2. The Strategy Design Pattern © Allan C. Milne v15.2.6

3. The one constant in software development … CHANGE.

4. 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.

5. 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.

6. The strategy pattern UML. MyClass … … … algorithm … … … DoIt() > AlgorithmFamily Algorithm() Class2 Algorithm() Class1 Algorithm()

7. Designing render behaviour. TiledMap … … … mRenderer … … … setRenderer (MapRenderer ) render() > MapRenderer render (TiledMap) ConsoleRenderer render (TiledMap) AudioRenderer render (TiledMap)

8. 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.

9. 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.

10. 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.

11. 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.

12. … 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.

13. The objects when executed … TiledMap map; map = new TiledMap (…, new ConsoleRenderer() ); map mRenderer … render( ) … (TiledMap object) Render(this) (ConsoleRenderer object) call TiledMap(…) constructor method

14. Now follow the code … … … … map.render(); map.setRenderer(new JPanelRenderer()); … … … map.render(); … … …