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Object Oriented Programming
Interfaces, , Callbacks Delegates and Events
Dr. Mike SpannDr. Mike Spann
m.spann@bham.ac.ukm.spann@bham.ac.uk
Contents
Introduction to interfacesIntroduction to interfaces Example – An Example – An IMeasureableIMeasureable interface interface Callback functionsCallback functions DelegatesDelegates Events and event handlersEvents and event handlers SummarySummary
Introduction to interfaces We have already seen how an abstract base class links We have already seen how an abstract base class links
related related derived class through inheritance derived class through inheritance
The overridden virtual methods of the abstract base The overridden virtual methods of the abstract base class must be implemented in the derived classesclass must be implemented in the derived classes
There are also occasions when we want There are also occasions when we want unrelatedunrelated classes to exhibit similar behaviourclasses to exhibit similar behaviour
For example we may want to be able to implement For example we may want to be able to implement a sorting algorithm on unrelated classesa sorting algorithm on unrelated classes
Introduction to interfaces
For example, we may want to sort objects of class Square on the basis of the length of their side
We could easily do this through We could easily do this through polymorphism by implementing an abstract polymorphism by implementing an abstract base class base class SortableSortable and using similar and using similar generic programming techniques we looked generic programming techniques we looked at in the last lectureat in the last lecture But, C# (along with Java) doesn’t But, C# (along with Java) doesn’t
support multiple inheritancesupport multiple inheritance
Introduction to interfaces
Shape
Square
Sortable
Introduction to interfaces
An interface is simply a list of public methods that any class which implements that interface must provide implementations of
Unrelated classes can implement a single interface In our example, we can define a ISortable
interface such that objects of any class which implements this interface can be sorted on the basis of some comparative measure
Our Square class can implement the ISortable interface using the length of side as the comparative measure
Introduction to interfaces
public interface ISortable{ int compareTo(ISortable s);}
Our ISortable interface contains a single public method compareTo() which defines how the comparison is made This method is overridden in classes which
implement this interface
Introduction to interfaces
compareTo(ISortable b) returns –1,0 or 1 depending on whether some chosen instance field f of a ISortable object is such that : this.f<b.f this.f==b.f this.f>b.f
We can implement this method in class Square to sort on the basis of length of side
Introduction to interfacespublic class Square : Shape, ISortable{ private int side;
public Square(int s) { side = s; }
public override void draw() { }
public override double area() { return side * side; }
public int compareTo(ISortable s) { Square sq=(Square)s;
if (side<sq.side) return -(1); if (side > sq.side) return 1;
return 0; }}
Introduction to interfaces
We can provide a ShellSort() method which implements the shell sort algorithm on arrays on ISortable objectsThe compareTo() method of the objects
implementing the interface is called inside ShellSort()
Introduction to interfacespublic class ArrayAlg{ public static void shellSort(ISortable[] a) { int n = a.Length; int incr = n / 2; while (incr >= 1) { for (int i = incr; i < n; i++) { ISortable temp = a[i]; int j = i; while (j >= incr && temp.compareTo(a[j - incr]) < 0) { a[j] = a[j - incr]; j -= incr; } a[j] = temp; } incr /= 2; } }}
Introduction to interfaces
To test our program we simply create an array of Square objects and pass it into the ShellSort() methodBecause of the cast in the compareTo()
method in Square, the correctly implemented compareTo() method is called through polymorphism
Introduction to interfaces
public class SortablesTest{ static void Main(string[] args) { Square[] s = new Square[3]; s[0] = new Square(3); s[0] = new Square(2); s[0] = new Square(1);
ArrayAlg.shellSort(s);// Sorts on length of side }}
An An IMeasureableIMeasureable interface interface
Suppose we have a DataSet class which computes simple statistics of numbers read from an input streamFor example, the average and maximum
DataSetaverage
maximum
Input stream
public class DataSet{ public DataSet() { sum = 0.0; maximum = 0.0; count = 0; }
public void add(double x) { sum += x; if (count == 0 || maximum < x) maximum = x; count++; }
public double getAverage() { if (count == 0) return 0; else return sum / count; }
public double getMaximum() { return maximum; }
private double sum, maximum; private int count;}
An An IMeasureableIMeasureable interface interface
Clearly we would have to modify the DataSet class if we wanted to get the average of a set of bank account balances or to find the coin with the highest value amongst a set DataSet is not re-useable as it stands However, if all classes that DataSet objects
operate on implement an IMeasurable interface, then the class becomes more flexible
An An IMeasureableIMeasureable interface interface
Thus getMeasure() for BankAccount objects return the balance and for Coin objects returns the coin value
public interface IMeasureable{
double getMeasure(); }
An An IMeasureableIMeasureable interface interface
public class BankAccount: IMeasureable{
private double balance;private int accountNumber;
.
.double getMeasure() { return balance;}
}
public class Coin: IMeasurable{ private double value;
.
.double getMeasure() { return value;}
}
An An IMeasureableIMeasureable interface interface
The IMeasurable interface expresses the commonality amongst objectsThe fact that each measurable objects can
return a value relating to its size DataSet objects can then be used to analyse
collections of objects of any class implementing this interface with minor modifications to the code
public class DataSet{ public DataSet() { sum = 0.0; count = 0; } public void add(IMeasureable x) { sum += x.getMeasure(); if (count == 0 || maximum.getMeasure() < x.getMeasure()) maximum = x; count++; }
public double getAverage() { if (count == 0) return 0; else return sum / count; }
public double getMaximum() { return maximum.getMeasure(); }
private IMeasureable maximum; private double sum; private int count;
}
An An IMeasureableIMeasureable interface interface
class MeasureablesTest{ static void Main(string[] args) { DataSet d=new DataSet(); Coin c1=new Coin(10); Coin c2=new Coin(20); d.add(c1); d.add(c2); double maxCoin=d.getMaximum(); System.Console.WriteLine("coin max= " + maxCoin); }}
Callback functionsCallback functions
The DataSet class is useful as a re-usable class but is still limited The IMeasurable interface can only be
implemented by user defined classes We can’t, for example, find the maximum of a
set of Rectangle objects as Rectangle is a pre-defined class
We can only measure an object in one way. For example, in the case of BankAccount objects, we can only measure it in terms of the balance
Callback functionsCallback functions
The solution is to delegate the measuring to a separate class rather than being the responsibility of the objects we are measuring We can create a separate IMeasurer interface
and implement a measure() method in objects implementing this interface
public interface IMeasurer{
double measure(Object anObject);
}
public class DataSetpublic class DataSet{{
public DataSet(IMeasurer m)public DataSet(IMeasurer m){{
measurer=m;measurer=m;}}
public void add(Object x)public void add(Object x){{
sum=sum+measurer.measure(x);sum=sum+measurer.measure(x);if (count==0 || if (count==0 || mamaximum<measurer.measure(x))ximum<measurer.measure(x))
maximum=measurer.measure(x);maximum=measurer.measure(x);count++;count++;
}}
public double getMaximum()public double getMaximum() {{ return maximum;return maximum; }}
private private IIMeasurer measurer;Measurer measurer; private double sum;private double sum; private int count;private int count; private double maximum;private double maximum;}}
Callback functionsCallback functions
Callback functionsCallback functions
A DataSet object makes a callback to the measure() method of an object implementing the IMeasurer interface when it needs to measure an object (such as checking a bank balance) This is in contrast to calling the getMeasure()
method of an object implementing the IMeasurable interface
We are now free to design any kind of measures on an object of any class For example, we can measure Square objects
by area We require a SquareMeasurer class which
implements the IMeasurer interface
Callback functionsCallback functions
public class Square : Shape{ private int side;
public Square(int s) { side = s; }
public override void draw() { }
public override double area() { return side * side; }
}
public class SquareMeasurer : IMeasurer{
public double measure(Object anObject){
Square sq=(Square) anObject; return sq.area();
}}
Callback functionsCallback functions
class MeasurersTest{ static void Main(string[] args) { IMeasurer m = new SquareMeasurer(); DataSet data = new DataSet(m);
// Add squares to the data set data.add(new Square(5)); // Callback to m.measure() data.add(new Square (30)); // Callback to m.measure()
// Get maximum double max = data.getMaximum(); }}
Callback functionsCallback functions
Adding square objects to data enforces callbacks to the measure() method of the SquareMeasurer object to be made
We have flexibility over our implementation of SquareMeasurer so that any feature of Square objects can be measured Or even defining several measurer classes to measure
different features Callbacks are used extensively in building graphical user
interfaces A callback function is added to an event object which is
called when the event is triggered A managed way of doing this is to encapsulate the
callback function into a delegate object
Delegates
A delegate object holds a reference to a method with a pre-defined signature A signature is simply the argument list and
return type of the method The keyword delegate specifies that we are
defining a delegate object For example we can define a delegate object
myDelegate which holds a method which returns void and takes an int and a double as arguments
public delegate void myDelegate(int arg1, double arg2)
Delegates A delegate object is initialized with a
(callback) method The method signature must match the
delegate signature
public delegate void myDelegate(int arg1, double arg2);
public class App{
public static void Main(){
myDelegate call = new myDelegate(aMethod);
// We can now treat call as a simple object and pass // it around as such
}
static void aMethod(int k, double x) {}}
Delegates
Delegate objects can be initialized with several method calls using the += operator The method calls can then be invoked in a chain by
passing the correct arguments to the delegate object Essentially it amounts to calling methods through a
proxy object and is a powerful mechanism for event handling as we shall see
Passing method calls into objects is then equivalent to passing delegate objects
Delegates
myDelegate(int,double)
call
aMethod()
anotherMethod()
Invoked bycall(1,2.0)
aMethod(1,2.0)
anotherMethod(1,2.0)
Delegates
public delegate void myDelegate(int arg1, double arg2);
public class App{
public static void Main(){
myDelegate call = new myDelegate(aMethod); call += new myDelegate(anotherMethod); call(1, 2.0);
}
static void aMethod(int k, double x) { System.Console.WriteLine("aMethod " + k + " " + x); }
static void anotherMethod(int k, double x) { System.Console.WriteLine("anotherMethod " + k + " " + x); }}
Delegates
Events and event handlers C# has built in support for event handling Event handling is usually used in GUI’s such
as to notify an application when a mouse click or button press has occurred
But any type (not just graphical types) can use events to allow notification of any kind
A type must register its interest in handling a specific event with an event handler with a pre-defined signatureThis is achieved using a delegate object
Events and event handlers
A type can define an event and a corresponding event handler which is a delegate object
public class MyType{
public delegate void EventHandler(int arg1, int arg2);
public event EventHandler myEvent; // defines myEvent // and corresponding // event handler
.}
Events and event handlers
An application can then register its interest in an event by adding an initialized delegate object to the event using the += operator
public class App{
public static void Main(){
MyType m = new MyType();m.myEvent += new MyType.EventHandler(myHandler);
}
public static void myHandler(int i1, int i2) {
// Event handler code }
}
Events and event handlers
Equivalent code is very common in GUI development where applications register their own event handlers to respond to events generated by graphical components such as button clicks Normally such code is automatically generated
if we are using visual programming techniques (‘drag and drop’)
However, it is still important to understand how it all works
Events and event handlers
For example the following code snippet registers a graphical applications event handler to respond to button clicks
public class MyGraphicalApp{
Button button = new Button();button.Click += new EventHandler(HandleButtonClick);
public void HandleButtonClick(Object sender EventArgs e) {
// Event handler code }
}
public class Button{
public delegate void EventHandler(.....);
public event EventHandler Click;..
}
Events and event handlers
For example, we can generate our own SafeArray class which fires an event on trying to access it beyond its bounds
We pass the array index into the event handler In our simple application, the event handler
simply prints out an error message In an embedded system application, the array
could be re-allocated to be a larger size in the event handler
Events and event handlerspublic class SafeArray{ public delegate void OutOfBoundsEventHandler(int arg1); public event OutOfBoundsEventHandler myOutOfBoundsEvent; private int[] data; private int numberOfElements=0;
public SafeArray(int n) { numberOfElements = n; data = new int[numberOfElements]; }
public int access(int elem) { if (elem < numberOfElements) return data[elem]; else myOutOfBoundsEvent(elem); // Fire an event
return 0; }}
Events and event handlers
public class App{
public static void Main(){
SafeArray s = new SafeArray(10);
s.myOutOfBoundsEvent += new SafeArray.OutOfBoundsEventHandler(myHandler);
s.access(7);
s.access(11); // Out of bounds event generated!
}
public static void myHandler(int i1) {
System.Console.WriteLine("Index " + i1 + " out of bounds "); }}
Summary
We have seen how objects of unrelated classes can exhibit similar behaviour by implementing an interface
Interfaces support generic programming and avoid multiple inheritance
We looked at a detailed example involving a IMeasurable interface We also introduced callbacks by having a separate measurer
object implementing an IMeasurer interface A callback function was made to a measure() method which
returned some measure on the object passed to it We have seen how delegates are objects which encapsulate
method calls These method calls can be chained using the += operator
We have seen how we can create types which can generate events and how applications can register their interest in events by initializing a delegate object with their own event handler
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