CODEWITHSUNDEEP
javadesign-patternsswtstack-overflowevent-listener
Adam Horvath
Adam Horvath

Reputation: 1286

SWT: Synchronously update field's text with ModifyListeners attached

Suggestions for a better title are welcome!

There are two fields (net- and gross price), both of which are editable. They have their own ModifyListeners attached, updating the other Text's value upon the percentage of applicable VAT.

The dialog So that's the theory.

The reality is that when any of the listeners set the other Text's text property, the other listener triggers, which triggers the caller Text's listener and so on till it reaches StackOverflow. :-)

Question: Is there a well-known way/pattern to avoid this behaviour?

I can only think of a field in the Dialog's class let's say:

boolean stopOverflow; // Stops Listeners from triggering each other

And make the listeners checking for it; setting it true/false whether we're already in a Modification event. Hoping SWT is single threaded.

A similar question can be found here:

How to limit ModifyListener for user interaction only

Upvotes: 1

Views: 173

Answers (2)

DRCrain
DRCrain

Reputation: 42

There's a more simple solution. It needs no additional field in the Dialog's class and is only one additional if.

private Text netPrice; // Let's say this holds your net-price text

netPrice.addModifyListener(new ModifyListener() {
    @Override
    public void modifyText(ModifyEvent e) {
        if (netPrice == Display.getCurrent().getFocusControl()) {
            //
            // Do your logic here, it's overflow safe
            //
        }
    }
});

Implement your gross-price Text the same way.

Upvotes: 1

greg-449
greg-449

Reputation: 111217

Calling removeModifyListener before doing the setText and then calling addModifyListener afterwards is one of the usual ways to approach this.

Keep the modify listener as a field in the dialog class so you can keep track of it:

private ModifyListener modifyListener = new MyModifyListener();

...

text.removeModifyListener(modifyListener);

text.setText(....);

text.addModifyListener(modifyListener).

Upvotes: 2

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