PatternStrategypdf

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Software Development and Test Laboratory ◆Department of Computer Science and Information Engineering ◆National Taipei University of Technology

/ 20

國立台北科技大學陳偉凱

• Define a family of algorithms.

–

Encapsulate each one (as an

object

).

–

Make them

interchangeable

(through

polymorphism).

–

Vary independently

from clients that use it

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Software Development and Test Laboratory ◆Department of Computer Science and Information Engineering ◆National Taipei University of Technology

/ 20

• Composition

keeps a list of words and handles

line breaking

of the words

3

Composition

Repair()

Simple

Tex

…

Object-Oriented Programming Strategy Pattern

• A possible design

Composition

Repair()

void Composition::Repair()

{

switch (CompositionType)

{

case SIMPLE:

…

break;

case TEX:

…

break;

case ARRAY:

…

break;

}

Line breaking

algorithm

Line breaking

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Software Development and Test Laboratory ◆Department of Computer Science and Information Engineering ◆National Taipei University of Technology

/ 20

• A better design

5

Composition

SimpleComposition()

TexComposition()

ArrayComposition()

Repair()

void Composition::Repair()

{

switch (CompositionType)

{

case SIMPLE:

SimpleComposition();

break;

case TEX:

TexComposition();

break;

case ARRAY:

ArrayComposition();

break;

}

Line breaking algorithm

Object-Oriented Programming Strategy Pattern

• Problems

–

Hard-wiring

line breaking algorithms

• Clients (Composition) get

bigger

and harder to

maintain.

• Different algorithms will be appropriate at different

times

–

The previous design requires

all algorithms

to exist

simultaneously

• Difficult to add new algorithms and vary existing ones

(modify algorithm



modify Composition).

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Software Development and Test Laboratory ◆Department of Computer Science and Information Engineering ◆National Taipei University of Technology

/ 20
7

編號

Q41:

好的軟體設計者必須考慮軟體如何

成長

與

改變

，

以及何種因素最可能成為改變的

焦點

。

A good (software) designer will consider how software will

grow and change and what elements are most likely to be

focal points for change.

Rebecca Wirfs-Brock and Alan McKean (2003)

Object-Oriented Programming Strategy Pattern

• Solution: apply Strategy pattern

Composition

Traverse ()

Repair ()

compositor

Compose ()

Compositor

Compose ()

SimpleCompositor

Compose ()

TexCompositor

Compose ()

ArrayCompositor

Compositor -> Compose ()

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Software Development and Test Laboratory ◆Department of Computer Science and Information Engineering ◆National Taipei University of Technology

/ 20

• Suppose we would like to evaluate the

efficiency of sorting algorithms, a design is:

9

Client (main)

+Sort()

+InsertionSort()

+SelectionSort()

+BubbleSort()

+QuickSort()

+...()

Array

void Array::Sort()

{

switch (sorter)

{

case INSERTION:

InsertionSort();

break;

case SELECTION:

SelectionSort();

break;

...

}

The same

problems

The same

problems

Object-Oriented Programming Strategy Pattern

• Solution: apply strategy pattern

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Software Development and Test Laboratory ◆Department of Computer Science and Information Engineering ◆National Taipei University of Technology

/ 20

• Many related classes

differ

only in their

behavior

.

• You need different

variants

of an algorithm.

• An algorithm uses

data

that clients

shouldn't

know

about. Avoid exposing complex,

algorithm-specific data structure to clients.

• A class defines many behaviors (use of

multiple conditional

(switch)

statements

).

11
Object-Oriented Programming Strategy Pattern

Context

ContextInterface()

Strategy

AlgorithmInterface()

ConcreteStrategyA

AlgorithmInterface()

ConcreteStrategyB

AlgorithmInterface()

ConcreteStrategyC

AlgorithmInterface()

Client

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Software Development and Test Laboratory ◆Department of Computer Science and Information Engineering ◆National Taipei University of Technology

/ 20

• Strategy (Compositor)

–

Declares an interface common to all supported

algorithms.

• ConcreteStrategy (SimpleCompositor ...)

–

Implements the algorithm using the strategy interface.

• Context (Composition)

–

Is configured with a ConcreteStrategy object.

–

Maintains a reference to a Strategy object.

–

May define an interface that lets Strategy access its

data.

13
Object-Oriented Programming Strategy Pattern

• Passing data (discussed later)

–

A context may

pass all

data required by the

algorithm.

–

Alternatively, the context can

pass itself

as an

argument to Strategy operations.

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Software Development and Test Laboratory ◆Department of Computer Science and Information Engineering ◆National Taipei University of Technology

/ 20

• Benefits:

–

Families of related algorithms

–

An alternative to subclassing

• subclassing context

–

Strategies eliminate conditional statements.

• no switch statements

–

A choice of implementations

• client can choose different strategies with different

time and space trade-offs

15
Object-Oriented Programming Strategy Pattern

• Drawbacks

–

Clients must be aware of different Strategies.

• So that a client can select the appropriate strategy

–

Communication overhead between Strategy and

Context.

–

Increased number of objects.

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Software Development and Test Laboratory ◆Department of Computer Science and Information Engineering ◆National Taipei University of Technology

/ 20

• Defining the Strategy and Context interfaces.

–

Context pass data to Strategy operations.

• Decoupled.

• Might pass unneeded data.

–

Context passed itself as an argument.

• Strategy requests exactly what it needs.

• Must define a more elaborate interface.

• Tightly coupled.

• Making Strategy objects optional

–

context implements a default Strategy

–

client select Strategy only when default is no good

17
Object-Oriented Programming Strategy Pattern

• Flyweight

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Software Development and Test Laboratory ◆Department of Computer Science and Information Engineering ◆National Taipei University of Technology

/ 20

• See “Sorter” sample code

19
Object-Oriented Programming Strategy Pattern

Client

Context

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PatternStrategypdf

<b>國立台北科技大學 陳偉凱</b>

•

Define a family of algorithms.

–

Encapsulate each one (as an

<b>object</b>

).

–

Make them

<b>interchangeable</b>

(through

polymorphism).

–

<b>Vary independently</b>

from clients that use it

•

<b>Composition</b>

keeps a list of words and handles

<b>line breaking </b>

of the words

<i><b>Composition</b></i>

<b>Repair()</b>

<b>Simple</b>

<b>Tex</b>

<b>…</b>

•

A possible design

<i><b>Composition</b></i>

<b>Repair()</b>

void Composition::Repair()

{

switch (CompositionType)

{

case SIMPLE:

…

break;

case TEX:

…

break;

case ARRAY:

…

break;

}

}

<b>Line breaking </b>

<b>algorithm</b>

<b>Line breaking </b>

•

A better design

<i><b>Composition</b></i>

<b>SimpleComposition()</b>

<b>TexComposition()</b>

<b>ArrayComposition()</b>

<b>Repair()</b>

void Composition::Repair()

{

switch (CompositionType)

{

case SIMPLE:

SimpleComposition();

break;

case TEX:

TexComposition();

break;

case ARRAY:

ArrayComposition();

break;

}

}

<b>Line breaking algorithm</b>

<b>Line breaking algorithm</b>

•

<b>Problems</b>

–

<b>Hard-wiring</b>

line breaking algorithms

•

Clients (Composition) get

<b>bigger</b>

<b>國立台北科技大學陳偉凱</b>