Factory Method Design Pattern in R

1. Definition

The Factory Method Design Pattern is a creational pattern that provides an interface for creating objects, but allows subclasses to alter the type of objects that will be created. Instead of calling a constructor directly, a factory method is used to create the object.

2. Problem Statement

Consider you're developing a data visualization library in R. You want to create different types of plots, such as bar, pie, and scatter plots. How can you structure your library to easily allow the addition of more plot types in the future without modifying the existing code?

3. Solution

The Factory Method Pattern provides a solution. You can define an interface for creating an object, but let subclasses decide which class to instantiate. The Factory Method lets a class defer instantiation to subclasses.

4. Real-World Use Cases

1. GUI libraries where each OS provides a different implementation of a button or window.

2. Data visualization libraries, where different methods might be used to represent data.

3. Connecting to different types of databases depending on the environment or configuration.

5. Implementation Steps

1. Define a creator interface or abstract class with a factory method.

2. Implement concrete creators that override the factory method to produce concrete products.

3. The client uses the creator, but relies on the factory method to produce instances.

6. Implementation in R Programming

# Step 1: Define the abstract creator
PlotFactory <- function() {
  list(
    createPlot = function() {
      stop("Abstract method. Should be implemented by concrete factories.")
    }
  )
}
# Step 2: Implement concrete creators
BarPlotFactory <- function() {
  factory <- PlotFactory()
  factory$createPlot <- function() {
    return("Drawing a Bar Plot!")
  }
  return(factory)
}
PiePlotFactory <- function() {
  factory <- PlotFactory()
  factory$createPlot <- function() {
    return("Drawing a Pie Plot!")
  }
  return(factory)
}
# Step 3: Client usage
createAndDrawPlot <- function(factory) {
  plot <- factory$createPlot()
  return(plot)
}
barFactory <- BarPlotFactory()
result1 <- createAndDrawPlot(barFactory)
pieFactory <- PiePlotFactory()
result2 <- createAndDrawPlot(pieFactory)
result1
result2

Output:

[1] "Drawing a Bar Plot!"
[1] "Drawing a Pie Plot!"

Explanation:

The Factory Method Pattern has been implemented in the context of a data visualization library in R.

1. We started by defining an abstract creator PlotFactory which contains a factory method createPlot which is just a placeholder here.

2. We then created two concrete factories BarPlotFactory and PiePlotFactory, both of which provide their implementation of the createPlot method.

3. The client function createAndDrawPlot takes a factory as an argument and uses it to produce and draw a plot.

The results show that depending on which concrete factory we provide to the client, a different type of plot is created.

7. When to use?

Use the Factory Method Pattern when:

1. A class can't anticipate the class of objects it must create.

2. A class wants its subclasses to specify the objects it creates.

3. Classes delegate responsibility to one of several helper subclasses, and you want to localize the knowledge of which helper subclass is the delegate.

While the Factory Method is a powerful design pattern, it's important to be wary of overcomplicating the client code by introducing numerous factory methods.