Appearance Mode (Facade Mode)

Definition

Here's a reference from Chapter 7 of Headfirst Design Patterns

Appearance patterns provide a unified interface for accessing a group of interfaces in a subsystem. Appearance defines a high-level interface that makes subsystems easier to use.

Advantages and disadvantages

The following references are from Reference 1

The Facade pattern is a typical application of the Dimiter's Law and has the following main advantages.

1. Decreases the coupling between the subsystem and the client so that changes in the subsystem do not affect the client class calling it.
2. Subsystem components are shielded from customers, reducing the number of objects processed by customers, and making subsystems easier to use.
3. It reduces compilation dependency in large software systems and simplifies the migration process between different platforms, since compiling a subsystem will not affect other subsystems or the appearance objects.

The main drawbacks of the Facade pattern are as follows.

1. Customers are not well restricted from using subsystem classes, and unknown risks can easily arise.
2. Adding a new subsystem may require modifying the appearance class or client source code, which violates the Open and Close Principle.

Use scenarios

The following references are from Reference 1

Appearance patterns are often considered in the following situations.

1. When building a hierarchical system, defining entry points for each layer in the subsystem using the appearance pattern simplifies the dependencies between subsystems.
2. When there are many subsystems in a complex system, appearance patterns can provide a simple interface for system design for external access.
3. When there is a strong connection between clients and multiple subsystems, the introduction of appearance patterns can separate them, thereby improving the independence and portability of subsystems.

Example

Here's an example code from Chapter 7 of the head first design pattern

//Appearance Role
public class HomeTheaterFacade {
  //Subsystem Role
   Amplifier amp;
  //Subsystem Role
   Tuner tuner;
  //Subsystem Role
   StreamingPlayer player;
  //Subsystem Role
   CdPlayer cd;
  //Subsystem Role
   Projector projector;
  //Subsystem Role
   TheaterLights lights;
  //Subsystem Role
   Screen screen;
  //Subsystem Role
   PopcornPopper popper;
 
   public HomeTheaterFacade(Amplifier amp, 
             Tuner tuner, 
             StreamingPlayer player, 
             Projector projector, 
             Screen screen,
             TheaterLights lights,
             PopcornPopper popper) {
 
      this.amp = amp;
      this.tuner = tuner;
      this.player = player;
      this.projector = projector;
      this.screen = screen;
      this.lights = lights;
      this.popper = popper;
   }
 
   public void watchMovie(String movie) {
      System.out.println("Get ready to watch a movie...");
      popper.on();
      popper.pop();
      lights.dim(10);
      screen.down();
      projector.on();
      projector.wideScreenMode();
      amp.on();
      amp.setStreamingPlayer(player);
      amp.setSurroundSound();
      amp.setVolume(5);
      player.on();
      player.play(movie);
   }
 
 
   public void endMovie() {
      System.out.println("Shutting movie theater down...");
      popper.off();
      lights.on();
      screen.up();
      projector.off();
      amp.off();
      player.stop();
      player.off();
   }

   public void listenToRadio(double frequency) {
      System.out.println("Tuning in the airwaves...");
      tuner.on();
      tuner.setFrequency(frequency);
      amp.on();
      amp.setVolume(5);
      amp.setTuner(tuner);
   }

   public void endRadio() {
      System.out.println("Shutting down the tuner...");
      tuner.off();
      amp.off();
   }
}

Client

public class HomeTheaterTestDrive {
   public static void main(String[] args) {
      Amplifier amp = new Amplifier("Amplifier");
      Tuner tuner = new Tuner("AM/FM Tuner", amp);
      StreamingPlayer player = new StreamingPlayer("Streaming Player", amp);
      CdPlayer cd = new CdPlayer("CD Player", amp);
      Projector projector = new Projector("Projector", player);
      TheaterLights lights = new TheaterLights("Theater Ceiling Lights");
      Screen screen = new Screen("Theater Screen");
      PopcornPopper popper = new PopcornPopper("Popcorn Popper");
 
      HomeTheaterFacade homeTheater = 
            new HomeTheaterFacade(amp, tuner, player, 
                  projector, screen, lights, popper);
 
      homeTheater.watchMovie("Raiders of the Lost Ark");
      homeTheater.endMovie();
   }
}

Reference resources

1Detailed Appearance Mode (Facade Mode)

2 Headfirst Design Mode