listening...

[abc123]

I want to monitor the state of some of my objects' fields' for change, and when it does change I want to do something, so instead of constantly checking if it had changed with if statements, etc, I thought it would be a good idea to add a propertyChangeListener() (or any other appropriate listener) to it... now, this object isn't related to graphics in anyway, its just an object so i don't know how to do it..

any ideas? hints on where to start?

[abc123]

figured this one out too.

[technobard]

Inquiring minds want to know, what was the solution? I didn't think it was possible without a gui.

[abc123]

well, this was my test code:

import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeSupport;
import java.beans.PropertyChangeEvent;
import javax.swing.event.SwingPropertyChangeSupport;

class test
{

	PropertyChangeSupport changeSupport = new PropertyChangeSupport(this);
	int x = 50;
	long y = 0;


	public void addPropertyChangeListener(String propertyName, PropertyChangeListener listener){
		if(listener == null){
			return;
		}
		if (changeSupport == null){
		    changeSupport = new PropertyChangeSupport(this);
		}
		changeSupport.addPropertyChangeListener(propertyName, listener);
    }

    public void firePropertyChange(String name, int oldVal, int newVal){
        if ((changeSupport != null) && (oldVal != newVal)) {
            changeSupport.firePropertyChange(name, new Integer(oldVal), new Integer(newVal));
		}
	}

	public void setX(int x){
		if(this.x == x){
			System.out.println("not changed.");
			return;
		}
		int oldx = this.x;
		this.x = x;
		changeSupport.firePropertyChange("x", oldx, x);
		System.out.println(x);
	}

	public static void main(String[] s){
		test t = new test();
		t.setX(50);
	}

	public test(){
		this.addPropertyChangeListener("x", new PropertyChangeListener(){ public void propertyChange(PropertyChangeEvent pce){ System.out.println("i was changed"); }});
	}

}

i basically looked through the source of JComponent, Container, etc, for how to do it in the java.src zip file

but upon realizing how its done, im not so sure if i need it anymore, because it really is only uesd to notify other objects of the its' states' changing, not itself, or something, and that is what that test prog is doing, but it may be useful.

[technobard]

Okay. So you create a manager object (changeSupport) and you register objects that have members that want to be monitored with the manager. Then you add/create listeners (PropertyChangeListener) to listen for events around the registered object member.

As part of the listener creation, you specify an action that occurs when a property changes by specifying the propertyChange(PropertyChangeEvent pce) method. Presumably you could modify this to include a method call on the listening object that would update totals, for example, that included the changed object.

I could see this being useful. I also understand why it is normally tied to a GUI since it would make sense to refresh a display after updating values.

Thanks for sharing the code.

[abc123]

yea, i decided it would only be useful for 3+ objects, A, B, C where A has the ".addEtc" and B and C do not know when the other(s) will be changing A so they need listeners... this is NOT the case with my objects, so i probably don't need it anyway, good to know still.