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 occurs, Tapestry calls the event handler method you've provided, if any, in the containing component's class.
Let's review a simple example. Here's a portion of the template for a page (let's call it "Chooser") that lets the user choose a number between 1 and 10:
Notice that Chooser.tml contains an ActionLink component. When rendered on the page, the ActionLink component creates a component event request URL, with the event type set to "action". In this case the URL might look like http://localhost:8080/chooser.select/3
This URL identifies the page that contains the component ("chooser"), the type of event (unless it is "action", the default and most common event type), the id of the component within the page ("select"), plus the additional context value ("3"). Additional context values, if any, are appended to the path.
There's no direct mapping from URL to a piece of code. Instead, when the user clicks on the link, the ActionLink component emits events. And then Tapestry ensures that the correct bit of code (your event handler method, see below) gets invoked for those events.
This demonstrates a critical difference between Tapestry and a more traditional, action oriented framework. The URL doesn't say what happens when the link is clicked, it identifies which component is responsible when the link is clicked.
Often, a navigation request (originating with the user) will spawn a number of flow-of-control requests. For example, an action event will trigger a form component, which will then emit 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.
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.
Tapestry does two things here:
- Because of the annotation, it identifies method valueChosen() as the method to invoke.
- When the link is clicked, it converts the context value from a string to an integer and passes it into the method.
|Added in 5.3|
In the above example, the valueChosen() method will be invoked when the default event, "action", occurs in the select 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:
The value attribute of the OnEvent annotation is the name of the event to match. The default event type is "action"; the ActionLink and Form components each use this event type. Alternatively, we could have used an EventLink component, in which case the name of the event is determined by the element's ID, rather than being "action".
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).
|You should usually specify exactly which component(s) you wish to receive events from. Using @OnEvent on a method and not specifying a specific component id means that the method will be invoked for events from any component.|
Event handler methods are normally given package-private visibility, to support testing, although technically they may have any visibility (even private).
A single event handler method may receive notifications from many different components.
As elsewhere, the comparison of event type and component id is case-insensitive.
As an alternative to the use of annotations, you may name your event handling methods following a certain convention, and Tapestry will find and invoke your methods just as if they were annotated.
This style of event handler methods start with the prefix "on", followed by the name of the action. You may then continue by adding "From" and a capitalized component id (remember that Tapestry is case insensitive about event names and component ids). So, for example, a method named onActionFromSelect(), if it exists, is invoked whenever an Action event is emitted by the select component.
The previous example may be rewritten as:
|Note from Howard: I've found that I prefer the naming convention approach, and reserve the annotation just for situations that don't otherwise fit.|
Main Article: Page Navigation
For page navigation events (originating in components such as EventLink, ActionLink and Form), the value returned from an event handler method determines how Tapestry will render a response.
- 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
See Page Navigation for more details.
In some cases, there may be multiple event handler methods matching a single event. In that case, Tapestry invokes them in the following order:
- Base class methods before sub-class methods.
- Matching methods within a class in alphabetical order.
- For a single method name with multiple overrides, by number of parameters, descending.
Of course, ordinarily would you not want to create more than one method to handle an event.
When a sub-class overrides an event handler method of a base class, the event handler method is only invoked once, along with any other base class methods. The subclass can change the implementation of the base class method via an override, but can't change the timing of when that method is invoked. See issue TAP5-51.
The context values (the context parameter to the EventLink or ActionLink component) can be any object. However, only a simple conversion to string occurs. (This is in contrast to Tapestry 4, which had an elaborate type mechanism with the odd name "DataSqueezer".)
Again, whatever your value is (string, number, date), it is converted into a plain string. This results in a more readable URL.
If you have multiple context values (by binding a list or array of objects to the context parameter of the EventLink or ActionLink), then each one, in order, will be added to the URL.
When an event handler method is invoked, the strings are converted back into values, or even objects. A ValueEncoder is used to convert between client-side strings and server-side objects. The ValueEncoderSource service provides the necessary value encoders.
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 cases, it is desirable 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:
The latter two should be avoided, they may be removed in a future release. In all of these cases, the context parameter acts as a freebie; it doesn't match against a context value as it represents all context values.
A parameter may be annotated with the @RequestParameter annotation; this allows a query parameter 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.
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.
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.
The event will bubble up the hierarchy, until it is aborted. The event is aborted when an event handler method returns 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).
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:
Instead, you may simply say:
Your event handler method may even declare that it "throws Exception" if that is more convenient.
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 emits a new event, of type "exception", passing the thrown exception as the context. In fact, the exception is wrapped inside a ComponentEventException, from which you may extract the event type and context.
The return value of the exception event handler replaces the return value of original event handler method. For the typical case (an exception thrown by an "activate" or "action" event), this will be a navigational response such as a page instance or page name.
This can be handy for handling cases where the data in the URL is incorrectly formatted.
In the above example, the navigational response is the page itself.
If there is no exception event handler, or the exception event handler returns null (or is void), then then the exception will be passed to the RequestExceptionHandler service, which (in default configuration) will be render the exception page.