How update attribute of p:remoteCommand works - jsf

Question

Primefaces 6.0. I understand that update attribute of p:remoteCommand should be used to specify clientIds of the components that should be updated by AJAX. I am trying to understand how PF works. In combination with DataTable it doesn't seem to work as expected. When I try to directly set update="form:dataTable:2:bColumn", it has no efect. However, doing this (commented out in the below code) RequestContext.getCurrentInstance().update("form:dataTable:2:bColumn"); will force PF to update the specified outputText.
Why is this happening? I will be happy for technical explanation - I am trying to find the answer by debugging PF Java/Javascript sources.
<h:form id="form">
<p:remoteCommand name="remoteCall"
action="#{grid4.onEdit}"
update="form:dataTable:2:bColumn"
/>
<p:dataTable id="dataTable"
var="gridItem"
value="#{grid4.gridItems}"
editable="true" editMode="cell"
>
<p:ajax event="cellEdit"
oncomplete="remoteCall()">
</p:ajax>
<p:column headerText="A">
<p:cellEditor>
<f:facet name="output"><h:outputText value="#{gridItem.a}" /></f:facet>
<f:facet name="input"><p:inputText value="#{gridItem.a}"/></f:facet>
</p:cellEditor>
</p:column>
<p:column headerText="B">
<h:outputText id="bColumn" value="#{gridItem.b}" />
</p:column>
</p:dataTable>
</h:form>
Bean
#ManagedBean
#ViewScoped
public class Grid4 {
private List<GridItem> gridItems = new ArrayList<>();
public Grid4() {
gridItems.add(new GridItem("1", "a","b"));
gridItems.add(new GridItem("2", "a","b"));
gridItems.add(new GridItem("3", "a","b"));
}
public void onEdit() {
System.out.println("onEdit()");
gridItems.get(2).setB("CHANGED VALUE");
// RequestContext.getCurrentInstance().update("form:dataTable:2:bColumn");
}
public List<GridItem> getGridItems() {
return gridItems;
}
public void setGridItems(List<GridItem> gridItems) {
this.gridItems = gridItems;
}
}

Answer 1

basically jsf ids an client side ids are two different things (check this answer and this post for a better understanding).
When you use RequestContext.getCurrentInstance().update("form:dataTable:2:bColumn"); that method use the client id to find the components that have to be updated, but in the case of the update property of p:remoteCommand it is expecting a jsf id, not the generated client id, so that´s why your update doesn't work. However, primefaces support jquery selectors to update components, so you could use a client side id on an update property like this update="#(#yourElementId)"

Answer 2

Let me start by mentioning that this is not specific to the p:remoteCommand. The reason for the behaviour you notice is rather simple although not directly obvious maybe since it is unfortunately not in the PrimeFaces documentation.
The update attribute in:
<p:remoteCommand name="remoteCall"
action="#{grid4.onEdit}"
update="form:dataTable:2:bColumn"
/>
uses a relative path if it does not start with a : and since the p:remoteCommand is already in the naming container with id='form', the form in the update attribute is superfluous and even makes it not work (run your app in dev mode, add a messages tag and see the errors).
So
<p:remoteCommand name="remoteCall"
action="#{grid4.onEdit}"
update="dataTable:2:bColumn"
/>
Should work, as should
<p:remoteCommand name="remoteCall"
action="#{grid4.onEdit}"
update=":form:dataTable:2:bColumn"
/>
The
RequestContext.getCurrentInstance().update("form:dataTable:2:bColumn");
is always absolute, so the colon is not needed here and it will find the element starting from the root (form 'prefix' is needed then)