Monday 27 July 2015

Temporarily disable or conditional executing of PrimeFaces AJAX calls


In JSF 2.0, the AJAX functionality is standardised in the specification and can be used in a consistent way between JSF implementations and component libraries like PrimeFaces.
With the addition of new functionality, new usage patterns become a habit. But new usages also brings more advanced scenarios with their own issues.

The situation I want to discuss today, is where you have the need for some partial screen update with AJAX but which you don't want in all situations. The conditional aspect which I referred to in the title of the text.

Problem description

Suppose you have the following screen area:

It contains the following elements from left to right
* A field to enter an existing code. When the end user leaves the field, the code is looked up.
* A second, read only field or label, where the description of the code is placed when it is an existing one.
* A button, which is only available when the first field doesn't contain an existing code and lets you add a new one.

Within the JSF page, the structure looks something like this.

<h:panelGroup id="codeArea">
    Code :
    <p:inputText value="#{codeBean.code}id="code">
        <p:ajax event="blur" listener="#{codeBean.onBlurCode}" update="codeArea"/>
    <p:inputText value="#{codeBean.codeDescription}" disabled="true" placeholder="description"/>
    <p:commandButton icon="fa fa-plus-circle" actionListener="#{codeBean.addCode}" update="codeArea" disabled="#{empty codeBean.description}"/>

With the ajax tag within the input field, we can perform the partial screen update when we leave the input field. We need to update the complete group as the button needs to become disabled as we enter an existing code.

Conditional executing

This construct works perfectly when you tab out of the input field.  The blur event triggers the AJAX execution and the server side code checks if the code exists. Due to the partial screen update the button becomes disabled when the code doesn't exists or enabled when the code isn't found.

But there is a scenario which fails; when the focus is in the input field and we click on the command button.  You will found out that the click on the button is ignored.  

If we inspect what is happening in the network console of the browser; we see that there is an AJAX request to the server which corresponds to the blur event of the field.  But there is no request which corresponds to the click on the button.
This is because the blur event replaced the DOM element in the browser with another, for the end user identical component. So the click is on an element which no longer exist on the screen after the AJAX  completes.

So how can we solve this? Luckily PrimeFaces uses JQuery on the client side.  So when we could disable the onblur event execution in the case when we click on the button, we have a solution. And the good thing is that we can achieve this, quite easily.

The mousedown event is executed before the blur event. This is can bee seen by the following testing code. (thanks to mudassir)

So by adding the following code:


We can remove all event handlers from the input field, and thus the click is executed.


Making the AJAX functionality can be easily achieved within PrimeFaces with the help of JQuery which is already available in the browser? In this text I showed you how you can disable the blur event in the case when you click a specific button but it will be active in other scenarios.

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