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NTUT CSIE

Bridge Pattern

Bridge Pattern

<b>CSIE Department, NTUT</b>
<b>Woei-Kae Chen</b>

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NTUT CSIE

Bridge pattern: Intent

Bridge pattern: Intent

z

Decouple

an

abstraction

from its

implementation

so that the two can

vary independently

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NTUT CSIE

Bridge pattern: Motivation

Bridge pattern: Motivation

z

When an abstraction can have several

possible implementations, the usual way to

accommodate them is to use inheritance.

Bridge pattern: Motivation

Bridge pattern: Motivation

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Two drawbacks

– It’s inconvenient to extend the Window abstractionto
cover different kinds of windows.

– It makes client code platform dependent(client
instantiates a concrete implementation)

<b>Window</b>

<b>XWindow</b> <b>PMWindow</b>

<b>Window</b>

<b>XWindow</b> <b>PMWindow</b> <b>IconWindow</b>

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NTUT CSIE

Bridge pattern: Motivation

Bridge pattern: Motivation

z

Bridge pattern: put the Window abstraction and its

implementation into separate class hierarchies

+DrawText()
+DrawRect()
<b>Window</b>
+DrawBorder()
<b>IconWindow</b>
+DrawCloseBox()
<b>TransientWindow</b>
DrawRect()
DrawText() DrawRect()
imp->DevDrawLine()
imp->DevDrawLine()
imp->DevDrawLine()
imp->DevDrawLine()
+DevDrawText()
+DevDrawLine()
<b>WindowImp</b>
+DevDrawText()
+DevDrawLine()
<b>XWindowImp</b>
+DevDrawText()
+DevDrawLine()
<b>PMWindowImp</b>
XDrawLine() XDrawString()
imp <b>bridge</b>
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NTUT CSIE

Bridge pattern: Applicability

Bridge pattern: Applicability

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Use the bridge pattern when

– You want to avoid a permanent binding between an
abstraction and its implementation

– Both the abstractions and their implementations should
be extensible by subclassing

– Changes in the implementation of an abstraction should
have no impact on clients – their code should not have
to be recompiled

– (C++) you want to hide the implementation of an
abstraction completely from clients (in C++, the
representation of a class is visible in the class interface)

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NTUT CSIE

Bridge pattern: Structure

Bridge pattern: Structure

+Operation()

<b>Abstraction</b>

<b>RefinedAbstraction</b>

imp->OperationImp()

+OperationImp()

<b>Implementor</b>

+OperationImp()

<b>ConcreteImplementorA</b>

imp

+OperationImp()

<b>ConcreteImplementorB</b>
<b>Client</b>

Bridge pattern: Participants

Bridge pattern: Participants

z

Abstraction (Window)

– Defines the abstraction’s interface

– Maintains a reference to an object of type Implementor

z

RefinedAbstraction (IconWindow)

– Extends the interface defined by Abstraction

z

Implementor (WindowImp)

– Defines the interface for implementation classes
(typically, the Implementor interface provides only
primitive operations, and Abstraction defines
higher-level operations)

z

ConcreteImplementor

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NTUT CSIE

Bridge pattern: Collaborations

Bridge pattern: Collaborations

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Abstraction forwards client requests to its

implementor object

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NTUT CSIE

Bridge pattern: Consequences

Bridge pattern: Consequences

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<i><b>Decoupling interface and implementation</b></i>

–

The implementation of an abstraction can be

configured at run-time.

–

Eliminates compile-time dependencies between

client and implementation

z

<i><b>Improved extensibility</b></i>

–

Abstraction and Implementor hierarchies can be

extended independently

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NTUT CSIE

Bridge pattern: Implementation

Bridge pattern: Implementation

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<i><b>Only one Implementor</b></i>

–

This is a degenerate case. There is a one-to-one

relationship between Abstraction and

Implementor –

still useful

, when a change in

the implementation of a class must not affect its

existing clients

z

<i><b>Creating the right implementor object</b></i>

–

If Abstraction knows about all

ConcreteImplementor classes, it can instantiate

one of them in its constructor

–

Or, we can introduce a factory object whose

duty is to encapsulate platform-specifics

Bridge pattern: Related Patterns

Bridge pattern: Related Patterns

z

An Abstract Factory can create and

configure a particular Bridge

z

The

Adapter

pattern is geared toward

making unrelated classes work together. It

is usually applied to systems

after

they’re

designed.

Bridge

, on the other hand, is used

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