€” an awesome book to use for getting your head around design patterns is "Head First Design Patterns" â€” look it up on Amazon]
The Stategy Pattern
Programmers often get into the habit of extending classes to change functionality â€” maybe because it is the easiest OOP function to understand. It can lead to problems though when you need to change implementations in a base class. I’ll use an example from a system I was writing the other day: the ubiquitous PHP email sender. The job I was doing required two types of email to be sent â€” a run-of-the-mill text email and a text email with an attachment.
Because I’m always trying to build my code libraries up, I decided to create set of classes that will do the job. Now, pre-design patterns, something like this may have happened:
Nothing wrong so far, but what happens if we want to add a HTML email? We could probably add another class below the attachments, because the HTML component is an attachment I guess, so we add the HTML file as one of the attachments at design time. This isn’t a great solution though, because one of the necessary bits to a HTML email is the HTML content, and you could theoretically remove it. How about we add another variable specifically for the HTML copy? That would also work, but what if the client decided that they wanted to select between HTML and text emails dynamically at run time? You would end up with something like this code wise:
(This is a bit of a contrived example, so run with me here)
now what happens if you want to add another type of email? You have to modify the case statement and test the whole thing all over again. Everyone hates testing – especially code that you know used to work that you had to change. Two keywords: CHANGE and TESTING. Minimise both and we as programmers are happy.
Now my solution was to create a class that has everything an email needs – To, From , Subject and a repository for headers (Plus a few other bits and pieces, which I won’t mention to reduce clutter). It also has a function called send(). Wow, ground breaking. Where the design gets a little strange, is there is another variable, called $emailType. This variable stores reference to a class of type EmailType (Well it pretends to, PHP4 doesn’t support interfaces). So, any class that implements EmailType can be stored in that variable. One of the abstract classes (again, let’s pretend it is abstract, PHP4 won’t understand) is the createMessage() function. This is where the magic occurs…
Each class that implements that interface know exaclty how it message needs to be contructed. The base email class doesn’t care – as long as it gets a string to tack on to the email it is happy. The creation of messages is de-coupled, meaning you can create a new email class without changing any exisiting code (As long as it implements the interface correctly).
This is an implementation of the strategy pattern!
Formal Description [Ganf of Four]: The strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangable. Strategy lets the algorithm vary independently from clients that use it.
Which is what we just did… More on design patterns later.