6 Components lifecycle

6.1 Lifecycle stages of a component

During its life a Wicket component goes through three basic stages:

1. Initialization: a component is instantiated by Wicket and prepared for the rendering phase.
2. Rendering: in this stage Wicket generates component markup. If a component contains children (i.e. is a subclass of MarkupContainer) it must first wait for them to be rendered before starting its own rendering.
3. Removing: this stage is triggered when a component is explicitly removed from its component hierarchy, i.e. when its parent invokes remove(component) on it. This stage is facultative and is never triggered for pages.

The following picture shows the state diagram of component lifecycle:

Once a component has been removed it can be added again to a container, but the initialization stage won't be executed again.

If you read the JavaDoc of class Component you will find a more detailed description of component lifecycle. However this description introduces some advanced topics we didn't covered yet hence, to avoid confusion, in this chapter some details have been omitted and they will be covered later in the next chapters.

For now you can consider just the simplified version of the lifecycle described above.

6.2 Hook methods for component lifecycle

Class Component comes with a number of hook methods that can be overridden in order to customize component behavior during its lifecycle. In the following table these methods are grouped according to the stage in which they are invoked (and they are sorted by execution order):

Now let's take a closer look at each stage and to at hook methods.

6.3 Initialization stage

This stage is performed at the beginning of the component lifecycle. During initialization, the component has already been inserted into its component hierarchy so we can safely access to its parent container or to its page with methods getParent() or getPage(). The only method triggered during this stage is onInitialize(). This method is a sort of “special” constructor where we can execute a custom initialization of our component.

Since onInitialize is similar to a regular constructor, when we override this method we have to call super.onInitialize inside its body, usually as first instruction.

6.4 Rendering stage

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 before the render phase starts. As stated in chapter 5.1, 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 before a component starts its rendering phase and it is our last chance to change its children hierarchy.

If we want add/remove children 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:

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("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”.

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 onComponentTag

Method onComponentTag(ComponentTag) is called to process 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:

<head>
  <meta charset="utf-8" />
  <title></title>
</head>
<body>		
  <h1 wicket:id="helloMessage"></h1>		
</body>

Java code:

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:

<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 chapter 5.2 how to use behavior AttributeModifier to manipulate the tag's attribute. In chapter 16.1 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:

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).

6.5 Removing stage

This stage is triggered when a component is removed from its component hierarchy. The only hook method for this phase is onRemove(). If our component still holds some resources needed during rendering phase, we can override this method to release them.

Once a component has been removed we are free to add it again to the same container or to a different one.

6.6 Summary

In this chapter we have seen which stages compose the lifecycle of Wicket components and which hook methods they provide. Overriding these methods we can dynamically modify the component hierarchy and we can enrich the behavior of our custom components.