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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:
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<p> Choose a number from 1 to 10:
<t:count end="10" value="index">
<a t:id="select" t:type="actionlink" context="index">${index}</t:comp>
</t:count>
</p>
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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.
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@OnEvent(component = "select")
void valueChosen(int value)
{
this.value = value;
}
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- 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.
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Starting in release 5.3, Tapestry will validate that the component, if any, identified for the event handler method actually exists in the containing component's template. This helps with typos in annotations (or in the naming conventions identified below).
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For some components, more than one type of event can occur, in which case you will want to be more specific:
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@OnEvent(value = "action", component = "select")
void valueChosen(int value)
{
this.value = value;
}
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The previous example may be rewritten as:
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void onActionFromSelect(int value)
{
this.value = value;
}
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In other words, there's no need to do this:
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void onActionFromRunQuery()
{
try
{
dao.executeQuery();
}
catch (JDBCException ex)
{
throw new RuntimeException(ex);
}
}
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Instead, you may simply say:
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void onActionFromRunQuery() throws JDBCException
{
dao.executeQuery();
}
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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.
Thus:
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Object onException(Throwable cause)
{
message = cause.getMessage();
return this;
}
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Component Events 