The StrategyDesign Pattern

© Allan C. Milne

v15.2.6

The one constant in software development …

CHANGE.

Inflexibility of rendering.

• Consider what requires to be changed if we wish to render a map on a different display technology …– The existing TiledMap code must be changed by

replacing the implementation of the render() method.

• Always a bad idea to change our existing code base to allow for new requirements:– may compromise related implementation and/or

functionality;– results in multiple versions;– complicates future maintenance.

The strategy design pattern …

• … defines a family of algorithms.• … encapsulates each algorithm.• … makes algorithms interchangeable.• … allows the algorithm to vary

independently of the clients that use it.

• In our scenario the family of algorithms are the different implementations of the map rendering behaviour.

The strategy pattern UML.

MyClass

… … …algorithm

… … …DoIt()

<<interface>> AlgorithmFamily

Algorithm()

Class2

Algorithm()

Class1

Algorithm()

Designing render behaviour.

TiledMap

… … …mRenderer

… … …setRenderer (MapRenderer)render()

<<interface>> MapRenderer

render (TiledMap)

ConsoleRenderer

render (TiledMap)

AudioRenderer

render (TiledMap)

Applies design principles …

• Identify aspects of the application that vary and separate them from what stays the same.

• Design to an interface, not an implementation.

• Favour composition over inheritance.

So how does this work?

• We create a TiledMap object and the constructor …– … assigns a supplied MapRenderer object.

• Thus each TiledMap type object may have its own rendering behaviour instantiated outside the TiledMap object.

• The aim here is to maintain the maximum flexibility in the design.

The flexibility of the design.

• Dynamically change the implementation of a behaviour of an object.

• Amend the concrete implementation of a behaviour.

• Allows us to add new implementations of a behaviour.

• All without changing any of the TiledMap class.

Implementing rendering …interface MapRenderer

{void render (TiledMap

aMap);}

class ConsoleRenderer implements MapRenderer { public void render (TiledMap aMap) { System.out.println (… …); … … … aMap.get(x,y) … … … }

} // end ConsoleRenderer class.

… and in the map class …

class tiledMap{ private MapRenderer mRenderer;

… … …

public void setRenderer (MapRenderer aRenderer) { mRenderer = aRenderer; }

public void render () { mRenderer.render (this);}

} // end tiledMap class.

The objects when executed … TiledMap map; map = new TiledMap (…, new ConsoleRenderer() );

map

mRenderer

…

render()

…(TiledMap object)

Render(this)

(ConsoleRenderer object)

call TiledMap(…) constructor method

Now follow the code …

… … … map.render();

map.setRenderer(new JPanelRenderer());

… … … map.render(); … … …