Bridge pattern

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. 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, and 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

The Bridge 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?
* 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 cannot 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 Unified Modeling Language class and sequence diagram below.

Structure

UML class and sequence diagram

In the above Unified Modeling Language class diagram, an abstraction (Abstraction) is not implemented 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

; 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

Example

C#

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

public class Bridge1 : IBridge

public class Bridge2 : IBridge

public interface IAbstractBridge

public class AbstractBridge : IAbstractBridge

The Bridge classes are the Implementation that uses the same interface-oriented architecture to create objects. On the other hand, the abstraction takes an instance of the implementation class and runs its method. Thus, they are completely decoupled from one another.

Crystal

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 #:# - radius: #"
end
end

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

C++

import std;

class DrawingAPI ;

class DrawingAPI01 : public DrawingAPI ;

class DrawingAPI02 : public DrawingAPI ;

class Shape ;

class CircleShape: public Shape ;

int main(int argc, char** argv)

Output:

API01.circle at 1.000000:2.000000 - radius: 3.075000
API02.circle at 5.000000:7.000000 - radius: 11.275000

Java

The following Java program defines a bank account that separates the account operations from the logging of these operations.

// Logger has two implementations: info and warning
@FunctionalInterface
interface Logger

abstract class AbstractAccount

class SimpleAccount extends AbstractAccount

public class BridgeDemo

It will output:

info: withdraw 75 result true
warning: withdraw 10 result true
warning: withdraw 100 result false

PHP

trait DrawingAPI

class DrawingAPI1 extends DrawingAPI

class DrawingAPI2 extends DrawingAPI

abstract class Shape(drawingAPI: DrawingAPI)

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

object BridgePattern

Python

"""
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 f"API1.circle at : - radius: "

class DrawingAPI2(DrawingAPI):
def draw_circle(self, x, y, radius):
return f"API2.circle at : - radius: "

class DrawingAPI3(DrawingAPI):
def draw_circle(self, x, y, radius):
return f"API3.circle at : - 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:
@staticmethod
def test():
shapes = CircleShape(1.0, 2.0, 3.0, DrawingAPI1()),
CircleShape(5.0, 7.0, 11.0, DrawingAPI2()),
CircleShape(5.0, 4.0, 12.0, DrawingAPI3()),

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

BridgePattern.test()

See also

* Adapter pattern
* Strategy pattern
* Template method pattern

References

External links

Bridge in UML and in LePUS3
(a formal modelling language)
* From:

{{DEFAULTSORT:Bridge Pattern
Software design patterns
Articles with example C Sharp code
Articles with example C++ code
Articles with example Java code