Component events are Tapestry's way of conveying a user's interactions with the web page, such as clicking links and submitting forms, to designated methods in your page and component classes. When a component event occursis triggered, Tapestry calls the event handler method you've provided, if any, in the containing component's class.

Let's review look at a simple example. Here's a portion of the template for a page (let's call it "ChooserReview") that lists documents and lets the user choose a number between 1 and 10:a user click to edit any one of them.

<p> Choose a number from 1Select document to 10edit:

     </p>
<t:countloop endsource="10documents" value="indexdocument">
    <div>
       <a t:id="select<t:actionlink" t:typeid="actionlinkedit" context="index">${index}</a>document.id"> ${document.name} </t:actionlink>       
    </t:count>div>
</p>t:loop>

Notice that ChooserReview.tml contains an ActionLink component . When rendered on the pagein a loop. For each rendering within the loop, the ActionLink component creates a component event request URL, with the event type set to "action". In this case the , each URL might look like:

    http://localhost:8080/chooserreview.selectedit/3

This URL identifies the page that contains the component ("chooserreview"), the type of event (unless it is "action", the default and most common event type), the the Component id of the component within the page ("selectedit"), plus and the additional context value ("3"). Additional context values, , the "id" property of the document). Additional context values, if any, are appended to the path. (The URL may also contain the event name, unless, as here, it is "action".)

There's no direct mapping from URL to a piece of code. Instead, when the user clicks on the link, the ActionLink component emits triggers events. And then Tapestry ensures that the correct bit bits of code (your event handler method, see below) gets get invoked for those events.

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Often, a navigation request (originating with the user) will spawn a number of flow-of-control requests. For example, a form component may trigger an action event will trigger a form component, which will then emit trigger notification events to announce when the form submission is about to be processed, and whether it was successful or not, and those event could be further handled by the page component.

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When a component event occurs, Tapestry invokes any event handler methods that you have identified for that event. You can identify your event handler methods via a naming convention (see Method Naming Convention below), or via the @OnEvent annotation.

  @OnEvent(component = "selectedit")
void  void valueChoseneditDocument(int valuedocId)
  {
    this.valueselectedId = valuedocId;
    // do something with the document here
}

Tapestry does two things here:

• Because of the annotation, it identifies method valueChoseneditDocument() as the method to invoke .whenever the component whose ID is "edit" triggers an event.
• Because there is a method parameter, when When the link is clicked , it converts the context value from a of the request is converted from a string to an integer and passes it into passed in as the method's value parameter.
  
  

In the above example, the valueChoseneditDocument() method will be invoked when the default event, "action", occurs in the select any event occurs in in the "edit" component (and has at least one context value).

For some components, more than one type of event can occur, in which case you will want to be more specific:

  @OnEvent(value = "action", component = "selectedit")
  void valueChoseneditDocument(int valuedocId)
  {
    this.valueselectedId = valuedocId;
    // do something with the document here
}

For the OnEvent annotation, the value attribute identifies the name of the event to match. We specified "action" because the ActionLink component emits triggers the "action" event, as noted in the Component Events section of its javadocs.

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<t:eventlink event="cleardelete" context="index">${index}</a>document.id"> ${document.name} </t:eventlink>

which is equivalent to:

<a t:type="eventlink" t:id="cleardelete" context="index">${index}document.id"> ${document.name} </a>

Note that if you omit the component part of the OnEvent annotation, then you'll receive notifications from all contained components, possibly including nested components (due to event bubbling).

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As elsewhere, the comparison of event type and component id ID is case-insensitive.

Method Naming Convention

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This style of event handler methods start with the prefix "on", followed by the name of the actionevent. You may then continue by adding "From" and a capitalized component id (remember that Tapestry is case insensitive about event names and component idsIDs). So, for example, a method named onActionFromSelectonValidateFromSave() , if it exists, is will be invoked whenever an Action a "Validate" event is emitted by the select componenttriggered by a component whose component ID is "save".

The previous example may be rewritten as:

  void onActionFromSelectonActionFromEdit(int valuedocId)
  {
    this.valueselectedId = valuedocId;
    // do something with the document here
}

...

• Null: For no value, or null, the current page (the page containing the component) will render the response.
• Page: For the name of a page, or a page class or page instance, a render request URL will be constructed and sent to the client as a redirect to that page.
• URL: For a java.net.URL, a redirect will be sent to the client. (In Tapestry 5.3.x and earlier, this only works for non-Ajax requests.)
• Zone body: In the case of an Ajax request to update a zone, the component event handler will return the new zone body, typically via an injected component or block.
• HttpError: For an HttpError, an error response is sent to the client.
• Link: For a Link, a redirect is sent to the client.
• Stream: For a StreamResponse, a stream of data is sent to the client
• boolean: true prevents the event from bubbling up further; false lets it bubble up. See Event Bubbling, below.

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As shown in the example above, most of the parameters passed to the event handler method are derived from the values provided in the event context. Each successive method parameter matches against a value provided in the event context (the context parameter of the ActionLink component; though many components have a similar context parameter).

In some many cases , it is desirable helpful to have direct access to the context (for example, to adapt to cases where there are a variable number of context values). The context values may be passed to an event handler method as parameter of the following types:

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The latter two should be avoided, they may be removed in a future release. In all of these cases, the context EventContext parameter acts as a freebie; it doesn't match against a context value as it represents all context values.

Accessing Request Query Parameters

A parameter may be annotated with the @RequestParameter annotation; this allows query parameters (?name1=value1&name2=value2, etc) to be extracted from the request, converted to the correct type, and passed to the method. Again, this doesn't count against the event context values.

Method Matching

An event handler method will only be invoked if the context contains at least as many values as the method has parameters. Methods with too many parameters will be silently skipped.

Tapestry will silently skip over a method if there are insufficient values in the context to satisfy the number of parameters requested.

EventContext parameters, and parameters annotated with @RequestParameter, do not count against this limit.

Method Ordering

When multiple methods match within the same class, Tapestry will invoke them in ascending alphabetical order. When there are multiple overrides of the same method name, Tapestry invokes them in descending order by number of parameters. In general, these situations don't happen ... in most cases, only a single method is required to handle a specific event form a specific component.

An event handler method may return the value true to indicate that the event has been handled; this immediately stops the search for additional methods in the same class (or in base classes) or in containing components.

Event Bubbling

The event will bubble up the component hierarchy, first to the containing component, then that component's containing component or page, and so on, until it is aborted by an event handler method returning true or a non-null value.

Returning a boolean value from an event handler method is special. Returning true will abort the event with no result; use this when the event is fully handled without a return value and no further event handlers (in the same component, or in containing components) should be invoked.

Returning false is the same as returning null; event processing will continue to look for more event handlers, in the same component or its parent.

When an event bubbles up from a component to its container, the origin of the event is changed to be the component. For example, a Form component inside a BeanEditForm component may fire a success event. The page containing the BeanEditForm may listen for that event, but it will be from the BeanEditForm component (which makes sense, because the id of the Form inside the BeanEditForm is part of the BeanEditForm's implementation, not its public interface).

If you want to handle events that have bubbled up from nested component, you'll soon find that you don't have easy access to the component ID of the firing component. In practical terms this means that you'll want to trigger custom events for the events emitted by those nested components (see Triggering Events, below), and use that custom event name in your event handler method.

Event Method Exceptions

Event methods are allowed to throw any exception (not just runtime exceptions). If an event method does throw an exception, Tapestry will catch the thrown exception and ultimately display the exception report page.

In other words, there's no need to do this:

Object onActionFromEdit(EventContext context)
{
    if (context.getCount() > 0) {
        this.selectedId = context.get(0);
        // do something with the document here
    } else {
        alertManager.warn("Please select a document.");
        return null;
    }
}

Accessing Request Query Parameters

A parameter may be annotated with the @RequestParameter annotation; this allows query parameters (?name1=value1&name2=value2, etc) to be extracted from the request, converted to the correct type, and passed to the method. Again, this doesn't count against the event context values.

See the example in the Link Components FAQ.

Method Matching

An event handler method will only be invoked if the context contains at least as many values as the method has parameters. Methods with too many parameters will be silently skipped.

Tapestry will silently skip over a method if there are insufficient values in the context to satisfy the number of parameters requested.

EventContext parameters, and parameters annotated with @RequestParameter, do not count against this limit.

Method Ordering

When multiple methods match within the same class, Tapestry will invoke them in ascending alphabetical order. When there are multiple overrides of the same method name, Tapestry invokes them in descending order by number of parameters. In general, these situations don't happen ... in most cases, only a single method is required to handle a specific event form a specific component.

An event handler method may return the value true to indicate that the event has been handled; this immediately stops the search for additional methods in the same class (or in base classes) or in containing components.

Event Bubbling

The event will bubble up the component hierarchy, first to the containing component, then that component's containing component or page, and so on, until it is aborted by an event handler method returning true or a non-null value.

Returning a boolean value from an event handler method is special. Returning true will abort the event with no result; use this when the event is fully handled without a return value and no further event handlers (in the same component, or in containing components) should be invoked.

Returning false is the same as returning null; event processing will continue to look for more event handlers, in the same component or its parent.

When an event bubbles up from a component to its container, the origin of the event is changed to be the component. For example, a Form component inside a BeanEditForm component may trigger a success event. The page containing the BeanEditForm may listen for that event, but it will be from the BeanEditForm component (which makes sense, because the id of the Form inside the BeanEditForm is part of the BeanEditForm's implementation, not its public interface).

If you want to handle events that have bubbled up from nested component, you'll soon find that you don't have easy access to the component ID of the firing component. In practical terms this means that you'll want to trigger custom events for the events triggered by those nested components (see Triggering Events, below), and use that custom event name in your event handler method.

Event Method Exceptions

Event methods are allowed to throw any exception (not just runtime exceptions). If an event method does throw an exception, Tapestry will catch the thrown exception and ultimately display the exception report page.

In other words, there's no need to do this:

  void onActionFromRunQuery()

  void onActionFromRunQuery()
  {
    try
    {
      dao.executeQuery();
    }
    catch (JDBCException ex)
    {
      throw new RuntimeException(ex);
    }
  }

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When an event handler method throws an exception (checked or runtime), Tapestry gives the component and its containing page a chance to handle the exception, before continuing on to report the exception.

Tapestry triggers Wiki Markup

{float:right|background=#eee|padding=0 1em}
    *JumpStart Demo:*
    [Handling A Bad Context|http://jumpstart.doublenegative.com.au/jumpstart/examples/infrastructure/handlingabadcontext/1]
{float}

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If there is no exception event handler, or the exception event handler returns null (or is void), then the exception will be passed to the RequestExceptionHandler service, which (in the default configuration) will render the exception page.

Triggering Events

If you want your own component to trigger events, just call the triggerEvent method of ComponentResources from within the your component class.

For example, the following emits triggers an "updateAll" event. A containing component can then respond to it, if desired, with an "onUpdateAll()" method in its own component class.

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The third parameter to triggerEvent is a ComponentEventCallback, which you'll only need to implement if you want to get the return value of the handler method. The return value of triggerEvent() says if the event was handled or not.