Ajax for valueChangeListener - ajax

Question

I'm using the p:ajax listener to handle value change events (because valueChangeListener is launched on form submit):
<p:ajax event="change" listener="#{bean.onNameChanged}"/>
Handle method:
public void onNameChanged(final AjaxBehaviorEvent event)
The problem is, I can't find in AjaxBehaviorEvent nor its class hierarchy the place to read the old value of the input. Neither could I find hint in google, how to get the old value...
How to access the old value in the p:ajax onChange event?

Answer 1

The problem is, I can't find in AjaxBehaviorEvent nor its class hierarchy the place to read the old value of the input. Neither could I find hint in google, how to get the old value...
Use a valueChangeListener.
Unfortunatelly, valueChangeListener is invoked before p:ajax, so I don't have actual data from forms in that method, so in theory I could use valueChangeListener to remember the old value and then wait for p:ajax to process...
Queue the value change event to the invoke application phase.
public void valueChangeListenerMethod(ValueChangeEvent event) {
if (event.getPhaseId() != PhaseId.INVOKE_APPLICATION) {
event.setPhaseId(PhaseId.INVOKE_APPLICATION);
event.queue();
return;
}
// Do your original job here.
// It will only be invoked when current phase ID is INVOKE_APPLICATION.
}

Answer 2

The ValueChangeListener should work this way:
The view:
<h:form>
<h:inputText value="#{sessionBean.hello}"
valueChangeListener="#{sessionBean.valueChangeListener}">
<p:ajax/>
</h:inputText>
</h:form>
The bean:
public void valueChangeListener(ValueChangeEvent e) {
System.out.println("valueChangeListener invoked:"
+ " OLD: " + e.getOldValue()
+ " NEW: " + e.getNewValue());
}
The above code will print if I change the text field from "hello" to "world":
valueChangeListener invoked: OLD: hello NEW: world

Answer 3

You could try the following:
Implement the value change event in your bean
public void processValueChange(ValueChangeEvent e){
//foo the bar
}
Define a valueChangeListener on your selection component
<p:selectOneMenu value="#{yourBean.value}" onchange="submit()" valueChangeListener="{#yourBean.processValueChange}">
The key piece there is the submit() bit that processes the enclosing form on change of the value. You can then getNewValue() and getOldValue() as necessary.
EDIT: Now that I think about it, I see no reason why you cannot leave your setup as-is and simply define the valueChangeListener. It should still be processed during the change event in the <p:ajax/>, in fact, it will be processed before the listener for the ajax event itself.

Answer 4

you can use this:
public void onNameChanged(AjaxBehaviorEvent event)
{
String myVal = (String) ((UIOutput) event.getSource()).getValue();
System.out.println("myVal: " + myVal);
}

Answer 5

Workaround is possible (tested with Primefaces 10):
<p:inputText id="name" value="bean.name">
<p:ajax event="valueChange" update="name"
listener="#{bean.onNameChanged}"
onstart="cfg.ext={params:[{name:'oldValue', value:'#{bean.name}'}]};"/>
</p:inputText>
update="name" is important, to get each time the new value into the javascript event handler.
Bean Method:
public void onNameChanged(final AjaxBehaviorEvent event) {
String oldValue = getFacesContext().getExternalContext().getRequestParameterMap()
.get("oldValue");
//Do with oldValue, whatever you want
}