Custom ValueChangeHandler GWT

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 wr开发者_JS百科ong. 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.

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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.

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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());
        }
    }
}

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

ValueChangeEvent.fire(hasValueChangeHandlerInstance, getValue());

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The ValueChangeEvent is generated for you by GWT. You can add one (or more) ValueChangeHandler to any class that implements the interface HasValueChangeHandlers. One of these classes is TextArea, so let's look at a little example:

    TextArea textArea = new TextArea();

    textArea.addValueChangeHandler(new ValueChangeHandler<String>() {

        @Override
        public void onValueChange(ValueChangeEvent<String> event) {
            // do something
        }
    });

When the value of the text area changes, GWT will automatically generate a ValueChangeEvent, which you can use in the part I marked with "do something".