wicket-user-guide/src/main/asciidoc/componentLifecycle/componentLifecycle_4.adoc - wicket - Git at Google


 This stage is reached each time a component is rendered, typically when a page is requested or when the component or one of its ancestors is refreshed via AJAX.

 === Method onConfigure

 Method _onConfigure()_ has been introduced in order to provide a good point to manage the component states such as its visibility or enabled state. This method is called on all components whose parent is visible.

 As stated in <<_hiding_or_disabling_a_component,chapter 6.1>>, _isVisible()_ and _isEnabled()_ are called multiple times when a page or a component is rendered, so it's highly recommended not to directly override these method, but rather to use _onConfigure()_ to change component states. On the contrary method _onBeforeRender_ (see the next paragraph) is not indicated for this task because it will not be invoked if component visibility is set to false.

 === Method onBeforeRender

 The most important hook method of this stage is probably _onBeforeRender()_. This method is called on all visible components before any of them are rendered. It is our last chance to change a component's state prior to rendering - no change to a component's state is allowed afterwards.

 If we want to add/remove child components this is the right place to do it. In the next example (project LifeCycleStages) we will create a page which alternately displays two different labels, swapping between them each time it is rendered:

 [source,java]
 ----
 public class HomePage extends WebPage
 {
 	private Label firstLabel;
 	private Label secondLabel;

 	public HomePage(){
 		firstLabel = new Label("label", "First label");
 		secondLabel = new Label("label", "Second label");

 		add(firstLabel);
 		add(new Link<Void>("reload"){
 			@Override
 			public void onClick() {
 			}
 		});
 	}

 	@Override
 	protected void onBeforeRender() {
 		if(contains(firstLabel, true))
 			replace(secondLabel);
 		else
 			replace(firstLabel);

 		super.onBeforeRender();
 	}
 }
 ----

 The code inside _onBeforeRender()_ is quite trivial as it just checks which label among _firstLabel_ and _secondLabel_ is currently inserted into the component hierarchy and it replaces the inserted label with the other one.

 This method is also responsible for invoking children _onBeforeRender()_. So if we decide to override it, we have to call _super.onBeforeRender()_. However, unlike _onInitialize()_, the call to superclass method should be placed at the end of method's body in order to affect children's rendering with our custom code.

 Please note that in the example above we can trigger the rendering stage pressing F5 key or clicking on link “reload”.

 WARNING: If we forget to call superclass version of methods _onInitialize()_ or _onBeforeRender()_, Wicket will throw an _IllegalStateException_ with the following message: +
 `_java.lang.IllegalStateException: org.apache.wicket.Component has not been properly initialized. Something in the hierarchy of <page class name> has not called super.onInitialize()/onBeforeRender() in the override of onInitialize()/ onBeforeRender() method_`

 === Method renderHead

 This method gives all components the possibility to add items to the page header through its argument of type _org.apache.wicket.markup.head.IHeaderResponse_

 === Method onRender

 This method does the actual rendering -- you will rarely have to implement it, since most components already contain a specific implementation to produce their markup.

 === Method onComponentTag

 Method _onComponentTag(ComponentTag)_ is called to process a component tag, which can be freely manipulated through its argument of type _org.apache.wicket.markup.ComponentTag_. For example we can add/remove tag attributes with methods _put(String key, String value)_ and _remove(String key)_, or we can even decide to change the tag or rename it with method _setName(String)_ (the following code is taken from project OnComponentTagExample):

 *Markup code:*

 [source,html]
 ----
 <head>
   <meta charset="utf-8" />
   <title></title>
 </head>
 <body>
   <h1 wicket:id="helloMessage"></h1>
 </body>
 ----

 *Java code:*

 [source,java]
 ----
 public class HomePage extends WebPage {
    public HomePage() {
       add(new Label("helloMessage", "Hello World"){
          @Override
          protected void onComponentTag(ComponentTag tag) {
             super.onComponentTag(tag);
             //Turn the h1 tag to a span
             tag.setName("span");
             //Add formatting style
             tag.put("style", "font-weight:bold");
          }
       });
     }
 }
 ----

 *Generated markup:*

 [source,html]
 ----
 <head>
   <meta charset="utf-8" />
   <title></title>
 </head>
 <body>
   <span wicket:id="helloMessage" style="font-weight:bold">Hello World</span>
 </body>
 ----

 Just like we do with _onInitialize_, if we decide to override _onComponentTag_ we must remember to call the same method of the super class because also this class may also customize the tag. Overriding _onComponentTag_ is perfectly fine if we have to customize the tag of a specific component, but if we wanted to reuse the code across different components we should consider to use a behavior in place of this hook method.

 We have already seen in <<_modifing_tag_attributes,chapter 6.2>> how to use behavior _AttributeModifier_ to manipulate the tag's attribute. In <<_enriching_components_with_behaviors,chapter 19.1>> we will see that base class _Behavior_ offers also a callback method named _onComponentTag(ComponentTag, Component)_ that can be used in place of the hook method _onComponentTag(ComponentTag)_.

 === Methods onComponentTagBody

 Method _onComponentTagBody(MarkupStream, ComponentTag)_ is called to process the component tag's body. Just like _onComponentTag_ it takes as input a _ComponentTag_ parameter representing the component tag. In addition, we also find a _MarkupStream_ parameter which represents the page markup stream that will be sent back to the client as response.

 _onComponentTagBody_ can be used in combination with the _Component_'s method _replaceComponentTagBody_ to render a custom body under specific conditions. For example (taken from project OnComponentTagExample) we can display a brief description instead of the body if the label component is disabled:

 [source,java]
 ----
 public class HomePage extends WebPage {
    public HomePage() {

       add(new Label("helloMessage", "Hello World"){
          @Override
          protected void onComponentTagBody(MarkupStream markupStream, ComponentTag tag) {

            if(!isEnabled())
                replaceComponentTagBody(markupStream, tag, "(the component is disabled)");
           else
                super.onComponentTagBody(markupStream, tag);
          }
       });
     }
 }
 ----

 Note that the original version of _onComponentTagBody_ is invoked only when we want to preserve the standard rendering mechanism for the tag's body (in our example this happens when the component is enabled).

 === Methods onAfterRender

 Called on each rendered component immediately after it has been rendered - _onAfterRender()_ will even be called when rendering failed with an exception.

	This stage is reached each time a component is rendered, typically when a page is requested or when the component or one of its ancestors is refreshed via AJAX.

	=== Method onConfigure

	Method _onConfigure()_ has been introduced in order to provide a good point to manage the component states such as its visibility or enabled state. This method is called on all components whose parent is visible.

	As stated in <<_hiding_or_disabling_a_component,chapter 6.1>>, _isVisible()_ and _isEnabled()_ are called multiple times when a page or a component is rendered, so it's highly recommended not to directly override these method, but rather to use _onConfigure()_ to change component states. On the contrary method _onBeforeRender_ (see the next paragraph) is not indicated for this task because it will not be invoked if component visibility is set to false.

	=== Method onBeforeRender

	The most important hook method of this stage is probably _onBeforeRender()_. This method is called on all visible components before any of them are rendered. It is our last chance to change a component's state prior to rendering - no change to a component's state is allowed afterwards.

	If we want to add/remove child components this is the right place to do it. In the next example (project LifeCycleStages) we will create a page which alternately displays two different labels, swapping between them each time it is rendered:

	[source,java]
	----
	public class HomePage extends WebPage
	{
	private Label firstLabel;
	private Label secondLabel;

	public HomePage(){
	firstLabel = new Label("label", "First label");
	secondLabel = new Label("label", "Second label");

	add(firstLabel);
	add(new Link<Void>("reload"){
	@Override
	public void onClick() {
	}
	});
	}

	@Override
	protected void onBeforeRender() {
	if(contains(firstLabel, true))
	replace(secondLabel);
	else
	replace(firstLabel);

	super.onBeforeRender();
	}
	}
	----

	The code inside _onBeforeRender()_ is quite trivial as it just checks which label among _firstLabel_ and _secondLabel_ is currently inserted into the component hierarchy and it replaces the inserted label with the other one.

	This method is also responsible for invoking children _onBeforeRender()_. So if we decide to override it, we have to call _super.onBeforeRender()_. However, unlike _onInitialize()_, the call to superclass method should be placed at the end of method's body in order to affect children's rendering with our custom code.

	Please note that in the example above we can trigger the rendering stage pressing F5 key or clicking on link “reload”.

	WARNING: If we forget to call superclass version of methods _onInitialize()_ or _onBeforeRender()_, Wicket will throw an _IllegalStateException_ with the following message: +
	`_java.lang.IllegalStateException: org.apache.wicket.Component has not been properly initialized. Something in the hierarchy of <page class name> has not called super.onInitialize()/onBeforeRender() in the override of onInitialize()/ onBeforeRender() method_`

	=== Method renderHead

	This method gives all components the possibility to add items to the page header through its argument of type _org.apache.wicket.markup.head.IHeaderResponse_

	=== Method onRender

	This method does the actual rendering -- you will rarely have to implement it, since most components already contain a specific implementation to produce their markup.

	=== Method onComponentTag

	Method _onComponentTag(ComponentTag)_ is called to process a component tag, which can be freely manipulated through its argument of type _org.apache.wicket.markup.ComponentTag_. For example we can add/remove tag attributes with methods _put(String key, String value)_ and _remove(String key)_, or we can even decide to change the tag or rename it with method _setName(String)_ (the following code is taken from project OnComponentTagExample):

	Markup code:

	[source,html]
	----
	<head>
	<meta charset="utf-8" />
	<title></title>
	</head>
	<body>
	<h1 wicket:id="helloMessage"></h1>
	</body>
	----

	Java code:

	[source,java]
	----
	public class HomePage extends WebPage {
	public HomePage() {
	add(new Label("helloMessage", "Hello World"){
	@Override
	protected void onComponentTag(ComponentTag tag) {
	super.onComponentTag(tag);
	//Turn the h1 tag to a span
	tag.setName("span");
	//Add formatting style
	tag.put("style", "font-weight:bold");
	}
	});
	}
	}
	----

	Generated markup:

	[source,html]
	----
	<head>
	<meta charset="utf-8" />
	<title></title>
	</head>
	<body>
	<span wicket:id="helloMessage" style="font-weight:bold">Hello World</span>
	</body>
	----

	Just like we do with _onInitialize_, if we decide to override _onComponentTag_ we must remember to call the same method of the super class because also this class may also customize the tag. Overriding _onComponentTag_ is perfectly fine if we have to customize the tag of a specific component, but if we wanted to reuse the code across different components we should consider to use a behavior in place of this hook method.

	We have already seen in <<_modifing_tag_attributes,chapter 6.2>> how to use behavior _AttributeModifier_ to manipulate the tag's attribute. In <<_enriching_components_with_behaviors,chapter 19.1>> we will see that base class _Behavior_ offers also a callback method named _onComponentTag(ComponentTag, Component)_ that can be used in place of the hook method _onComponentTag(ComponentTag)_.

	=== Methods onComponentTagBody

	Method _onComponentTagBody(MarkupStream, ComponentTag)_ is called to process the component tag's body. Just like _onComponentTag_ it takes as input a _ComponentTag_ parameter representing the component tag. In addition, we also find a _MarkupStream_ parameter which represents the page markup stream that will be sent back to the client as response.

	_onComponentTagBody_ can be used in combination with the _Component_'s method _replaceComponentTagBody_ to render a custom body under specific conditions. For example (taken from project OnComponentTagExample) we can display a brief description instead of the body if the label component is disabled:

	[source,java]
	----
	public class HomePage extends WebPage {
	public HomePage() {

	add(new Label("helloMessage", "Hello World"){
	@Override
	protected void onComponentTagBody(MarkupStream markupStream, ComponentTag tag) {

	if(!isEnabled())
	replaceComponentTagBody(markupStream, tag, "(the component is disabled)");
	else
	super.onComponentTagBody(markupStream, tag);
	}
	});
	}
	}
	----

	Note that the original version of _onComponentTagBody_ is invoked only when we want to preserve the standard rendering mechanism for the tag's body (in our example this happens when the component is enabled).

	=== Methods onAfterRender

	Called on each rendered component immediately after it has been rendered - _onAfterRender()_ will even be called when rendering failed with an exception.