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Replace IContextProvider<T, C> with standard Java 8 java.util.function.Function<C, T> which is virtually identical.

As a consequence IPageManagerProvider, IPageRendererProvider and IRequestCycleProvider now override #apply() method instead of #get().

Deprecate org.apache.wicket.protocol.http.documentvalidation.HtmlDocumentValidator 

Jira
server ASF JIRA serverId 5aa69414-a9e9-3523-82ec-879b028fb15b key WICKET-6119

Tests based on HtmlDocumentValidator are very fragile. They start to fail as soon as there is a single character change somewhere in the page markup.

We believe that there are very few users of this API. It is recommended to use TagTester and WicketTestCase#executeTest() instead.

Deprecate org.apache.wicket.model.AbstractReadOnlyModel

Use an anonymous instance of IModel instead. Since Wicket 8.0 IModel doesn't require providing implementation of #setObject(Object) method.

AbstractChoice#getChoices() has been made final. If the application needs to provide different choices for each render then it should override AbstractChoice#getChoicesModel() instead. The application code would be almost the same as before, it will just need to wrap the final List result in an IModel, most probably ListModel.

ListenerInterfaceRequestHandler simplification

Jira
server ASF JIRA columns key,summary,type,created,updated,due,assignee,reporter,priority,status,resolution serverId 5aa69414-a9e9-3523-82ec-879b028fb15b key WICKET-6137

RequestListenerInterface was removed:

• IResourceListener, IBehaviorListener, IOnChangeListener, ILinkListener are replaced by the generic method IRequestListener#onRequest()
• ListenerInterfaceRequestHandler was renamed to ListenerRequestHandler
• Component's and Behavior's #canCallListenerInterface() were renamed to #canCallListener()
• PageSettings#getCallListenerInterfaceAfterExpiry() was renamed to #getCallListenerAfterExpiry.

A Component or Behavior can now implement IRequestListener once only, thus removing the need to include an identifier (e.g. "ILinkListener") in the URL.

If you implemented IResourceListener previously, you have to override IRequestListener#rendersPage() now to return false.

wantOnSelectionChangedNotifications moved to FormComponentUpdatingBehavior 

Jira
server ASF JIRA columns key,summary,type,created,updated,due,assignee,reporter,priority,status,resolution serverId 5aa69414-a9e9-3523-82ec-879b028fb15b key WICKET-6348

Change notification was moved from CheckBox, DropDownChoice, RadioChoice, CheckGroup/Check and RadioGroup/Radio into a new behavior FormComponentUpdatingBehavior.

Instead of subclasses the component, this behavior can now be added to the component:

// Wicket 7.x
new CheckBox("id", model) {
	protected boolean wantOnSelectionChangedNotifications() {
		return true;
        }

	protected void onSelectionChanged(Boolean newSelection) {
		// do something, page will be rerendered;
	}
};


// Wicket 8.x
new CheckBox("id", model)
.add(new FormComponentUpdatingBehavior() {
	protected void onUpdate() {
		// do something, page will be rerendered;
	}

    protected void onError(RuntimeException ex) {
        super.onError(ex);
    }
});

As with AjaxFormComponentUpdatingBehavior any error during processing of the form component can now be handled in #onError().

Renderers are IDetachable now 

Jira
server ASF JIRA columns key,summary,type,created,updated,due,assignee,reporter,priority,status,resolution serverId 5aa69414-a9e9-3523-82ec-879b028fb15b key WICKET-6347

Renderers (IChoiceRendere, IOptionRenderer and IAutoCompleteRenderer now take part in detachment as other Wicket concepts like components and models. The owning component is responsible to detach it.