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This section overviews the attributes that are common for Ajax components such as a4j:support, a4j:commandButton, a4j:jsFunction, a4j:poll, a4j:push and so on. Also, most RichFaces components with built-in Ajax support have these attributes for a similar purpose.
Ajax components attributes help RichFaces to expose its features. Most of the attributes have default values. Thus, you can start working with RichFaces without knowing the usage of these attribute. However, their usage allows to tune the required Ajax behavior very smoothly.
reRender is a key attribute. It allows to point to the area(s) on a page that should be updated as a response on Ajax interaction. The value of the reRender attribute is an id of the JSF component or an id list. This is a simple example:
reRender uses UIComponent.findComponent() algorithm (with some additional exceptions) to find the component in the component tree. As you see, the algorithm presumes several steps. Each other step is used if the previous step is not successful. Therefore, you can define how fast the component is found mentioning it more precisely. The following example shows the difference in approaches (both buttons will work successfully):
You can use JSF EL expression as a value of the reRender attribute. It might be a property of types Set, Collection, Array or simple String. The EL for reRender is resolved right before the Render Response phase. Hence, you can calculate what should be re-rendered on any previous phase during the Ajax request processing.
Most common problem with using reRender is pointing it to the component that has a 'rendered' attribute. Note, that JSF does not mark the place in the browser DOM where the outcome of the component should be placed in case the 'rendered' condition returns false. Therefore, after the component becomes rendered during the Ajax request, RichFaces delivers the rendered code to the client, but does not update a page, because the place for update is unknown. You need to point to one of the parent components that has no 'rendered' attribute. As an alternative, you can wrap the component with a4j:outputPanel layout="none".
ajaxRendered attribute of the a4j:outputPanel set to true allows to define the area of the page that will be re-rendered even if it is not pointed in the reRender attribute explicitly. It might be useful if you have an area on a page that should be updated as a response on any Ajax request. For example, the following code allows to output error messages regardless of what Ajax request causes the Validation phase failed.
limitToList attribute allows to dismiss the behavior of the a4j:outputPanel ajaxRendered attribute. limitToList = "true" means to update only the area(s) that mentioned in the reRender attribute explicitly. All output panels with ajaxRendered="true" will be ignored.
Queue and Traffic Flood Protection
eventsQueue attribute defines the name of the queue that will be used to order upcoming Ajax requests. By default, RichFaces does not queue Ajax requests. If events are produced simultaneously, they will come to the server simultaneously. JSF implementations (especially, the very first ones) does not guaranty that the request that comes first will be served or passed into the JSF lifecycle first. The order how the server side data will be modified in case of simultaneous request might be unpredictable. Usage of eventsQueue attribute allows to avoid possible mess. Define the queue name explicitly, if you expect intensive Ajax traffic in your application.
The next request posted in the same queue will wait until the previos one is not processed and Ajax Response is returned back if the eventsQueue attribute is defined. In addition, Richfaces starts to remove from the queue 'similar' requests. 'Similar' requests are the requests produced by the same event. For example, according to the following code, only the newest request will be sent to the server if a user types very fast and has typed the several characters already before the previous Ajax Response is back.
requestDelay attribute defines the time (in ms.) that the request will be wait in the queue before it is ready to send. When the delay time is over, the request will be sent to the server or removed if the newest 'similar' request is in a queue already .
ignoreDupResponses attribute orders to ignore the Ajax Response produced by the request if the newest 'similar' request is in a queue already. ignoreDupResponses="true" does not cancel the request while it is processed on the server, but just allows to avoid unnecessary updates on the client side if the response loses the actuality.
Defining the eventsQueue along with requestDelay allows to protect against unnecessary traffic flood and synchronizes Ajax requests order. If you have several sources of Ajax requests, you can define the same queue name there. This might be very helpful if you have Ajax components that invoke request asynchronously from the ones produced by events from users. For example a4j:poll or a4j:push. In case the requests from such components modify the same data, the synchronization might be very helpful.
Data Processing Options
RichFaces uses form based approach for Ajax request sending. This means each time, when a user click an Ajax button or a4j:poll produces an asynchronous request, the data from the closest JSF form is submitted with the XMLHTTPRequest object. The form data contains the values from the form input element and auxiliary information such as state saving data
ajaxSingle - boolean attribute which provides possibility to limit JSF tree processing(decoding, conversion/validation, value applying) to the component which send the request only.
In other words when ajaxSingle attribute is equal true, it orders to process only a value of the current component (along with f:param or a4j:actionparam values if any). In case of a4j:support, it will be a value of the parent component.
process attribute allows you to define the components to be processed additionally with the component which is marked as ajaxSingle or wrapped to a region. Imagine you need to process only two fields in the different parts of view. If you wrap the first to a region or make nested support component ajaxSingle - the second component will not be processed and if you do not define the first component in this way - all the view will be processed. In this case the next simple constructions should be used:
Here when the name field looses focus, Ajax request will be sent. And only two fields (name and additionally email) will be processed (decoding, conversion/validation, value applying will be executed). The same is for the email field blur event.
immediate attribute has the same purpose as any other non-Ajax-JSF component. The default ActionListener should be executed immediately (i.e. during the Apply Request Values phase of a request processing lifecycle), rather than waiting until the Invoke Application phase. Using immediate="true" is one of the ways to have some data model values updated when other cannot be updated because of a problem with passing the Validation phase successfully. This might be important inside the h:dataTable like components where using a4j:region is impossible due to the h:dataTable component architecture.
bypassUpdates attribute allows to bypass the Update Model phase. It might be useful if you need to check user input against the available validator, but not to update the model with those data. Note, that an action will be invoked at the end of the Validation phase only if the Validation phase is passed successfully. The listeners of the Application phase will not be invoked in any case.
Action and Navigation
Ajax component is similar to any other non-Ajax JSF component like h:commandButton. It allows to submit the form. You can use action and actionListener attribute to invoke the action method and define the action event.
action method must return null if you want to have an Ajax Response with a partual page update. This is regular mode called "Ajax request generates Ajax Response". In case of action does not return null, but the action outcome that matches one of navigation rules, RichFaces starts to work in "Ajax request generates Non-Ajax Response" mode. This mode might be helpful in two major cases:
onclick attribute is similar to the 'onsubmit', but for clickable components such as a4j:commandLink and a4j:commandButton. If it returns false, the Ajax request is canceled also.
data attribute allows to get the additional data from the server during an Ajax call. You can use JSF EL to point the property of the managed bean and its value will be serialized in JSON format and be available on the client side. You can refer to it using the 'data' variable. For example:
Richfaces allows to serialize not only primitive types into JSON format, but also complex types including arrays and collections. The beans should be serializable to be refered with 'data'.