Bridge pattern

From Wikipedia, the free encyclopedia
Jump to: navigation, search

The bridge pattern is a design pattern used in software engineering that is meant to "decouple an abstraction from its implementation so that the two can vary independently", introduced by the Gang of Four.[1] The bridge uses encapsulation, aggregation, and can use inheritance to separate responsibilities into different classes.

When a class varies often, the features of object-oriented programming become very useful because changes to a program's code can be made easily with minimal prior knowledge about the program. The bridge pattern is useful when both the class and what it does vary often. The class itself can be thought of as the abstraction and what the class can do as the implementation. The bridge pattern can also be thought of as two layers of abstraction.

When there is only one fixed implementation, this pattern is known as the Pimpl idiom in the C++ world.

The bridge pattern is often confused with the adapter pattern. In fact, the bridge pattern is often implemented using the object adapter pattern, e.g. in the Java code below.

Variant: The implementation can be decoupled even more by deferring the presence of the implementation to the point where the abstraction is utilized.

Overview[edit]

The Bridge [2] design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.

What problems can the Bridge design pattern solve? [3]

  • An abstraction and its implementation should be defined and extended independently from each other.
  • A compile-time binding between an abstraction and its implementation should be avoided so that an implementation can be selected at run-time.

When using subclassing, different subclasses implement an abstract class in different ways. But an implementation is bound to the abstraction at compile-time and can't be changed at run-time.

What solution does the Bridge design pattern describe?

  • Separate an abstraction (Abstraction) from its implementation (Implementor) by putting them in separate class hierarchies.
  • Implement the Abstraction in terms of (by delegating to) an Implementor object.

This enables to configure an Abstraction with an Implementor object at run-time.
See also the UML class and sequence diagram below.

Structure[edit]

UML class and sequence diagram[edit]

A sample UML class and sequence diagram for the Bridge design pattern. [4]

In the above UML class diagram, an abstraction (Abstraction) isn't implemeted as usual in a single inheritance hierarchy. Instead, there is one hierarchy for an abstraction (Abstraction) and a separate hierarchy for its implementation (Implementor), which makes the two independent from each other. The Abstraction interface (operation()) is implemented in terms of (by delegating to) the Implementor interface (imp.operationImp()).
The UML sequence diagram shows the run-time interactions: The Abstraction1 object delegates implementation to the Implementor1 object (by calling operationImp() on Implementor1), which performs the operation and returns to Abstraction1.

Class diagram[edit]

Bridge UML class diagram.svg

Abstraction (abstract class)
defines the abstract interface
maintains the Implementor reference.
RefinedAbstraction (normal class)
extends the interface defined by Abstraction
Implementor (interface)
defines the interface for implementation classes
ConcreteImplementor (normal class)
implements the Implementor interface
Bridge in LePUS3 (legend)

Example[edit]

C#[edit]

Bridge pattern compose objects in tree structure. It decouples abstraction from implementation. Here abstraction represents the client from which the objects will be called. An example implemented in C# is given below

// Helps in providing truly decoupled architecture
public interface IBridge
{
    void Function1();
    void Function2();
}

public class Bridge1 : IBridge
{
    public void Function1()
    {
        Console.WriteLine("Bridge1.Function1");
    }

    public void Function2()
    {
        Console.WriteLine("Bridge1.Function2");
    }
}

public class Bridge2 : IBridge
{
    public void Function1()
    {
        Console.WriteLine("Bridge2.Function1");
    }

    public void Function2()
    {
        Console.WriteLine("Bridge2.Function2");
    }
}

public interface IAbstractBridge
{
    void CallMethod1();
    void CallMethod2();
}

public class AbstractBridge : IAbstractBridge
{
    public IBridge bridge;

    public AbstractBridge(IBridge bridge)
    {
        this.bridge = bridge;
    }

    public void CallMethod1()
    {
        this.bridge.Function1();
    }

    public void CallMethod2()
    {
        this.bridge.Function2();
    }
}

As you can see, the Bridge classes are the Implementation that uses the same interface-oriented architecture to create objects. On the other hand, the abstraction takes an object of the implementation phase and runs its method. Thus, it makes completely decoupled from one another.

Crystal[edit]

abstract class DrawingAPI
  abstract def draw_circle(x : Float64, y : Float64, radius : Float64)
end

class DrawingAPI1 < DrawingAPI
  def draw_circle(x : Float, y : Float, radius : Float)
    "API1.circle at #{x}:#{y} - radius: #{radius}"
  end
end

class DrawingAPI2 < DrawingAPI
  def draw_circle(x : Float64, y : Float64, radius : Float64)
    "API2.circle at #{x}:#{y} - radius: #{radius}"
  end
end

abstract class Shape
  protected getter drawing_api : DrawingAPI

  def initialize(@drawing_api)
  end

  abstract def draw
  abstract def resize_by_percentage(percent : Float64)
end

class CircleShape < Shape
  getter x : Float64
  getter y : Float64
  getter radius : Float64

  def initialize(@x, @y, @radius, drawing_api : DrawingAPI)
    super(drawing_api)
  end

  def draw
    @drawing_api.draw_circle(@x, @y, @radius)
  end

  def resize_by_percentage(percent : Float64)
    @radius *= (1 + percent/100)
  end
end

class BridgePattern
  def self.test
    shapes = [] of Shape
    shapes << CircleShape.new(1.0, 2.0, 3.0, DrawingAPI1.new)
    shapes << CircleShape.new(5.0, 7.0, 11.0, DrawingAPI2.new)

    shapes.each do |shape|
      shape.resize_by_percentage(2.5)
      puts shape.draw
    end
  end
end

BridgePattern.test

Output

API1.circle at 1.0:2.0 - radius: 3.075
API2.circle at 5.0:7.0 - radius: 11.275

Java[edit]

The following Java (SE 6) program illustrates a 'shape'.

/** "Implementor" */
interface DrawingAPI {
    public void drawCircle(final double x, final double y, final double radius);
}

/** "ConcreteImplementor"  1/2 */
class DrawingAPI1 implements DrawingAPI {
    public void drawCircle(final double x, final double y, final double radius) {
        System.out.printf("API1.circle at %f:%f radius %f\n", x, y, radius);
    }
}

/** "ConcreteImplementor" 2/2 */
class DrawingAPI2 implements DrawingAPI {
    public void drawCircle(final double x, final double y, final double radius) {
        System.out.printf("API2.circle at %f:%f radius %f\n", x, y, radius);
    }
}

/** "Abstraction" */
abstract class Shape {
    protected DrawingAPI drawingAPI;

    protected Shape(final DrawingAPI drawingAPI){
        this.drawingAPI = drawingAPI;
    }

    public abstract void draw();                                 // low-level
    public abstract void resizeByPercentage(final double pct);   // high-level
}

/** "Refined Abstraction" */
class CircleShape extends Shape {
    private double x, y, radius;
    public CircleShape(final double x, final double y, final double radius, final DrawingAPI drawingAPI) {
        super(drawingAPI);
        this.x = x;  this.y = y;  this.radius = radius;
    }

    // low-level i.e. Implementation specific
    public void draw() {
        drawingAPI.drawCircle(x, y, radius);
    }
    // high-level i.e. Abstraction specific
    public void resizeByPercentage(final double pct) {
        radius *= (1.0 + pct/100.0);
    }
}

/** "Client" */
class BridgePattern {
    public static void main(final String[] args) {
        Shape[] shapes = new Shape[] {
            new CircleShape(1, 2, 3, new DrawingAPI1()),
            new CircleShape(5, 7, 11, new DrawingAPI2())
        };

        for (Shape shape : shapes) {
            shape.resizeByPercentage(2.5);
            shape.draw();
        }
    }
}

It will output:

API1.circle at 1.000000:2.000000 radius 3.075000
API2.circle at 5.000000:7.000000 radius 11.275000

PHP[edit]

interface DrawingAPI {
    function drawCircle($x, $y, $radius);
}

class DrawingAPI1 implements DrawingAPI {
    public function drawCircle($x, $y, $radius) {
        echo "API1.circle at $x:$y radius $radius.\n";
    }
}

class DrawingAPI2 implements DrawingAPI {
    public function drawCircle($x, $y, $radius) {
        echo "API2.circle at $x:$y radius $radius.\n";
    }
}

abstract class Shape {
    protected $drawingAPI;

    public abstract function draw();
    public abstract function resizeByPercentage($pct);

    protected function __construct(DrawingAPI $drawingAPI) {
        $this->drawingAPI = $drawingAPI;
    }
}

class CircleShape extends Shape {
    private $x;
    private $y;
    private $radius;

    public function __construct($x, $y, $radius, DrawingAPI $drawingAPI) {
        parent::__construct($drawingAPI);
        $this->x = $x;
        $this->y = $y;
        $this->radius = $radius;
    }

    public function draw() {
        $this->drawingAPI->drawCircle($this->x, $this->y, $this->radius);
    }

    public function resizeByPercentage($pct) {
        $this->radius *= $pct;
    }
}

class Tester {
    public static function main()  {
        $shapes = array(
            new CircleShape(1, 3, 7,  new DrawingAPI1()),
            new CircleShape(5, 7, 11, new DrawingAPI2()),
        );

        foreach ($shapes as $shape) {
            $shape->resizeByPercentage(2.5);
            $shape->draw();
        }
    }
}

Tester::main();

Output:

API1.circle at 1:3 radius 17.5
API2.circle at 5:7 radius 27.5

Scala[edit]

trait DrawingAPI {
  def drawCircle(x: Double, y: Double, radius: Double)
}

class DrawingAPI1 extends DrawingAPI {
  def drawCircle(x: Double, y: Double, radius: Double) = println(s"API #1 $x $y $radius")
}

class DrawingAPI2 extends DrawingAPI {
  def drawCircle(x: Double, y: Double, radius: Double) = println(s"API #2 $x $y $radius")
}

abstract class Shape(drawingAPI: DrawingAPI) {
  def draw()
  def resizePercentage(pct: Double)
}

class CircleShape(x: Double, y: Double, var radius: Double, drawingAPI: DrawingAPI)
    extends Shape(drawingAPI: DrawingAPI) {

  def draw() = drawingAPI.drawCircle(x, y, radius)

  def resizePercentage(pct: Double) { radius *= pct }
}

object BridgePattern {
  def main(args: Array[String]) {
    Seq (
	new CircleShape(1, 3, 5, new DrawingAPI1),
	new CircleShape(4, 5, 6, new DrawingAPI2)
    ) foreach { x =>
        x.resizePercentage(3)
        x.draw()			
      }	
  }
}

Python[edit]

"""
Bridge pattern example.
"""
from abc import ABCMeta, abstractmethod


NOT_IMPLEMENTED = "You should implement this."


class DrawingAPI:
    __metaclass__ = ABCMeta

    @abstractmethod
    def draw_circle(self, x, y, radius):
        raise NotImplementedError(NOT_IMPLEMENTED)


class DrawingAPI1(DrawingAPI):
    def draw_circle(self, x, y, radius):
        return "API1.circle at {0}:{1} - radius: {2}".format(x, y, radius)


class DrawingAPI2(DrawingAPI):
    def draw_circle(self, x, y, radius):
        return "API2.circle at {0}:{1} - radius: {2}".format(x, y, radius)


class Shape:
    __metaclass__ = ABCMeta

    drawing_api = None

    def __init__(self, drawing_api):
        self.drawing_api = drawing_api

    @abstractmethod
    def draw(self):
        raise NotImplementedError(NOT_IMPLEMENTED)

    @abstractmethod
    def resize_by_percentage(self, percent):
        raise NotImplementedError(NOT_IMPLEMENTED)


class CircleShape(Shape):
    def __init__(self, x, y, radius, drawing_api):
        self.x = x
        self.y = y
        self.radius = radius
        super(CircleShape, self).__init__(drawing_api)

    def draw(self):
        return self.drawing_api.draw_circle(
            self.x, self.y, self.radius
        )

    def resize_by_percentage(self, percent):
        self.radius *= (1 + percent/100)


class BridgePattern(object):
    @staticmethod
    def test():
        shapes = [
            CircleShape(1.0, 2.0, 3.0, DrawingAPI1()),
            CircleShape(5.0, 7.0, 11.0, DrawingAPI2())
        ]

        for shape in shapes:
            shape.resize_by_percentage(2.5)
            print(shape.draw())


BridgePattern.test()

See also[edit]

References[edit]

  1. ^ Gamma, E, Helm, R, Johnson, R, Vlissides, J: Design Patterns, page 151. Addison-Wesley, 1995
  2. ^ Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides (1994). Design Patterns: Elements of Reusable Object-Oriented Software. Addison Wesley. pp. 151ff. ISBN 0-201-63361-2. 
  3. ^ "The Bridge design pattern - Problem, Solution, and Applicability". w3sDesign.com. Retrieved 2017-08-12. 
  4. ^ "The Bridge design pattern - Structure and Collaboration". w3sDesign.com. Retrieved 2017-08-12. 

External links[edit]