Custom ValueChangeHandler GWT

Question

I need to write a custom ValueChangeHandler and call out onValueChange(ValueChangeEvent). However I don't understand how to write a ValueChangeEvent.

Maybe I understand the entire GWT event system wrong. Can anyone help?

Edit: I am asking how to create my own class that dispatches the ValueChangeEvent. I understand how to listen to it.

The constructor of ValueChangeEvent is not visible and I can't create it.

Answer 1

If you want to fire a ValueChangeEvent you must implement the interface HasValueChangeHandlers by or your class or somewhere in the class.

A simple implementation would be to use the EventBus:

EventBus bus = new SimpleEventBus();

@Override
public void fireEvent(GwtEvent<?> event) {
    bus.fireEvent(event);
}

@Override
public HandlerRegistration addValueChangeHandler(ValueChangeHandler<T> handler) {
    return bus.addHandler(ValueChangeEvent.getType(), handler);
}

Note you need to substitute T with the type you want to fire.

Because you can't create a ValueChangeEvent directly dispatching an event is done via the fire method:

ValueChangeEvent.fire(this, value);

Where this refers to the class/field implementing the HasValueChangeHandlers and value refers to the value that has been changed and you want to dispatch the event.

Answer 2

Actually, instead of creating a new EventBus or HandlerManager, as your widget will be a subclass of Wiget, it the standard way would be to use the Widget.addHandler(handler, eventType) method. Here's a minimal example:

    public class MyWidget<T> extends Composite implements HasValueChangeHandlers<T>, HasValue<T> {

    private T value;

    public MyWidget() {
        // Initialize stuff
    }

    @Override
    public HandlerRegistration addValueChangeHandler(final ValueChangeHandler<T> handler) {
        return addHandler(handler, ValueChangeEvent.getType());
    }

    @Override
    public T getValue() {
        return value;
    }

    @Override
    public void setValue(T value) {
        setValue(value, false);
    }

    @Override
    public void setValue(T value, boolean fireEvents) {
        this.value = value;
        if (fireEvents) {
            ValueChangeEvent.fire(this, getValue());
        }
    }
}

Answer 3

Really late to answer, but I've just solved this problem like this:

ValueChangeEvent.fire(hasValueChangeHandlerInstance, getValue());