Session 07: C# Design Patterns Composite Pattern Iterator Pattern Interfaces in the libraries FEN 2013-04-01 AK - IT: Softwarekonstruktion

Example: Composite-pattern Composite: Graphical editor, Word processor, Inventories etc.. (What will a Show()-method look like on Shape, Circle, Picture etc.) FEN 2013-04-01 AK - IT: Softwarekonstruktion

AK - IT: Softwarekonstruktion public abstract class Shape { private int id; private string color; private int xpos; private int ypos; public void MoveTo(Position newPos){ // PRE none // POST pos'=newPos } public abstract void Show(); // POST the shape is displayed public abstract void Hide(); // POST the shape is hidden   public abstract float Area(); // POST Computes the area with 4 digits. }//end Shape FEN 2013-04-01 AK - IT: Softwarekonstruktion

AK - IT: Softwarekonstruktion public class Circle: Shape{ private int r; //radius public void Show(){ //PRE none //POST the circle is drawn //Must be implemented using appropriate //graphical routines } public void Hide(){ //PRE none //POST the circle is hidden // More operations }//end Circle FEN 2013-04-01 AK - IT: Softwarekonstruktion

Static type Shape – and Picture is a Shape itself! public class Picture : Shape{ List<Shape> pictList; // operations for adding, deleting etc. shapes public void Show(){ //PRE none //POST The composite shape is drawn for(Shape s : pictList) s.Show(); } public float Area() { float result=0; for(int i=0;i<pictList.Count;i++) { result= result+pictList[i].Area(); return result; }//end Picture Static type Dynamic type definesshow() FEN 2013-04-01 AK - IT: Softwarekonstruktion

AK - IT: Softwarekonstruktion Composite Pattern The concept of patterns originates from architecture (Christopher Alexander, 1977): “Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice” (Christopher Alexander e. a.: “A Pattern Language”. Oxford University Press, New York, 1977.) FEN 2013-04-01 AK - IT: Softwarekonstruktion

AK - IT: Softwarekonstruktion (OO) Design Patterns A well known and widely accepted concept in software engineering Developed in the early 1990s and published by Gamma e.a. (“Gang of Four”, GoF) in 1995: “(…) design patterns (…) are descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context.” (Erich Gamma e.a.:”Design Patterns. Elements of Reusable Object-Oriented Software”. Addison-Wesley. 1995.) FEN 2013-04-01 AK - IT: Softwarekonstruktion

The benefits of patterns A pattern captures a proven good design: A pattern is based on experience A pattern is discovered – not invented It introduces a new (and higher) level of abstraction, which makes it easier: to talk and reason about design on a higher level to document and communicate design One doesn’t have to reinvent solutions over and over again Patterns facilitate reuse not only of code fragments, but of ideas. FEN 2013-04-01 AK - IT: Softwarekonstruktion

Patterns as a learning tool It is often said that good skills in software construction require experience and talent …and neither can be taught or learned at school Patterns capture experience (and talent) in a way that is communicable and comprehensible …and hence experience can be taught (and learned) So one should put a lot of effort in studying patterns FEN 2013-04-01 AK - IT: Softwarekonstruktion

AK - IT: Softwarekonstruktion More Examples Organisational Structures demos\RecursiveDirectoryTraverse A directory is a collection of elements which are either files or directories themselves. FEN 2013-04-01 AK - IT: Softwarekonstruktion

AK - IT: Softwarekonstruktion Exercises Exercise 4 on Session07.docx. FEN 2013-04-01 AK - IT: Softwarekonstruktion

AK - IT: Softwarekonstruktion Interfaces An interface may be seen as a purely abstract class. Interface: only method signatures, no implementation! (All methods are abstract) An interface represents a design. Example: Designing an object that can be used for iterating over a data structure. Also as generic public interface IEnumerator { void Reset(); // reset iterator to beginning bool MoveNext(); // advance to next element object Current { get; } // retrieve current element } FEN 2013-04-01 AK - IT: Softwarekonstruktion

Example – Iterator Pattern This piece of client code iterates over any data structure that implements IEnumerator: data structure iterator IEnumerator iter; iter = ...; // get ref to iterator object iter.Reset(); while ( iter.MoveNext() ) MessageBox.Show( iter.Current.ToString() ); See it work! FEN 2013-04-01 AK - IT: Softwarekonstruktion

Different Collections List: Array-based (static) Supports access by position Uses extra memory Expensive when re-allocating LinkedList: Linked list (dynamic, no extra memory) Doesn’t support access by position. HashSet (Dictionary): Array-based (static) Supports fast access (in average) by key – slow in worst case Uses extra memory Expensive when re-allocating SortedSet (SortedDictionary): Search tree (dynamic, no extra memory) Supports fast access (logarithmic) in average and worst case. FEN 2013-04-01 AK - IT: Softwarekonstruktion

In .NET interfaces are used heavily. IComparable ICloneable IDisposable IEnumerable & IEnumerator IList ISerializable IDBConnection, IDBCommand, IDataReader etc. FEN 2013-04-01 AK - IT: Softwarekonstruktion

AK - IT: Softwarekonstruktion Typical Design in APIs Specification in the interface. Implementation in the concrete classes. Operations with common implementation are implemented in the abstract class (no code duplication). Operation specific for the different concrete classes are left abstract in the abstract class. FEN 2013-04-01 AK - IT: Softwarekonstruktion

AK - IT: Softwarekonstruktion 8. Interfaces January 2003 Example: Sorting Goal: To write a generic Sort() method as the one found in System.Array Recall that System.Array class contains a static Sort() method that can sort any kind of array… FEN 2013-04-01 AK - IT: Softwarekonstruktion Denmark .NET Workshop © 2003 Joe Hummel

Step 1: Define the interface Sorting requires a way of comparison of objects: returns < 0 if this object < obj parameter returns == 0 if this object = obj parameter returns > 0 if this object > obj parameter public interface IComparable<T> { int CompareTo(T obj); } FEN 2013-04-01 AK - IT: Softwarekonstruktion

Step 2: Classes must implement the interface 8. Interfaces Step 2: Classes must implement the interface January 2003 Objects That are to be sorted must implement IComparable><> Example: sort Student objects on id public class Student : Icomparable<Student> { private int m_ID; . int IComparable.CompareTo(Student other) return this.m_ID – other.m_ID; } interface Student type Recall from earlier lecture that Student class inherited from Person class… FEN 2013-04-01 AK - IT: Softwarekonstruktion Denmark .NET Workshop © 2003 Joe Hummel

Step 3: Clients program towards the interface 8. Interfaces January 2003 Step 3: Clients program towards the interface Sort assumes that the elements in the array a implement IComparable: public class Array { public static void Sort(Array a) IComparable icobj; for (int i = 0; i < a.Length-1; i++) { for (int j = i+1; j < a.Length; j++) { icobj = (IComparable) a.GetValue(i); if (icobj.CompareTo(a.GetValue(j)) > 0) swap(a, i, j); }//for } Not a very efficient sort, but you get the idea — this method will sort any array of objects, as long as those objects implement IComparable. FEN 2013-04-01 AK - IT: Softwarekonstruktion Denmark .NET Workshop © 2003 Joe Hummel

AK - IT: Softwarekonstruktion Step 4: test! Example: sort an array of Student objects View Code FEN 2013-04-01 AK - IT: Softwarekonstruktion

AK - IT: Softwarekonstruktion Exercises Exercise 5 on Session07.docx. FEN 2013-04-01 AK - IT: Softwarekonstruktion