PollingDiv

A piece of the screen that keeps itself up to date. Nothing on the page does the updating: the browser asks the server every couple of seconds whether anything changed, and this component gets asked the same question.

public class QueueLength extends PollingDiv {
    private Span m_count;

    @Override public void createContent() throws Exception {
        m_count = new Span();
        add(m_count);
    }

    @Override public void checkForChanges() throws Exception {
        m_count.setText(queue.size() + " waiting");
    }
}

• The one method
• Registering
• What it costs

The one method

checkForChanges() is called on every poll - by default every two and a half seconds - and whatever it changes goes to the browser as an ordinary delta.

The default implementation rebuilds the whole component. That is the simplest thing that works and the wrong thing for anything but the smallest div: it throws the built tree away, so the delta is large, the screen flickers and anything the user was doing inside it is lost.

Registering

There is nothing to register and nothing to clean up: the component adds itself to the conversation's pollers when it is added to a page and removes itself when it is taken off. Put it on the screen and it polls; remove it and the polling stops, along with the requests.

What it costs

A request every couple of seconds, per browser, for as long as the page is open. That is cheap for one user and not cheap for a thousand, so it is worth being deliberate:

• a value that changes on its own - a queue length, a job's state, a temperature
  • is what this is for;
• a value that changes because the user did something does not need it; the click that changed it is already a request;
• a long job with an end is an AsyncContainer, which stops polling when it finishes.