- another example of a structural pattern

Nov 2005 MSc Slide

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- Another Example of a Structural Pattern

Alternative way of adding Functionality to an existing class (alternative to a derived class)

Favors Composition over Inheritance

Often referred to as a Wrapper

Advantage: No need for several levels of Inheritence (reduces Complexity)

Decorator Pattern

Nov 2005 MSc Slide

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Instead of:

Decorator Pattern

X

Y

Z Z p = new Z();

X Y Z

Z p=new Z(new Y(new X()));

Can add functionality by adding more Objects

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Typically used to change appearanceof a Visual Component

One big advantage is that you can dynamically change appearance/functionality of a component

Decorator Pattern

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Decorator Pattern

Want to decorate a Button

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using System;using System.Windows.Forms;using System.Drawing;

abstract class Decorator : Panel { protected Control ctrl; public Decorator(Control c) {

this.Size=c.Size; ctrl=c; Controls.Add(c); c.Paint += new PaintEventHandler( paint); }

virtual public void paint(object sender, PaintEventArgs pe){}}

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class SlashDecorator : Decorator {

public SlashDecorator(Control c):base(c) {}

override public void paint(object sender, PaintEventArgs pe){ Pen aPen = new Pen(Color.Black , 1); Graphics g = pe.Graphics;

int x2 = this.Size.Width; int y2 = this.Size.Height; g.DrawLine(aPen,0,0,x2,y2);

} }

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class GFrame:Form {private Button cbutton=new Button();private Button dbutton=new Button();private SlashDecorator d;public GFrame():base(){

cbutton.Text="cbutton"; dbutton.Text="dbutton"; d=new SlashDecorator(dbutton); cbutton.SetBounds(10,10,cbutton.Size.Width, cbutton.Size.Height); d.SetBounds(100,10,d.Size.Width,d.Size.Height); Controls.Add(cbutton); Controls.Add(d);

}}

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public class Test92{public static void Main(string[] args){

Application.Run(new GFrame());}

}

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Example 2:

This time we have a Decorated Label

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using System;using System.Windows.Forms;using System.Drawing;

abstract class Decorator : Panel { protected Control ctrl; public Decorator(Control c) {

this.Size=c.Size; ctrl=c; Controls.Add(c); c.Paint += new PaintEventHandler( paint); }

virtual public void paint(object sender, PaintEventArgs pe){}}

As before

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class CoolDecorator : Decorator {

public CoolDecorator(Control c):base(c) { }

override public void paint(object sender, PaintEventArgs pe){ Pen aPen = new Pen(Color.Red , 2);

Graphics g = pe.Graphics; int x2 = this.Size.Width;

int y2 = this.Size.Height; g.DrawRectangle(aPen,0,0,x2,y2);

} }

Draw a Rectangle

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class GFrame:Form {private Label l=new Label();private CoolDecorator c;

public GFrame():base(){ l.Text="MyLabel"; c=new CoolDecorator(l); c.SetBounds(10,10,c.Size.Width, c.Size.Height-5); Controls.Add(c);

}}

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public class Test92{public static void Main(string[] args){

Application.Run(new GFrame());}

}

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Now to add Dynamic Behaviour, change Colour to Blue

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class CoolDecorator : Decorator { bool mouse_over=false;

public CoolDecorator(Control c):base(c) { EventHandler evh = new EventHandler(mouseEnter); ctrl.MouseEnter+= evh; ctrl.MouseLeave += new EventHandler(mouseLeave); } public void mouseEnter(object sender, EventArgs e){ mouse_over = true; ctrl.Refresh (); } public void mouseLeave(object sender, EventArgs e){ mouse_over = false; this.Refresh (); }

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override public void paint(object sender, PaintEventArgs pe){ Pen aPen = new Pen(Color.Red , 2); Pen bPen = new Pen(Color.Blue , 2); Graphics g = pe.Graphics;

int x2 = this.Size.Width; int y2 = this.Size.Height; g.DrawRectangle(aPen,0,0,x2,y2); if (mouse_over == true){ g.DrawRectangle(bPen,0,0,x2,y2);} } }

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UML Class DiagramControl

CoolDecorator SlashDecorator

GFrame

Decorator Control

Panel

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Big Advantage of Decorator(Wrapper) is that you can addfunctionality by adding Objects

Label lab=new Label(); lab.Text="Example"; SlashDecorator c=new SlashDecorator(lab); Controls.Add(c);

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Combining Decorators

c=new CoolDecorator(b);Normally:

Want: c=new SlashDecorator(new CoolDecorator(b));

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Combining Decorators

c=new CoolDecorator(null, b);Normal Wrapper:

Embedded Wrapper:

sd=new SlashDecorator(new CoolDecorator(null,b),b);

In practice new a ref to parent and underlying control:

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abstract class Decorator : Panel {protected Control ctrl; public Decorator(Control c) {

this.Size=c.Size; ctrl=c; Controls.Add(c); c.Paint += new PaintEventHandler( paint); }

virtual public void paint(object sender, PaintEventArgs pe){}}

As before

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class CoolDecorator : Decorator { bool mouse_over=false; SlashDecorator sdec;

public CoolDecorator(SlashDecorator sdec,Control c) :base(c) { this.sdec = sdec; if (sdec !=null) c.Paint += new PaintEventHandler( sdec.paint); EventHandler evh = new EventHandler(mouseEnter); ctrl.MouseEnter+= evh; MouseEventHandler(mouseMove); ctrl.MouseLeave += new EventHandler(mouseLeave); }

override public void paint(object sender, …

As before

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class SlashDecorator : Decorator { private CoolDecorator cd=null;

public SlashDecorator(CoolDecorator cd,Control c) :base(c) {

this.cd=cd; if (cd !=null)

c.Paint += new PaintEventHandler( cd.paint);}

override public void paint(object sender, …

As before

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class GFrame:Form {private Label l=new Label();private SlashDecorator sd;

public GFrame():base(){ l.Text="MyLabel";

sd=new SlashDecorator(new CoolDecorator(null,l),l);

sd.SetBounds(100,10,sd.Size.Width,sd.Size.Height); Controls.Add(sd); this.Refresh();}

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Exercise 0:

Modify the first Decorator (test0.java) so its in the following format

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Exercise 1: Modify Exercise 0 to include some dynamic Behaviour

Red X