mari göransson - kau - datavetenskap - davd11 1 java beans - events and properties -

Mari Göransson - KaU - Datavetenskap - DAVD11

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Java BeansJava BeansJava BeansJava Beans

- Events and Properties -- Events and Properties -


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Properties• Simple properties• Bound properties• Constrained properties


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Simple Properties• Named attributes that can be read

or written by calling appropriate methods on the JavaBean

• E.g. Getter: String getTextValue()• E.g. Setter: setTextValue(String)• -> property is textValue


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Bound Properties• When changes to the property are

of interest to other beans.• The component provides a change

notification service: public void addPropertyChangeListener(PropertyChangeListener l) public void removePropertyChangeListener(PropertyChangeListener l)


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Bound Properties• Mostly, the listeners are handled

by the class PropertyChangeSupport.

• When a change occurs a PropertyChangeEvent is fired containing the old and new value.


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Constrained Properties• When a bean (listener) may want

to validate a change and reject the change if it is inappropriate.


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Event-mechanism models

• The basic event-delegation mechanism is a specialized version of the Observer pattern.

• JavaBeans introduced another version of the observer pattern; the property change listener.


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Java Event Handling and the Observer

Pattern• The observer pattern is used when

an object wants to be informed of a state change in another object.

• In java, the observer object listens for events to happen.


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Pattern


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Pattern


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JavaBeans and Events


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Property Change Listeners as Observers

• This is a closer representation of the Observer pattern: each observer listens for changes to an attribute in the subject and is informed when the state changes.


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PropertyChangeListeners

• The registered set of listeners is managed within the PropertyChangeSupport class.

• When the watched property changes the support class informs all registered listeners of the old and new value.


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Swing• The Swing components have a

similar event model where many of their properties are bound.


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Bound Property Structure

source

<<use>>

changes

<<create>>

*

PropertyChangeSupport

+addPropertyChangeListener:+removePropertyChangeListener:+firePropertyChange:

SerializableMyJavaBean

+addPropertyChangeListener:+removePropertyChangeListener:+setData:

SerializableMyBoundPropertyListener

+propertyChange:

EventListenerinterface

PropertyChangeListener

EventObjectPropertyChangeEvent

+getPropertyName:String+getNewValue:Object+getOldValue:Object

Object

Or inherit from it

By the classesimplementingthe interface


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Implementing Bound Property Support

• Import the java.beans package. This gives you access to the PropertyChangeSupport class.


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• Instantiate a PropertyChangeSupport object:private PropertyChangeSupport changes =

new PropertyChangeSupport(this);

• This object maintains the property change listener list and fires property change events on your behalf.


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• Implement methods to maintain the property change listener list. Since PropertyChangeSupport implements these methods, you merely wrap calls to the property-change support object's methods:


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public void addPropertyChangeListener( PropertyChangeListener l){ changes.addPropertyChangeListener(l);}

public void removePropertyChangeListener( PropertyChangeListener l){ changes.removePropertyChangeListener(l);}


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• Modify a property's setter method to fire a property change event when the property is changed. Public void setText(String value){ String oldValue = text; text = value; changes.firePropertyChange(”text", oldValue, value);}


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• The firePropertyChange method bundles its parameters into a PropertyChangeEvent object, and calls propertyChange(PropertyChangeEvent pce) on each registered listener.


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Implementing Bound Property Listeners

• Implement the PropertyChangeListener interface.

• public class MyClass implements java.beans.PropertyChangeListener,

java.io.Serializable


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Implementing Bound Property Listeners

• Implement the propertyChange method in the listener.

• public void propertyChange(PropertyChangeEvent e)

• Example taken from: http://java.sun.com/docs/books/tutorial/javabeans/properties/bound.html

mari göransson - kau - datavetenskap - davd11 1 java beans - events and properties -

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