Today, as we continue our journey through our Guide to Software Design Patterns, we'll keep on the path of Creational design by taking a closer look at the factory method pattern. The factory method is similar to the simple factory pattern that we explored previously, except the factory method provides a simple way to further abstract the underlying class (and associated logic) from the client that is making use of our factory.
In this article we'll cover what the factory method is, using both real world examples and functional C# code, so let's get going!
In the Real World
Just as with the simple factory article we published previously, the factory method pattern hinges on that core concept of a factory. Like real world factories, our code should be able to make use of an intermediary factoryclass of some sort, which eases the rapid production of objects for the client (i.e. the developer making use of our classes). If setup correctly, the client can easily use of code without knowing (or caring) about how everything works behind the scenes; the factory should take care of the work for us.
To continue with my love of books, for our real world example of implementing the factory method we'll explore the relationships between authors and publishers. There are many different types of authors, including those that specialize in fiction and those that prefer nonfiction. And, just like authors, different publishers may opt to focus on publishing the works of authors that specialize in particular fields or writing styles.
For example, a newspaper is a type of publisher that primarily focuses on publishing nonfiction authors. Similarly, a blog that wants funny cat gifs and articles with titles like Top 10 Zaniest Things You Never Knew Were So Zany! is likely to publish stuff from fiction authors. The publication process may not only differ from one publisher to the other, but also the types of authors working for them will be dramatically different.
However, some things will remain the same. For example, a publisher will always perform a few basic tasks, such as hiring an appropriate author and publishing. Therefore, the baseline concept of a publisher acts as the factory method for this process, abstracting and separating the types of publishers from the types of authors.
How It Works In Code
Our code example continues with the publisher and author analogy. First, here's the code example in full, after which we'll break down the components to illustrate what is going on:
using Utility;namespace Airbrake.DesignPatterns.FactoryMethod
{
class Program
{
static void Main(string[] args)
{
// Create a Blog and publish.
var blog = new Blog();
blog.Publish();// Create a Newspaper and publish.
var newspaper = new Newspaper();
newspaper.Publish();
}
}interface IAuthor
{
void Write();
}class FictionAuthor : IAuthor
{
public void Write()
{
Logging.Log($"I'm an {this.ToString()} and I write fiction!");
}
}class NonfictionAuthor : IAuthor
{
public void Write()
{
Logging.Log($"I'm an {this.ToString()} and I write nonfiction!");
}
}abstract class Publisher
{
abstract public IAuthor HireAuthor();public void Publish()
{
IAuthor author = HireAuthor();
author.Write();
}
}class Blog : Publisher
{
public override IAuthor HireAuthor()
{
return new FictionAuthor();
}
}class Newspaper : Publisher
{
public override IAuthor HireAuthor()
{
return new NonfictionAuthor();
}
}
}using System;
using System.Diagnostics;namespace Utility
{
/// <summary>
/// Houses all logging methods for various debug outputs.
/// </summary>
public static class Logging
{
/// <summary>
/// Outputs to <see cref="System.Diagnostics.Debug.WriteLine"/> if DEBUG mode is enabled,
/// otherwise uses standard <see cref="Console.WriteLine"/>.
/// </summary>
/// <param name="value">Value to be output to log.</param>
public static void Log(object value)
{
#if DEBUG
Debug.WriteLine(value);
#else
Console.WriteLine(value);
#endif
}
}
}
The basic goal here of our factory method is to separate the logic of hiring an appropriate type of author from the particular type of publisher. Therefore, we start with a basic IAuthor interface, which contains the baseline of what our Author sub-classes must include: the method to Write(). From there, we create two unique types of Authors, the FictionAuthor and the NonfictionAuthor, both of which must include the Write() method:
interface IAuthor
{
void Write();
}class FictionAuthor : IAuthor
{
public void Write()
{
Logging.Log($"I'm an {this.ToString()} and I write fiction!");
}
}class NonfictionAuthor : IAuthor
{
public void Write()
{
Logging.Log($"I'm an {this.ToString()} and I write nonfiction!");
}
}
Now we create our Publisher class, which contains the core component of our factory method pattern, and is where the crossover between Author and Publisher takes place:
abstract class Publisher
{
abstract public IAuthor HireAuthor();
public void Publish()
{
IAuthor author = HireAuthor();
author.Write();
}
}
In this case, we don't want the client to be able to create an instance of a basic Publisher class, so we make it abstract. We then include a HireAuthor() method, which returns our IAuthor interface. Finally, the fundamental purpose of a publisher is to publish work, so we add the Publish() method, inside which we instantiate a generic IAuthor using the HireAuthor() method, before forcing our author to Write().
The last part of our structure is to create some non-abstract publications, in this case Blog and Newspaper. Since both inherit from Publisher, we must include (and override) the HireAuthor() method. These HireAuthor()methods each return an appropriate type of Author: the FictionAuthor and NonfictionAuthor, respectively:
class Blog : Publisher
{
public override IAuthor HireAuthor()
{
return new FictionAuthor();
}
}class Newspaper : Publisher
{
public override IAuthor HireAuthor()
{
return new NonfictionAuthor();
}
}
What's cool about this factory method pattern is that the implementation (and logic) of Blog.HireAuthor() and Newspaper.HireAuthor() can be different. However, since Publisher.Publish() simply accepts whatever IAuthoris provided, and issues a Write() method command, the client doesn't need to know what's going on behind the scenes here. For example, here's a bit of our example code that makes use of both types of Publishers:
static void Main(string[] args)
{
// Create a Blog and publish.
var blog = new Blog();
blog.Publish();// Create a Newspaper and publish.
var newspaper = new Newspaper();
newspaper.Publish();
}
The client is able to freely create a Blog and a Newspaper, issuing the Publish() command for both, without knowing how the factory method logic works in the background. The result, as expected, is that our output shows that appropriate types of IAuthors were generated for each, in accordance with the type of author each type of publication expects:
I'm an Airbrake.DesignPatterns.FactoryMethod.FictionAuthor and I write fiction!
I'm an Airbrake.DesignPatterns.FactoryMethod.NonfictionAuthor and I write nonfiction!
While this is just a simple example of the factory method in action, it hopefully illustrates the purpose and power of this pattern. Our Publisher class could easily be further expanded and improved, and so long as it still makes use of the IAuthor's ability to Write() somewhere, any inherited classes from Publisher can continue to be referenced as in our above example, without ever knowing how the authorship or writing process works behind the scenes.