Design Pattern: Facade Pattern

Summary

Pattern Name	Facade Pattern
Pattern Type	Structural Pattern
Scope	Object
Tagline	Unified interface to make subsystems easier to use
Use cases	1. When a simple interface is required for a complex system (distributed into subsystems) 2. When underlying complexity needs to be hidden from the client
Related Patterns	Abstract Factory Mediator
Difficulty Level	Easy
Implementations

Definition

Facade pattern adds a new layer of interface to a system, on top of the underlying complex subsystems interfaces.

Facade means – “the front of a building“.
Like, when we look at the front of a building, we do not get a complete idea of the actual structure of the building or other factors of the building.
Similarly, Facade pattern works as the front of a system, and the client does not have any idea what complexities are there in that underlying system.

Say, one part of the implementation of a system is divided into multiple subsystems. Then using and managing multiple subsystems directly from the client is difficult.

To reduce the complexity we can add a new layer to the implementation, which will communicate with the subsystems. This new layer works as an entry point for the client. The client can interact with this new layer, instead of communicating with the subsystems directly. That way the client does not need to be aware of the subsystems and gets a common interface.

All request from the client (the class which is using the subsystems) goes to the Facade, and then the facade talks to(uses the subsystem classes) the subsystems, get the result, and then return that to the client. So, the number of classes (or subsystems) that the client deals with is drastically reduced.

The subsystem modules/classes are not aware of the Facade, so no change is required in the subsystems. Everything is handled by the Facade.

There is no restriction to using the subsystem class directly from the client, the client can use the classes directly (but that is not the ideal case, as the Facade is implemented).

A Facade does not need to implement all the functionalities that the subsystem classes have. It only needs to implement the functionalities that the client(s) need.

Key points of Facade pattern are:

Facade pattern can be implemented at any point of the development process, when we realize that the system (or part of the system) has become very complex.

Use Cases

Here are the use cases of Facade Pattern –

Implementation

Here are the steps to implement Facade:

1. Create a new class.
2. Implement functionality that is required by the client class(es) in this new class.
3. Make sure to implement all functionality required by the client, so that the client does not have to interact with the subsystem classes directly.

Examples

Example #1: Travel Plan

For example, let’s consider a system that represents a travel/transport process. For the implementation, we have a class (or maybe multiple classes) to operate a Car, and other classes for handling Map, Toll, and Weather:

• Car – handles the operation of a car, like, engine start/stop, driving, fuel calculation, etc.
• Map – handles the operation for getting the full route and what should be the next move.
• Toll – calculates toll at a certain part of travel.
• Weather – to get weather information.

We will create a Facade class named TravelFacade and will use the classes mentioned above in this facade.

Check the class diagram for this:

Here is the Pseudocode for this implementation –

Car Class

Car class implements the engine start/stop, go straight/left/right, etc.

class Car

    function startEngine()
        // Implement functionality
    end function

    function stopEngine()
       // Implement functionality
    end function

    function goStraight()
       // Implement functionality
    end function

    function goLeft()
       // Implement functionality
    end function

    function goRight()
       // Implement functionality
    end function

    function getDistanceTravelled()
       // Implement functionality
    end function

end class

Map Class

Map class has functions to get direction, distance, get information about certain locations, etc.

class Map

    var startLat
    var startLng
    
    var endLat
    var endLng

    // define constructor
    function Map(startLatParam, startLngParam, endLatParam, endLngParam)
        set startLat = startLatParam
        set startLng = startLngParam
        set endLat = endLatParam
        set endLng = endLngParam
    end function

    function getCurrentLocation()
        // Implement functionality
    end function

    function getNextMove()
        // Implement functionality
    end function

    function getFullRoute()
        // Implement functionality
    end function

    function getLocationDetails(lat, lng)
        // Implement functionality
    end function

end class

Toll Class

Toll class is used to get the toll points and amount of toll.

class Toll

    function getTollPoints(lat, lng)
        // Implement functionality
    end function

    function getTollAmount(tollPointId)
        // Implement functionality
    end function

    function getTotalToll(lat, lng)
        // Implement functionality
    end function

end class

Weather Class

Weather class is for getting the weather information:

class Weather

    function getWeatherInfo(lat, lng)
        // Implement functionality
    end function

end class

Travel Facade

Let’s create TravelFacade class and implement all required functions:

class TravelFacade
    var float startLat
    var float startLng
    
    var float endLat
    var float endLng

    var Map map
    var Toll toll
    var Car car
    var Weather weather

    // define constructor
    function TravelFacade(startLatParam, startLngParam, endLatParam, endLngParam)
        set startLat = startLatParam
        set startLng = startLngParam
        set endLat = endLatParam
       set endLng = endLngParam
        
        // Initialize classes
        set map = new Map(startLat, startLng, endLat, endLng)
        set car = new Car()
        set toll = new Toll()
        set weather = new Weather()

    end function    

    function getRoute()
        return map.getFullRoute()
    end function

   function getLocationInfo(lat, lng)
        map.getLocationDetails(lat, lng)
        weather.getWeatherInfo(lat, lng)
    end function

    function getCurrentLocation()
        return map.getCurrentLocation()
    end function

    function operateCar()
        set fullRoute = map.getFullRoute()

        car.startEngine();

        loop point in fullRoute
            set nextMove = map.getNextMove()

            if (nextMove == "straight") {
                car.goStraight()
            } else if (nextMove == "left") {
                car.goLeft()
            } else if (nextMove == "right") {
                car.goRight()
            }
        end loop

        car.stopEngine();
    end function

    function getTotalTollAmount(lat, lng)
        return toll.getTotalToll(lat, lng))
    end function

end class

Code Implementations

Use the following links to check Facade pattern implementation in specific programming languages.