PatternStrategypdf

Software Development and Test Laboratory ◆ Department of Computer Science and Information Engineering ◆ National Taipei University of Technology. / 20 • Strategy (Compositor)[r]

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

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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 independentlyfrom clients that use it

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/ 20 • Compositionkeeps 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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• 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 Line breaking algorithm

Object-Oriented Programming Strategy Pattern

• Problems

–Hard-wiringline breaking algorithms

• Clients (Composition) get biggerand 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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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)

• Solution: apply Strategy pattern Composition

Traverse () Repair ()

compositor

Compose ()

Compositor

Compose ()

SimpleCompositor

Compose ()

TexCompositor

Compose ()

ArrayCompositor

Compositor -> Compose ()

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• 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

• Solution: apply strategy pattern

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• Many related classes differonly in their behavior.

• You need different variants of an algorithm. • An algorithm uses datathat 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

ConcreteStrategyB

ConcreteStrategyC

AlgorithmInterface() Client

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/ 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.

• 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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• 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

• 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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• 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

• Flyweight

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• See “Sorter” sample code

Client Client

Context Context

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