Bridge
Table of content
Bridge pattern is useful when a code often changes for an implementation as well as for a use of code. In your application, you should have provider classes and client classes:
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Each client class can interact with each provider class. However, if the implementation changes, the method signatures of the Provider interface may change and all the client classes have to change. In the same way, if the client classes need a new interface, we need to rewrite all the providers. The solution is to add a bridge, that is to say a class that will be called by the clients, that contains a reference to the providers and forward the client call to the providers.
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In the future, the two interfaces (client/bridge and bridge/provider) may change independently and the bridge may trans-code the call order.
Examples
It's hard to find an example in a library as this pattern is designed for versatile specifications and a library does not change constantly between two versions.
Cost
The cost of this pattern is the same as the adapter. It can be planned at design time but the best way to decide to add it is the experience feedback. Implement it when you have frequently changed an interface in a short time.
Creation
Its implementation is easy but can be expensive. It depends on the complexity of the interface. The more methods you have, the more expensive it will be.
Maintenance
If you always update the client/bridge interface and the bridge/provider interface the same way, it would be more expensive than if you do not implement the design pattern.
Removal
This pattern can be easily removed as automatic refactoring operations can easily remove its existence.
Advices
- Put the bridge term in the name of the bridge class to indicate the use of the pattern to the other developers.
Implementation
Implementation in Kotlin
/** "Implementor" */ fun interface MusicPlayer { fun play(song: String): String } /** "ConcreteImplementor" 1/4 */ val defaultPlayer = MusicPlayer{ "Reproducing $it in ${MusicFormat.NONE}" } /** "ConcreteImplementor" 2/4 */ val mp3Player = MusicPlayer{ "Reproducing $it in ${MusicFormat.MP3}" } /** "ConcreteImplementor" 3/4 */ val mp4Player = MusicPlayer{ "Reproducing $it in ${MusicFormat.MP4}" } /** "ConcreteImplementor" 4/4 */ val vlcPlayer = MusicPlayer{ "Reproducing \"$it\" in ${MusicFormat.VLC}" } /** "Abstraction" */ abstract class MusicPlayerUI { var musicPlayer : MusicPlayer = defaultPlayer abstract fun playMusic(song: String, musicFormat: MusicFormat) } /** "Refined Abstraction" */ class MyMusicPlayerUI : MusicPlayerUI() { override fun playMusic(song: String, musicFormat: MusicFormat) { musicPlayer = when(musicFormat) { MusicFormat.NONE -> defaultPlayer MusicFormat.MP3 -> mp3Player MusicFormat.MP4 -> mp4Player MusicFormat.VLC -> vlcPlayer } println(musicPlayer.play(song)) } } enum class MusicFormat{ NONE,MP3,MP4,VLC } /** "Client" */ object BridgePattern { @JvmStatic fun main(args: Array<String>) { val musicPlayer = MyMusicPlayerUI() musicPlayer.playMusic("The best song", MusicFormat.MP4) } } Implementation in Java
The following Java (SE 6) program illustrates the bridge pattern.
/** * Implementor */ interface DrawingAPI { public void drawCircle(double x, double y, double radius); } /** * ConcreteImplementor 1/2 */ class DrawingAPI1 implements DrawingAPI { public void drawCircle(double x, double y, 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(double x, double y, 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(DrawingAPI drawingAPI) { this.drawingAPI = drawingAPI; } public abstract void draw(); // low-level public abstract void resizeByPercentage(double pct); // high-level } /** * Refined Abstraction */ class CircleShape extends Shape { private double x, y, radius; public CircleShape(double x, double y, double radius, 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(double pct) { radius *= pct; } } /** * Client */ class BridgePattern { public static void main(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 7.5000000 API2.circle at 5.000000:7.000000 radius 27.500000
Implementation in PHP
interface DrawingAPI { function drawCircle($dX, $dY, $dRadius); } class DrawingAPI1 implements DrawingAPI { public function drawCircle($dX, $dY, $dRadius) { echo "API1.circle at ".$dX.":".$dY." radius ".$dRadius."<br/>"; } } class DrawingAPI2 implements DrawingAPI { public function drawCircle($dX, $dY, $dRadius) { echo "API2.circle at ".$dX.":".$dY." radius ".$dRadius."<br/>"; } } abstract class Shape { protected $oDrawingAPI; public abstract function draw(); public abstract function resizeByPercentage($dPct); protected function __construct(DrawingAPI $oDrawingAPI) { $this->oDrawingAPI = $oDrawingAPI; } } class CircleShape extends Shape { private $dX; private $dY; private $dRadius; public function __construct( $dX, $dY, $dRadius, DrawingAPI $oDrawingAPI ) { parent::__construct($oDrawingAPI); $this->dX = $dX; $this->dY = $dY; $this->dRadius = $dRadius; } public function draw() { $this->oDrawingAPI->drawCircle( $this->dX, $this->dY, $this->dRadius ); } public function resizeByPercentage($dPct) { $this->dRadius *= $dPct; } } class Tester { public static function main() { $aShapes = array( new CircleShape(1, 3, 7, new DrawingAPI1()), new CircleShape(5, 7, 11, new DrawingAPI2()), ); foreach ($aShapes as $shapes) { $shapes->resizeByPercentage(2.5); $shapes->draw(); } } } Tester::main(); Output:
API1.circle at 1:3 radius 17.5 API2.circle at 5:7 radius 27.5
Implementation in C#
C#
The following C# program illustrates the "shape" example given above and will output:
API1.circle at 1:2 radius 7.5 API2.circle at 5:7 radius 27.5
using System; /** "Implementor" */ interface IDrawingAPI { void DrawCircle(double x, double y, double radius); } /** "ConcreteImplementor" 1/2 */ class DrawingAPI1 : IDrawingAPI { public void DrawCircle(double x, double y, double radius) { System.Console.WriteLine("API1.circle at {0}:{1} radius {2}", x, y, radius); } } /** "ConcreteImplementor" 2/2 */ class DrawingAPI2 : IDrawingAPI { public void DrawCircle(double x, double y, double radius) { System.Console.WriteLine("API2.circle at {0}:{1} radius {2}", x, y, radius); } } /** "Abstraction" */ interface IShape { void Draw(); // low-level (i.e. Implementation-specific) void ResizeByPercentage(double pct); // high-level (i.e. Abstraction-specific) } /** "Refined Abstraction" */ class CircleShape : IShape { private double x, y, radius; private IDrawingAPI drawingAPI; public CircleShape(double x, double y, double radius, IDrawingAPI drawingAPI) { this.x = x; this.y = y; this.radius = radius; this.drawingAPI = drawingAPI; } // low-level (i.e. Implementation-specific) public void Draw() { drawingAPI.DrawCircle(x, y, radius); } // high-level (i.e. Abstraction-specific) public void ResizeByPercentage(double pct) { radius *= pct; } } /** "Client" */ class BridgePattern { public static void Main(string[] args) { IShape[] shapes = new IShape[2]; shapes[0] = new CircleShape(1, 2, 3, new DrawingAPI1()); shapes[1] = new CircleShape(5, 7, 11, new DrawingAPI2()); foreach (IShape shape in shapes) { shape.ResizeByPercentage(2.5); shape.Draw(); } } } C# using generics
The following C# program illustrates the "shape" example given above and will output:
API1.circle at 1:2 radius 7.5 API2.circle at 5:7 radius 27.5
using System; /** "Implementor" */ interface IDrawingAPI { void DrawCircle(double x, double y, double radius); } /** "ConcreteImplementor" 1/2 */ struct DrawingAPI1 : IDrawingAPI { public void DrawCircle(double x, double y, double radius) { System.Console.WriteLine("API1.circle at {0}:{1} radius {2}", x, y, radius); } } /** "ConcreteImplementor" 2/2 */ struct DrawingAPI2 : IDrawingAPI { public void DrawCircle(double x, double y, double radius) { System.Console.WriteLine("API2.circle at {0}:{1} radius {2}", x, y, radius); } } /** "Abstraction" */ interface IShape { void Draw(); // low-level (i.e. Implementation-specific) void ResizeByPercentage(double pct); // high-level (i.e. Abstraction-specific) } /** "Refined Abstraction" */ class CircleShape<T> : IShape where T : struct, IDrawingAPI { private double x, y, radius; private IDrawingAPI drawingAPI = new T(); public CircleShape(double x, double y, double radius) { 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(double pct) { radius *= pct; } } /** "Client" */ class BridgePattern { public static void Main(string[] args) { IShape[] shapes = new IShape[2]; shapes[0] = new CircleShape<DrawingAPI1>(1, 2, 3); shapes[1] = new CircleShape<DrawingAPI2>(5, 7, 11); foreach (IShape shape in shapes) { shape.ResizeByPercentage(2.5); shape.Draw(); } } } Implementation in Python
The following Python program illustrates the "shape" example given above and will output:
API1.circle at 1:2 7.5 API2.circle at 5:7 27.5
# Implementor class DrawingAPI: def drawCircle(x, y, radius): pass # ConcreteImplementor 1/2 class DrawingAPI1(DrawingAPI): def drawCircle(self, x, y, radius): print("API1.circle at %f:%f radius %f" % (x, y, radius)) # ConcreteImplementor 2/2 class DrawingAPI2(DrawingAPI): def drawCircle(self, x, y, radius): print("API2.circle at %f:%f radius %f" % (x, y, radius)) # Abstraction class Shape: # low-level def draw(self): pass # high-level def resizeByPercentage(self, pct): pass # Refined Abstraction class CircleShape(Shape): def __init__(self, x, y, radius, drawingAPI): self.__x = x self.__y = y self.__radius = radius self.__drawingAPI = drawingAPI # low-level i.e. Implementation specific def draw(self): self.__drawingAPI.drawCircle(self.__x, self.__y, self.__radius) # high-level i.e. Abstraction specific def resizeByPercentage(self, pct): self.__radius *= pct def main(): shapes = [ CircleShape(1, 2, 3, DrawingAPI1()), CircleShape(5, 7, 11, DrawingAPI2()) ] for shape in shapes: shape.resizeByPercentage(2.5) shape.draw() if __name__ == "__main__": main() Implementation in Ruby
An example in Ruby.
class Abstraction def initialize(implementor) @implementor = implementor end def operation raise 'Implementor object does not respond to the operation method' unless @implementor.respond_to?(:operation) @implementor.operation end end class RefinedAbstraction < Abstraction def operation puts 'Starting operation... ' super end end class Implementor def operation puts 'Doing necessary stuff' end end class ConcreteImplementorA < Implementor def operation super puts 'Doing additional stuff' end end class ConcreteImplementorB < Implementor def operation super puts 'Doing other additional stuff' end end normal_with_a = Abstraction.new(ConcreteImplementorA.new) normal_with_a.operation # Doing necessary stuff # Doing additional stuff normal_with_b = Abstraction.new(ConcreteImplementorB.new) normal_with_b.operation # Doing necessary stuff # Doing other additional stuff refined_with_a = RefinedAbstraction.new(ConcreteImplementorA.new) refined_with_a.operation # Starting operation... # Doing necessary stuff # Doing additional stuff refined_with_b = RefinedAbstraction.new(ConcreteImplementorB.new) refined_with_b.operation # Starting operation... # Doing necessary stuff # Doing other additional stuff Implementation in Scala
A Scala implementation of the Java drawing example with the same output.
/** "Implementor" */ trait DrawingAPI { def drawCircle(x:Double, y:Double, radius:Double) } /** "ConcreteImplementor" 1/2 */ class DrawingAPI1 extends DrawingAPI { def drawCircle(x:Double, y:Double, radius:Double) { printf("API1.circle at %f:%f radius %f\n", x, y, radius) } } /** "ConcreteImplementor" 2/2 */ class DrawingAPI2 extends DrawingAPI { def drawCircle(x:Double, y:Double, radius:Double) { printf("API2.circle at %f:%f radius %f\n", x, y, radius) } } /** "Abstraction" */ trait Shape { def draw() // low-level def resizeByPercentage(pct:Double) // high-level } /** "Refined Abstraction" */ class CircleShape(var x:Double, var y:Double, var radius:Double, val drawingAPI:DrawingAPI) extends Shape { // low-level i.e. Implementation specific def draw() = drawingAPI.drawCircle(x, y, radius) // high-level i.e. Abstraction specific def resizeByPercentage(pct:Double) = radius *= pct } /** "Client" */ val shapes = List( new CircleShape(1, 2, 3, new DrawingAPI1), new CircleShape(5, 7, 11, new DrawingAPI2) ) shapes foreach { shape => shape.resizeByPercentage(2.5) shape.draw() } Implementation in D
An example in D.
import std.stdio; /** "Implementor" */ interface DrawingAPI { void drawCircle(double x, double y, double radius); } /** "ConcreteImplementor" 1/2 */ class DrawingAPI1: DrawingAPI { void drawCircle(double x, double y, double radius) { writefln("\nAPI1.circle at %f:%f radius %f", x, y, radius); } } /** "ConcreteImplementor" 2/2 */ class DrawingAPI2: DrawingAPI { void drawCircle(double x, double y, double radius) { writefln("\nAPI2.circle at %f:%f radius %f", x, y, radius); } } /** "Abstraction" */ interface Shape { void draw(); // low-level void resizeByPercentage(double pct); // high-level } /** "Refined Abstraction" */ class CircleShape: Shape { this(double x, double y, double radius, DrawingAPI drawingAPI) { this.x = x; this.y = y; this.radius = radius; this.drawingAPI = drawingAPI; } // low-level i.e. Implementation specific void draw() { drawingAPI.drawCircle(x, y, radius); } // high-level i.e. Abstraction specific void resizeByPercentage(double pct) { radius *= pct; } private: double x, y, radius; DrawingAPI drawingAPI; } int main(string[] argv) { auto api1 = new DrawingAPI1(); auto api2 = new DrawingAPI2(); auto c1 = new CircleShape(1, 2, 3, api1); auto c2 = new CircleShape(5, 7, 11, api2); Shape[4] shapes; shapes[0] = c1; shapes[1] = c2; shapes[0].resizeByPercentage(2.5); shapes[0].draw(); shapes[1].resizeByPercentage(2.5); shapes[1].draw(); return 0; } Implementation in Perl
This example in Perl uses the MooseX::Declare module.
# Implementor role Drawing::API { requires 'draw_circle'; } # Concrete Implementor 1 class Drawing::API::1 with Drawing::API { method draw_circle(Num $x, Num $y, Num $r) { printf "API1.circle at %f:%f radius %f\n", $x, $y, $r; } } # Concrete Implementor 2 class Drawing::API::2 with Drawing::API { method draw_circle(Num $x, Num $y, Num $r) { printf "API2.circle at %f:%f radius %f\n", $x, $y, $r; } } # Abstraction role Shape { requires qw( draw resize ); } # Refined Abstraction class Shape::Circle with Shape { has $_ => ( is => 'rw', isa => 'Any' ) for qw( x y r ); has api => ( is => 'ro', does => 'Drawing::API' ); method draw() { $self->api->draw_circle( $self->x, $self->y, $self->r ); } method resize(Num $percentage) { $self->{r} *= $percentage; } } my @shapes = ( Shape::Circle->new( x=>1, y=>2, r=>3, api => Drawing::API::1->new ), Shape::Circle->new( x=>5, y=>7, r=>11, api => Drawing::API::2->new ), ) $_->resize( 2.5 ) and $_->draw for @shapes; Implementation in Delphi
The following Delphi program illustrates the "shape" example given above and will output:
API1.circle at 1:2 7.5 API2.circle at 5:7 27.5
program Bridge; {$APPTYPE CONSOLE} {$R *.res} uses System.SysUtils; type ///implementator TDrawingAPI = class abstract procedure DrawCircle(x, y, radius: double); virtual; abstract; end; //ConcreteImplementator 1 TDrawingAPI1 = class(TDrawingAPI) procedure DrawCircle(x, y, radius: double); override; end; //ConcreteImplementator 2 TDrawingAPI2 = class(TDrawingAPI) procedure DrawCircle(x, y, radius: double);override; end; //Abstraction TShape = class abstract procedure Draw();virtual; abstract;// low-level (i.e. Implementation-specific) procedure ResizeByPercentage(pct: double);virtual; abstract;// high-level (i.e. Abstraction-specific) end; //Refined Abstraction TCircleShape = class(TShape) strict private x, y, radius: double; drawingAPI: TDrawingAPI; public constructor Create(x, y, radius: double; drawingAPI: TDrawingAPI); procedure Draw;override; procedure ResizeByPercentage(pct: double);override; end; { TDeawingAPI1 } procedure TDrawingAPI1.DrawCircle(x, y, radius: double); begin WriteLn('API1.circle at '+FloatToStr(x)+' : '+FloatToStr(y)+' radius '+FloatToStr(radius)); end; { TDeawingAPI } procedure TDrawingAPI2.DrawCircle(x, y, radius: double); begin WriteLn('API2.circle at '+FloatToStr(x)+' : '+FloatToStr(y)+' radius '+FloatToStr(radius)); end; { TCircleShape } constructor TCircleShape.Create(x, y, radius: double; drawingAPI: TDrawingAPI); begin self.x := x; self.y := y; self.radius := radius; self.drawingAPI := drawingAPI; end; procedure TCircleShape.Draw; begin drawingAPI.DrawCircle(x, y, radius); end; procedure TCircleShape.ResizeByPercentage(pct: double); begin radius := radius * pct; end; var shapes: array of TShape; shape: TShape; begin try { TODO -oUser -cConsole Main : Insert code here } SetLength(shapes, 2); shapes[0] := TCircleShape.Create(1, 2, 3, TDrawingAPI1.Create); shapes[1] := TCircleShape.Create(5, 7, 11, TDrawingAPI2.Create); for shape in shapes do begin shape.ResizeByPercentage(2.5); shape.Draw; end; WriteLn(#13#10+'Press any key to continue..'); ReadLn; shapes[0].Free; shapes[1].Free; except on E: Exception do Writeln(E.ClassName, ': ', E.Message); end; end.
