Page History

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• Can lead to difficult to find bugs if there is mismatch between the client side Ajax ID attribute and the server side control ID attribute. Dynamic stateless pages could also lead to difficulties if an Ajax control is added inside another control's listener and subsequent requests won't have the Ajax control present in the server-side component tree.
• Implementation is more complex than for Page Methods
• The stateless nature of Click makes Ajax applications inherently tricky, because if a non-Ajax request is made to the server, all Ajax related state will be undone. This can be alleviated by using Stateful pages, but since requests to stateful pages are serialized, this creates another, albeit small, problem in that parallel Ajax requests to a stateful page won't work. In other words if the browser makes two Ajax requests to a stateful Page where the first request is very resource intensive and takes 10 seconds to execute, the second Ajax request will have to wait until the first request finishes. Although this might not be a common use case, it is worth taking into consideration.

Outstanding Issues:

• A control can only have a single listener defined. How will a control handle multiple incoming events? For example a field might handle two Ajax events, "onblur" for validation purposes, and "onkeypress" for auto-completion. The Event object passed into the listener could be inspected, but this leads to if-else logic. This brings up the idea of having multiple listeners per control.
  • Pro: Each listener could be registered for a specific event, and will only be triggered for Ajax requests of that event type.
  • Con: Having multiple listeners per control could increase the conceptual complexity and could lead to an increase in maintenance. For example, if the client-side code neglects to pass the event parameter, no listener will be invoked. The user will now have to deal with whether the control ID or event request parameter or both are missing.

3. Behaviors

Behaviors is a concept borrowed from Wicket and JSF. A behavior is attached to a Control and will be notified by the Control for specific events such as onInit, onProcess, onRender, getHeadElements. A behavior can also act as a listener and receive events, just like a Control does.

In terms of Ajax, a Behavior could be added to a Control and will inject the necessary JavaScript and Ajax code needed by the control. Further, the Behavior will act as a listener and will receive the incoming request. So unlike Proposal 2 above, the Control will not be invoked for Ajax requests.

To make things more conrete, lets look at an example:

public class MyPage extends Page {
  public void onInit() {

  }
}