Write Ajax handlers as lambdas

Use this when an anonymous subclass with a single overridden method is more code than the handler itself. The common Ajax behaviors have static factory methods that take a lambda; the lambda receives the AjaxRequestTarget.

Create a behavior from a lambda

Label preview = new Label("preview", nameModel);
preview.setOutputMarkupId(true);
add(preview);

TextField<String> name = new TextField<>("name", nameModel);
name.add(OnChangeAjaxBehavior.onChange(target -> target.add(preview)));
add(name);

The factory methods and the method each one implements:

The lambdas are SerializableConsumer and SerializableBiConsumer instances: they are stored with the page, so everything they capture has to be serializable. Capture components and models, not services; inject services into the page instead.

Stop a timer from inside its lambda

The lambda does not get the behavior, so keep a reference when you need it:

AbstractAjaxTimerBehavior poll = AbstractAjaxTimerBehavior.onTimer(Duration.ofSeconds(2), target -> {
    target.add(status);
});
status.add(poll);

add(new AjaxLink<Void>("pause") {
    @Override
    public void onClick(AjaxRequestTarget target) {
        poll.stop(target);
    }
});

When you need more than one callback

A factory method implements one callback. An AjaxFormSubmitBehavior that also handles validation errors, or a behavior that overrides updateAjaxAttributes, still needs a subclass:

quantity.add(new AjaxFormSubmitBehavior("change") {
    @Override
    protected void onSubmit(AjaxRequestTarget target) {
        target.add(result);
    }

    @Override
    protected void onError(AjaxRequestTarget target) {
        target.add(feedback);
    }
});

Ajax links and buttons from lambdas

wicket-core has no factory methods for Ajax components; AjaxLink, AjaxButton and the others are subclassed. The exception is IndicatingAjaxButton.onSubmit(id, (button, target) -> ...) in wicket-extensions.

The WicketStuff module wicketstuff-lambda-components has a ComponentFactory with methods such as ajaxLink, ajaxButton, ajaxCheckBox and ajaxSubmitLink:

add(ComponentFactory.ajaxLink("refresh", (link, target) -> target.add(table)));
add(ComponentFactory.ajaxSubmitLink("save",
    (link, target) -> target.add(result),
    (link, target) -> target.add(feedback)));

It is a separate dependency with its own release cycle; see WicketStuff.