Façade defines a higher-level interface that makes the subsystem easier to use. With a Facade you can take a complex subsystem and make it easier to use by implementing a Facade class that provides a more reasonable interface. Its similar to wrapping but it is designed around simpler access to complex systems while still leaving the sub-system functions exposed if needed. A sort of delegation of tasks to a single object.
Example: Below is a image of a Complex set of tasks that relate to a HomeTheater system. These are all the tasks you would need to do separately in order to use you appliances.
However a Facade can simplify the operations down to a few small function calls.
For example: The Facade class treats the home theatre components as a subsystem, and calls on the subsystem to implement its watchMovie() method.
The Client i.e.: The Remote Control invokes calls to methods in the Facade, not on the subsystem. So we just invoke the watchMovie() method and it communicates with the lights, DVD player, projector, amplifier, screen and pop-corn maker for us.
Remember Facades don't necessarily encapsulate subsystem classes because you should still have access to the lower level function(s), they merely provide a simplified interface to their functionality.
A Facade Pattern allows us to de-couple the client implementation from the subsystem.
So if you buy a bigger and better HomeTheater system and you coded your Client to the Facade rather than the subsystem then nothing should change.
Here's a code snippet from Head First that demonstrates the Facade in Action.
Cut and Paste it into a Console Project and step through it to better see how it works.
namespace Martins.Facade.Pattern
{
class HomeTheaterTestDrive
{
static void Main(string[] args)
{
Amplifier amp = new Amplifier("Top-O-Line Amplifier");
Tuner tuner = new Tuner("Top-O-Line AM/FM Tuner", amp);
DvdPlayer dvd = new DvdPlayer("Top-O-Line DVD Player", amp);
CdPlayer cd = new CdPlayer("Top-O-Line CD Player", amp);
Projector projector = new Projector("Top-O-Line Projector", dvd);
TheaterLights lights = new TheaterLights("Theater Ceiling Lights");
Screen screen = new Screen("Theater Screen");
PopcornPopper popper = new PopcornPopper("Popcorn Popper");
// Facade Class
HomeTheaterFacade homeTheater =
new HomeTheaterFacade(amp, tuner, dvd, cd,
projector, screen, lights, popper);
//The WatchMovie() method wraps alot of functions.
homeTheater.WatchMovie("Raiders of the Lost Ark");
homeTheater.EndMovie();
}
}
#region Facade
public class HomeTheaterFacade
{
private Amplifier amp;
private Tuner tuner;
private DvdPlayer dvd;
private CdPlayer cd;
private Projector projector;
private TheaterLights lights;
private Screen screen;
private PopcornPopper popper;
public HomeTheaterFacade(Amplifier amp,
Tuner tuner,
DvdPlayer dvd,
CdPlayer cd,
Projector projector,
Screen screen,
TheaterLights lights,
PopcornPopper popper)
{
this.amp = amp;
this.tuner = tuner;
this.dvd = dvd;
this.cd = cd;
this.projector = projector;
this.screen = screen;
this.lights = lights;
this.popper = popper;
}
public void WatchMovie(string movie)
{
Console.WriteLine("Get ready to watch a movie...");
popper.On();
popper.Pop();
lights.Dim(10);
screen.Down();
projector.On();
projector.WideScreenMode();
amp.On();
amp.SetDvd(dvd);
amp.SetSurroundSound();
amp.SetVolume(5);
dvd.On();
dvd.Play(movie);
}
public void EndMovie()
{
Console.WriteLine("\nShutting movie theater down...");
popper.Off();
lights.On();
screen.Up();
projector.Off();
amp.Off();
dvd.Stop();
dvd.Eject();
dvd.Off();
}
public void ListenToCd(string cdTitle)
{
Console.WriteLine("Get ready for an audiopile experence...");
lights.On();
amp.On();
amp.SetVolume(5);
amp.SetCd(cd);
amp.SetStereoSound();
cd.On();
cd.Play(cdTitle);
}
public void EndCd()
{
Console.WriteLine("Shutting down CD...");
amp.Off();
amp.SetCd(cd);
cd.Eject();
cd.Off();
}
public void ListenToRadio(double frequency)
{
Console.WriteLine("Tuning in the airwaves...");
tuner.On();
tuner.SetFrequency(frequency);
amp.On();
amp.SetVolume(5);
amp.SetTuner(tuner);
}
public void EndRadio()
{
Console.WriteLine("Shutting down the tuner...");
tuner.Off();
amp.Off();
}
}
#endregion
#region Subsystem Components
public class Tuner
{
string description;
Amplifier amplifier;
double frequency;
public Tuner(string description, Amplifier amplifier)
{
this.description = description;
this.amplifier = amplifier;
}
public void On()
{
Console.WriteLine(description + " on");
}
public void Off()
{
Console.WriteLine(description + " off");
}
public void SetFrequency(double frequency)
{
Console.WriteLine(description + " setting frequency to " + frequency);
this.frequency = frequency;
}
public void SetAm()
{
Console.WriteLine(description + " setting AM mode");
}
public void SetFm()
{
Console.WriteLine(description + " setting FM mode");
}
public override string ToString()
{
return description;
}
}
public class Amplifier
{
string description;
Tuner tuner;
DvdPlayer dvd;
CdPlayer cd;
public Amplifier(string description)
{
this.description = description;
}
public void On()
{
Console.WriteLine(description + " on");
}
public void Off()
{
Console.WriteLine(description + " off");
}
public void SetStereoSound()
{
Console.WriteLine(description + " stereo mode on");
}
public void SetSurroundSound()
{
Console.WriteLine(description + " surround sound on (5 speakers, 1 subwoofer)");
}
public void SetVolume(int level)
{
Console.WriteLine(description + " setting volume to " + level);
}
public void SetTuner(Tuner tuner)
{
Console.WriteLine(description + " setting tuner to " + dvd);
this.tuner = tuner;
}
public void SetDvd(DvdPlayer dvd)
{
Console.WriteLine(description + " setting DVD player to " + dvd);
this.dvd = dvd;
}
public void SetCd(CdPlayer cd)
{
Console.WriteLine(description + " setting CD player to " + cd);
this.cd = cd;
}
public override string ToString()
{
return description;
}
}
public class TheaterLights
{
string description;
public TheaterLights(string description)
{
this.description = description;
}
public void On()
{
Console.WriteLine(description + " on");
}
public void Off()
{
Console.WriteLine(description + " off");
}
public void Dim(int level)
{
Console.WriteLine(description + " dimming to " + level + "%");
}
public override string ToString()
{
return description;
}
}
public class Screen
{
string description;
public Screen(string description)
{
this.description = description;
}
public void Up()
{
Console.WriteLine(description + " going up");
}
public void Down()
{
Console.WriteLine(description + " going down");
}
public override string ToString()
{
return description;
}
}
public class Projector
{
string description;
DvdPlayer dvdPlayer;
public Projector(string description, DvdPlayer dvdPlayer)
{
this.description = description;
this.dvdPlayer = dvdPlayer;
}
public void On()
{
Console.WriteLine(description + " on");
}
public void Off()
{
Console.WriteLine(description + " off");
}
public void WideScreenMode()
{
Console.WriteLine(description + " in widescreen mode (16x9 aspect ratio)");
}
public void TvMode()
{
Console.WriteLine(description + " in tv mode (4x3 aspect ratio)");
}
public override string ToString()
{
return description;
}
}
public class PopcornPopper
{
string description;
public PopcornPopper(string description)
{
this.description = description;
}
public void On()
{
Console.WriteLine(description + " on");
}
public void Off()
{
Console.WriteLine(description + " off");
}
public void Pop()
{
Console.WriteLine(description + " popping popcorn!");
}
public override string ToString()
{
return description;
}
}
public class DvdPlayer
{
string description;
int currentTrack;
Amplifier amplifier;
string movie;
public DvdPlayer(string description, Amplifier amplifier)
{
this.description = description;
this.amplifier = amplifier;
}
public void On()
{
Console.WriteLine(description + " on");
}
public void Off()
{
Console.WriteLine(description + " off");
}
public void Eject()
{
movie = null;
Console.WriteLine(description + " eject");
}
public void Play(string movie)
{
this.movie = movie;
currentTrack = 0;
Console.WriteLine(description + " playing \"" + movie + "\"");
}
public void Play(int track)
{
if (movie == null)
{
Console.WriteLine(description + " can't play track " + track + " no dvd inserted");
}
else
{
currentTrack = track;
Console.WriteLine(description + " playing track " + currentTrack + " of \"" + movie + "\"");
}
}
public void Stop()
{
currentTrack = 0;
Console.WriteLine(description + " stopped \"" + movie + "\"");
}
public void Pause()
{
Console.WriteLine(description + " paused \"" + movie + "\"");
}
public void SetTwoChannelAudio()
{
Console.WriteLine(description + " set two channel audio");
}
public void SetSurroundAudio()
{
Console.WriteLine(description + " set surround audio");
}
public override string ToString()
{
return description;
}
}
public class CdPlayer
{
string description;
int currentTrack;
Amplifier amplifier;
string title;
public CdPlayer(string description, Amplifier amplifier)
{
this.description = description;
this.amplifier = amplifier;
}
public void On()
{
Console.WriteLine(description + " on");
}
public void Off()
{
Console.WriteLine(description + " off");
}
public void Eject()
{
title = null;
Console.WriteLine(description + " eject");
}
public void Play(string title)
{
this.title = title;
currentTrack = 0;
Console.WriteLine(description + " playing \"" + title + "\"");
}
public void Play(int track)
{
if (title == null)
{
Console.WriteLine(description + " can't play track " + currentTrack +
", no cd inserted");
}
else
{
currentTrack = track;
Console.WriteLine(description + " playing track " + currentTrack);
}
}
public void Stop()
{
currentTrack = 0;
Console.WriteLine(description + " stopped");
}
public void Pause()
{
Console.WriteLine(description + " paused \"" + title + "\"");
}
public override string ToString()
{
return description;
}
}
#endregion
}
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