As more and more businesses (including Category 4) take up the use of the computer language PHP 5, then those of us programming in PHP have more need to consider object oriented programming patterns. Support for object oriented programming was weak in PHP 4 but is robust in PHP 5.

One issue that becomes important in any OOP language is how some of your code should depend on other parts of your code. That is, how do you manage your dependencies? Martin Fowler has a terrific article up about the design pattern known as “Dependency Injection”. Though it doesn’t use PHP in its examples, the discussion of the pattern is worth reading for anyone designing a new framework, or thinking about the weaknesses of current frameworks:

One of the entertaining things about the enterprise Java world is the huge amount of activity in building alternatives to the mainstream J2EE technologies, much of it happening in open source. A lot of this is a reaction to the heavyweight complexity in the mainstream J2EE world, but much of it is also exploring alternatives and coming up with creative ideas. A common issue to deal with is how to wire together different elements: how do you fit together this web controller architecture with that database interface backing when they were built by different teams with little knowledge of each other.A number of frameworks have taken a stab at this problem, and several are branching out to provide a general capability to assemble components from different layers. These are often referred to as lightweight containers, examples include PicoContainer, and Spring….

In this example I’m writing a component that provides a list of movies directed by a particular director. This stunningly useful function is implemented by a single method.

class MovieLister…
public Movie[] moviesDirectedBy(String arg) {
List allMovies = finder.findAll();
for (Iterator it = allMovies.iterator(); it.hasNext();) {
Movie movie = (Movie) it.next();
if (!movie.getDirector().equals(arg)) it.remove();
}
return (Movie[]) allMovies.toArray(new Movie[allMovies.size()]);
}

The implementation of this function is naive in the extreme, it asks a finder object (which we’ll get to in a moment) to return every film it knows about. Then it just hunts through this list to return those directed by a particular director. This particular piece of naivety I’m not going to fix, since it’s just the scaffolding for the real point of this article.

The real point of this article is this finder object, or particularly how we connect the lister object with a particular finder object. The reason why this is interesting is that I want my wonderful moviesDirectedBy method to be completely independent of how all the movies are being stored. So all the method does is refer to a finder, and all that finder does is know how to respond to the findAll method. I can bring this out by defining an interface for the finder.

public interface MovieFinder {
List findAll();
}

Now all of this is very well decoupled, but at some point I have to come up with a concrete class to actually come up with the movies. In this case I put the code for this in the constructor of my lister class.

class MovieLister…
private MovieFinder finder;
public MovieLister() {
finder = new ColonDelimitedMovieFinder(”movies1.txt”);
}

The name of the implementation class comes from the fact that I’m getting my list from a colon delimited text file. I’ll spare you the details, after all the point is just that there’s some implementation.

Now if I’m using this class for just myself, this is all fine and dandy. But what happens when my friends are overwhelmed by a desire for this wonderful functionality and would like a copy of my program? If they also store their movie listings in a colon delimited text file called “movies1.txt” then everything is wonderful. If they have a different name for their movies file, then it’s easy to put the name of the file in a properties file. But what if they have a completely different form of storing their movie listing: a SQL database, an XML file, a web service, or just another format of text file? In this case we need a different class to grab that data. Now because I’ve defined a MovieFinder interface, this won’t alter my moviesDirectedBy method. But I still need to have some way to get an instance of the right finder implementation into place.