Facade Design Pattern in R

1. Definition

The Facade Design Pattern provides a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use. In essence, it's like simplifying a complex library or system with a friendlier, easier-to-use interface.

2. Problem Statement

Imagine you're using a complex library with dozens of classes and methods to render a video. However, for your needs, you only require a simple "play" functionality. Navigating this library and figuring out the sequence of calls for such a simple task can be overkill.

3. Solution

Implement the Facade Pattern. Create a 'VideoPlayerFacade' class that has a simple 'play' method. Internally, this method would handle all the complex calls to the library, but for an external user, playing a video is just a one-method call.

4. Real-World Use Cases

1. Computer's BIOS serving as a facade between the OS and the hardware.

2. Customer support serving as a facade between a company's departments and the customers.

3. Modern game consoles, which hide the complexity of things like graphic rendering and sound processing behind a simpler interface.

5. Implementation Steps

1. Identify the complicated subsystem that needs simplification.

2. Define the methods you want to expose as part of the facade.

3. Implement the facade, making calls to the subsystem where needed.

6. Implementation in R Programming

# Complex subsystem classes
Renderer <- function() {
  list(
    prepare = function() {
      return("Preparing Renderer...")
    },
    renderFrame = function() {
      return("Rendering Frame...")
    }
  )
}
SoundSystem <- function() {
  list(
    loadSound = function() {
      return("Loading Sounds...")
    },
    playSound = function() {
      return("Playing Sound...")
    }
  )
}
# Facade
VideoPlayerFacade <- function() {
  renderer <- Renderer()
  sound <- SoundSystem()
  list(
    play = function() {
      output <- c()
      output <- c(output, renderer$prepare())
      output <- c(output, sound$loadSound())
      output <- c(output, renderer$renderFrame())
      output <- c(output, sound$playSound())
      return(output)
    }
  )
}
# Client code
player <- VideoPlayerFacade()
output <- player$play()

Output:

[1] "Preparing Renderer..." "Loading Sounds..."    "Rendering Frame..."   "Playing Sound..."

Explanation:

1. We started with a couple of hypothetical subsystem classes Renderer and SoundSystem. These are representative of a more complex system that deals with video playback.

2. The VideoPlayerFacade is our facade class. It has a simple play method which, when called, takes care of invoking the right methods in the correct order from our subsystem.

3. The client, which could be any part of your application that wants to play a video, now only interacts with the VideoPlayerFacade and doesn't need to know anything about the underlying complexity.

7. When to use?

Use the Facade Pattern when:

1. You want to provide a simple interface to a complex subsystem.

2. You want to decouple clients from the components of a complex subsystem, making it easier to upgrade or modify the subsystem independently.

3. When you want to layer your subsystems. Every subsystem can be wrapped in its own facade.

The Facade pattern simplifies interaction with complex systems and promotes loose coupling by keeping the subsystems separate from their clients.