Chapter 13' Graphical User Interface Concepts: Part 1 Delegates

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Chapter 13. Graphical User Interface Concepts
Part 1Delegates
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Delegates

• Simplest description type-safe function pointers
• Declaring a delegate defines a class that derives
  from System.MulticastDelegate, which derives from
  System.Delegate

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Delegates

• Sometimes useful to pass methods as arguments to
  other methods
• Delegates are sets of references to methods
• Delegate objects can be passed to methods
  methods can then invoke the methods the delegate
  objects refer to
• Delegates that contain one method are known as
  singlecast delegates and are created or derived
  from class Delegate
• Delegates that contain multiple methods are known
  as multicast delegates and are created or derived
  from class MulticastDelegate

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Delegates

• Delegates must be declared before use
• A delegate declaration specifies the parameters
  and return type of the methods the delegate can
  refer to
• Methods who can be referred to by a delegate,
  must have the same signature as the delegate
• Delegate instances can then be created to refer
  to a method(s)
• Once a delegate instance is created, the method
  it refers to can be invoked

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Delegates

• Delegates maintain a list of methods to be called
  when the delegate is invoked.
• To insure type safety, the signature (return type
  and arguments) of acceptable callback
• methods is specified when the delegate class is
  defined.
• We instantiate a delegate by "wrapping" the
  callback method.
• We can add methods to the callback chain or
  remove methods from the chain.

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Using Delegate

• First must declare one.
• The delegates declaration specifies a method
  header (parameters and return value).
• Methods whose references will be contained within
  a delegate object must have the same method
  header as that defined in the delegate
  declaration.
• Second create methods that have this signature.
• The third step is to create a delegate instance
  that contains a reference to that method.
• Finally we can invoke the method reference that
  it contains. We show this process in our next
  example.

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• using System
• delegate int MathOp(int x, int y) //Declare
  delegate type
• class Class1
• static void Main(string args)
• MathOp f1 new MathOp(Class1.Add)
  //Instantiate
• MathOp f2 new MathOp(Class1.Mult)//delegates
• Console.WriteLine(DoSomeMath(f1, 2, 3))
• Console.WriteLine(DoSomeMath(f2, 2, 3))
• public static int DoSomeMath(MathOp fn, int x,
  int y)
• return fn(x, y) //Invoke the delegate
• public static int Add(int a, int b)
• return a b

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• class DemoClass
• private string name
• private int number
• public DemoClass(string str, int x)
• name str
• number x
• public void ShowMyName()
• Console.WriteLine("My name is " this.name)
• public void ShowMyNumber()
• Console.WriteLine("My number is "
  this.number)

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• delegate void DemoOp() //Declare delegate type
• static void Main(string args)
• //Create some objects
• DemoClass dc1 new DemoClass("Demo 1", 1)
• DemoClass dc2 new DemoClass("Demo 2", 2)
• //Declare an empty delegate
• DemoOp demoDel null
• //Append a method to list
• demoDel new DemoOp(dc1.ShowMyNumber)
• //Append another one
• demoDel new DemoOp(dc2.ShowMyName)
• //Invoke the delegate
• demoDel()

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• class DemoClass
• private string name
• private int number
• public DemoClass(string str, int x)
• name str
• number x
• public void ShowMyName()
• Console.WriteLine("My name is " this.name)
• public void ShowMyNumber()
• Console.WriteLine("My number is "
  this.number)

What is "this" when the method is called via
a delegate?
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• delegate void DemoOp() //Declare delegate type
• static void Main(string args)
• //Create some objects
• DemoClass dc1 new DemoClass("Demo 1", 1)
• DemoClass dc2 new DemoClass("Demo 2", 2)
• //Declare an empty delegate
• DemoOp demoDel null
• //Append a method to list
• demoDel new DemoOp(dc1.ShowMyNumber)
• //Append another one
• demoDel new DemoOp(dc2.ShowMyName)
• //Invoke the delegate
• demoDel()

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Delegates Behind the Scenes