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marți, 27 ianuarie 2015

Use JSF VisitTree API to check a component in the component tree

In this post, you can see how to check a component in the component tree by using the UIComponent.visitTree, declared as follows:

public boolean visitTree(VisitContext context,VisitCallback callback)

First we write a class, named FindByIdVisitCallback, as below:

public class FindByIdVisitCallback implements VisitCallback {

 private final String id;       //can be null
 private final String clientId; //can be null

 public FindByIdVisitCallback(String clientId, String id) {
  this.clientId = clientId;
  this.id = id;
 }

 @Override
 public VisitResult visit(VisitContext context, UIComponent target) {

  if ((id == null) && (clientId == null)) {
       return VisitResult.REJECT;
  }

  if (target.getId().equals(id) || 
         target.getClientId(context.getFacesContext()).equals(clientId)) {
      return VisitResult.COMPLETE;
  }

  return VisitResult.ACCEPT;
}

Now, when we want to use it we follow two simple steps:

·         create an instance of FindByIdVisitCallback class and pass the id/clientId of the component to check
·         call the visitTree() method and capture the boolean result

Note If the result returned by the visitTree() is false, then the checked component doesn't exist, and if it returns true, then the component exist in the indicated tree/sub-tree. This method DOES NOT return an UIComponent instance, this is why I have avoided the term "find component" and prefer "check component". Can be used just for check if a component exist or not in the indicated tree/sub-tree. So, you can check by manually typing the id/clientId, or use something like: myComponent.getId()/myComponent.getClientId(). Nevertheless, you can find the component instance by declaring outside the visitTree(), an UIComponent foundComponent;and, when it is found by the passed id/clientId set it as foundComponent=target.

Examples:
JSF page snippet
...
<h:body> 
 <h:graphicImage id="myImageId" url="http://1.bp.blogspot.com/-
  gtjrtr50mMY/VL0IcD0hbrI/AAAAAAAAA-I/8fmOugRBkvU/s1600/omniutil.png"/>
 <h:form id="myFormId">
  <h:panelGrid id="panelId">             
   Some text
   <h:outputText id="myOutputId" value="..." />
   <h:panelGroup id="myDivId" layout="block">
    <h:inputText id="myInputId" value="..."/>                                     
   </h:panelGroup>
   <h:commandButton id="myButtonId" value="Click me!" />           
  </h:panelGrid>
 </h:form>
</h:body>
...

·         search from UIViewRoot the component with clientId myImageId

FindByIdVisitCallback findByIdVisitCallback = new FindByIdVisitCallback(null, "myImageId");
//or
FindByIdVisitCallback findByIdVisitCallback = new FindByIdVisitCallback("myImageId", null);

boolean result = UIViewRoot.visitTree(VisitContext.createVisitContext(FacesContext), findByIdVisitCallback);

·         search from UIViewRoot the component with id, myInputId / clientId, myFormId:myInputId

FindByIdVisitCallback findByIdVisitCallback = new FindByIdVisitCallback(null, "myInputId");
//or - this will perform faster
FindByIdVisitCallback findByIdVisitCallback = new FindByIdVisitCallback("myFormId:myInputId", null);

boolean result = UIViewRoot.visitTree(VisitContext.createVisitContext(FacesContext), findByIdVisitCallback);

Note  Whenever you can, is better to use clientIds instead of ids, because the search perform faster.
Note   The Visit Tree API has the big advantage that inputs in iterating components (UIData/UIRepeat) will be consulted on a per-iteration basis (otherwise, are consulted only once).

You can see another implementation of Visit Tree API in the OmniFaces Highlight component. There, OmniFaces use this API to traverse a form children, and accomplish some specific tasks for children that are instances of UIInput and are not valid.

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