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From "Bob Harner (Confluence)" <conflue...@apache.org>
Subject [CONF] Apache Tapestry > Component Events
Date Mon, 14 Oct 2013 11:15:00 GMT
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    <h2><a href="https://cwiki.apache.org/confluence/display/TAPESTRY/Component+Events">Component
Events</a></h2>
    <h4>Page <b>edited</b> by             <a href="https://cwiki.apache.org/confluence/display/~bobharner">Bob
Harner</a>
    </h4>
        <div id="versionComment">
        <b>Comment:</b>
        "then then" fixed per M. MGelbana<br />
    </div>
        <br/>
                         <h4>Changes (1)</h4>
                                 
    
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                    <table class="diff" cellpadding="0" cellspacing="0">
    
            <tr><td class="diff-snipped" >...<br></td></tr>
            <tr><td class="diff-unchanged" >In the above example, the navigational
response is the page itself. <br> <br></td></tr>
            <tr><td class="diff-changed-lines" >If there is no exception event
handler, or the exception event handler returns null (or is void), then <span class="diff-deleted-words"style="color:#999;background-color:#fdd;text-decoration:line-through;">then</span>
the exception will be passed to the [RequestExceptionHandler|http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/services/RequestExceptionHandler.html]
service, which (in default configuration) will be render the exception page. <br></td></tr>
    
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    </div>                            <h4>Full Content</h4>
                    <div class="notificationGreySide">
        <h1><a name="ComponentEvents-ComponentEvents"></a>Component Events</h1>

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<p><b>Component events</b> 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.</p>

<p>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:</p>

<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="theme: Default; brush: xml; gutter: false" style="font-size:12px; font-family:
ConfluenceInstalledFont,monospace;">
&lt;p&gt; Choose a number from 1 to 10:

    &lt;t:count end="10" value="index"&gt;
        &lt;a t:id="select" t:type="actionlink" context="index"&gt;${index}&lt;/t:comp&gt;
    &lt;/t:count&gt;
&lt;/p&gt;
</pre>
</div></div>

<p>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 <tt><a href="http://localhost:8080/chooser.select/3"
class="external-link" rel="nofollow">http://localhost:8080/chooser.select/3</a></tt></p>

<p>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"). <em>Additional
context values, if any, are appended to the path.</em></p>

<p>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.</p>

<p>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
<em>which component is responsible</em> when the link is clicked.</p>

<p>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.</p>

<h1><a name="ComponentEvents-EventHandlerMethods"></a>Event Handler Methods</h1>

<p>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 @<a href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/annotations/OnEvent.html"
class="external-link" rel="nofollow">OnEvent</a> annotation.</p>

<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="theme: Default; brush: java; gutter: false" style="font-size:12px; font-family:
ConfluenceInstalledFont,monospace;">
  @OnEvent(component = "select")
  void valueChosen(int value)
  {
    this.value = value;
  }
</pre>
</div></div>

<p>Tapestry does two things here:</p>

<ul>
	<li>Because of the annotation, it identifies method valueChosen() as the method to
invoke.</li>
	<li>When the link is clicked, it converts the context value from a string to an integer
and passes it into the method.</li>
</ul>




<div class='panelMacro'><table class='infoMacro'><colgroup><col width='24'><col></colgroup><tr><td
valign='top'><img src="/confluence/images/icons/emoticons/information.gif" width="16"
height="16" align="absmiddle" alt="" border="0"></td><td><b>Added in
5.3</b><br /></td></tr></table></div>
<div style="border-right: 20px solid #D8E4F1;border-left: 20px solid #D8E4F1;">
<p>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).</p>
</div>

<p>In the above example, the valueChosen() method will be invoked when the default event,
"action", occurs in the <tt>select</tt> component (and has at least one context
value).</p>

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

<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="theme: Default; brush: java; gutter: false" style="font-size:12px; font-family:
ConfluenceInstalledFont,monospace;">
  @OnEvent(value = "action", component = "select")
  void valueChosen(int value)
  {
    this.value = value;
  }
</pre>
</div></div>

<p>The <tt>value</tt> 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".</p>

<p>If you omit the <tt>component</tt> part of the OnEvent annotation, then
you'll receive notifications from <em>all</em> contained components, possibly
including nested components (due to event bubbling).</p>

<div class='panelMacro'><table class='tipMacro'><colgroup><col width='24'><col></colgroup><tr><td
valign='top'><img src="/confluence/images/icons/emoticons/check.gif" width="16" height="16"
align="absmiddle" alt="" border="0"></td><td>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 <em>any</em>
component.</td></tr></table></div>

<p>Event handler methods are normally given package-private visibility, to support testing,
although technically they may have any visibility (even private).</p>

<p>A single event handler method may receive notifications from many different components.</p>

<p>As elsewhere, the comparison of event type and component id is case-insensitive.</p>

<h2><a name="ComponentEvents-MethodNamingConvention"></a>Method Naming Convention</h2>

<p>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.</p>

<p>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 <tt>Action</tt>
event is emitted by the <tt>select</tt> component.</p>

<p>The previous example may be rewritten as:</p>

<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="theme: Default; brush: java; gutter: false" style="font-size:12px; font-family:
ConfluenceInstalledFont,monospace;">
  void onActionFromSelect(int value)
  {
    this.value = value;
  }
</pre>
</div></div>

<div class='panelMacro'><table class='infoMacro'><colgroup><col width='24'><col></colgroup><tr><td
valign='top'><img src="/confluence/images/icons/emoticons/information.gif" width="16"
height="16" align="absmiddle" alt="" border="0"></td><td>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.</td></tr></table></div>

<h2><a name="ComponentEvents-MethodReturnValues"></a>Method Return Values</h2>

<p>Main Article: <a href="/confluence/display/TAPESTRY/Page+Navigation" title="Page
Navigation">Page Navigation</a></p>

<p>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.</p>

<ul>
	<li><b>Null</b>: For no value, or null, the current page (the page containing
the component) will render the response.</li>
	<li><b>Page</b>: 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.</li>
	<li><b>URL</b>: 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.)</li>
	<li><b>Zone body</b>: 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.</li>
	<li><b>HttpError</b>: For an <a href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/services/HttpError.html"
class="external-link" rel="nofollow">HttpError</a>, an error response is sent to
the client.</li>
	<li><b>Link</b>: For a <a href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/Link.html"
class="external-link" rel="nofollow">Link</a>, a redirect is sent to the client.</li>
	<li><b>Stream</b>: For a <a href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/StreamResponse.html"
class="external-link" rel="nofollow">StreamResponse</a>, a stream of data is sent
to the client</li>
</ul>


<p>See <a href="/confluence/display/TAPESTRY/Page+Navigation" title="Page Navigation">Page
Navigation</a> for more details.</p>

<h2><a name="ComponentEvents-MultipleMethodMatches"></a>Multiple Method
Matches</h2>

<p>In some cases, there may be multiple event handler methods matching a single event.
 In that case, Tapestry invokes them in the following order:</p>

<ul>
	<li>Base class methods before sub-class methods.</li>
	<li>Matching methods within a class in alphabetical order.</li>
	<li>For a single method name with multiple overrides, by number of parameters, descending.</li>
</ul>


<p>Of course, ordinarily would you <em>not</em> want to create more than
one method to handle an event.</p>

<p>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 <em>implementation</em> of the base class method via an override, but can't
change the <em>timing</em> of when that method is invoked. See <a href="https://issues.apache.org/jira/browse/TAP5-51"
class="external-link" rel="nofollow">issue TAP5-51</a>.</p>

<h1><a name="ComponentEvents-EventContext"></a>Event Context</h1>

<p>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".)</p>

<p>Again, whatever your value is (string, number, date), it is converted into a plain
string. This results in a more readable URL.</p>

<p>If you have multiple context values (by binding a list or array of objects to the
<em>context</em> parameter of the EventLink or ActionLink), then each one, in
order, will be added to the URL.</p>

<p>When an event handler method is invoked, the strings are converted back into values,
or even objects. A <a href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/ValueEncoder.html"
class="external-link" rel="nofollow">ValueEncoder</a> is used to convert between
client-side strings and server-side objects. The <a href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/services/ValueEncoderSource.html"
class="external-link" rel="nofollow">ValueEncoderSource</a> service provides the
necessary value encoders.</p>

<p>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).</p>

<p>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:</p>

<ul>
	<li><a href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/EventContext.html"
class="external-link" rel="nofollow">EventContext</a></li>
	<li>Object[]</li>
	<li>List&lt;Object&gt;</li>
</ul>


<p>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 <em>all</em> context values.</p>

<h2><a name="ComponentEvents-AccessingRequestQueryParameters"></a>Accessing
Request Query Parameters</h2>

<p>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.</p>


<h2><a name="ComponentEvents-MethodMatching"></a>Method Matching</h2>

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

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

<p>EventContext parameters, and parameters annotated with @RequestParameter, do not
count against this limit.</p>

<h2><a name="ComponentEvents-MethodOrdering"></a>Method Ordering</h2>

<p>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.</p>

<p>An event handler method may return the value <tt>true</tt> 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.</p>

<h1><a name="ComponentEvents-EventBubbling"></a>Event Bubbling</h1>

<p>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.</p>

<p>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.</p>

<p>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.</p>

<p>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).</p>

<h1><a name="ComponentEvents-EventMethodExceptions"></a>Event Method Exceptions</h1>

<p>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.</p>

<p>In other words, there's no need to do this:</p>

<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="theme: Default; brush: java; gutter: false" style="font-size:12px; font-family:
ConfluenceInstalledFont,monospace;">
  void onActionFromRunQuery()
  {
    try
    {
      dao.executeQuery();
    }
    catch (JDBCException ex)
    {
      throw new RuntimeException(ex);
    }
  }
</pre>
</div></div>

<p>Instead, you may simply say:</p>

<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="theme: Default; brush: java; gutter: false" style="font-size:12px; font-family:
ConfluenceInstalledFont,monospace;">
  void onActionFromRunQuery() throws JDBCException
  {
    dao.executeQuery();
  }
</pre>
</div></div>

<p>Your event handler method may even declare that it "throws Exception" if that is
more convenient.</p>

<h1><a name="ComponentEvents-InterceptingEventExceptions"></a>Intercepting
Event Exceptions</h1>

<p>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.</p>

<p>Tapestry emits a new event, of type "exception", passing the thrown exception as
the context. In fact, the exception is wrapped inside a <a href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/runtime/ComponentEventException.html"
class="external-link" rel="nofollow">ComponentEventException</a>, from which you
may extract the event type and context.</p>

<p>Thus:</p>

<div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent">
<pre class="theme: Default; brush: java; gutter: false" style="font-size:12px; font-family:
ConfluenceInstalledFont,monospace;">
  Object onException(Throwable cause)
  {
    message = cause.getMessage();

    return this;
  }
</pre>
</div></div>

<p>The return value of the exception event handler <em>replaces</em> 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 <a href="/confluence/display/TAPESTRY/Page+Navigation"
title="Page Navigation">navigational response</a> such as a page instance or page
name.</p>

<p>This can be handy for handling cases where the data in the URL is incorrectly formatted.</p>

<p>In the above example, the navigational response is the page itself.</p>

<p>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 <a href="http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/services/RequestExceptionHandler.html"
class="external-link" rel="nofollow">RequestExceptionHandler</a> service, which (in
default configuration) will be render the exception page.</p>

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