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.
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?[1]
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?
Abstraction) from its implementation (Implementor) by putting them in separate class hierarchies.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.
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 foran 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 diagramshows 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.
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
public class Bridge1 : IBridge
public class Bridge2 : IBridge
public interface IAbstractBridge
public class AbstractBridge : IAbstractBridgeThe 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.
class DrawingAPI1 < DrawingAPI def draw_circle(x : Float, y : Float, radius : Float) "API1.circle at #:# - radius: #" endend
class DrawingAPI2 < DrawingAPI def draw_circle(x : Float64, y : Float64, radius : Float64) "API2.circle at #:# - radius: #" endend
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) endend
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 endend
BridgePattern.test
Output
API1.circle at 1.0:2.0 - radius: 3.075 API2.circle at 5.0:7.0 - radius: 11.275
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
The following Java program defines a bank account that separates the account operations from the logging of these operations.
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
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
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