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https://issues.apache.org/jira/browse/WICKET-6246?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15505701#comment-15505701
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Thomas Chrenko commented on WICKET-6246:
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The stackoverflow.com question where it all started:
http://stackoverflow.com/questions/39511880/apache-wicket-push-websocket-component-update-while-ajax-request-rendering
> WebSocket request while Ajax request leads to error regarding
> HtmlHeaderCotnainer
> ---------------------------------------------------------------------------------
>
> Key: WICKET-6246
> URL: https://issues.apache.org/jira/browse/WICKET-6246
> Project: Wicket
> Issue Type: Bug
> Components: wicket
> Affects Versions: 6.24.0
> Reporter: Thomas Chrenko
> Labels: header, header-contribution
> Attachments: quickstart.zip
>
>
> There is a problem with header contribution with leads to a "Cannot replace a
> component which has not been added: id='_header_'...." error.
> Short:
> - Add at least two components to an AjaxRequestTarget
> - The (at least) second component must initiate a WebSocket push update of
> another component in onConfigure/onBeforeRender (maybe others)
> - Exception at the end of ajax request (WebSocket request finishes without
> problem)
> Long:
> AbstractAjaxResponse iterates all components to update in
> writeComponents(Response, String).
> For each component writeComponent(Response, String, Component, String) is
> called in sub class XmlAjaxResponse which calls
> writeHeaderContribution(Response, Component) of AbstractAjaxResponse.
> This method creates the HtmlHeaderContainer and adds is to the page when
> rendering of first component added to target starts.
> Second component rendering (onConfigure/onBeforeRender) send a component
> update via websocket push to the client.
> The websocket request replaces the HtmlHeaderContainer added to the page and
> at the end of the AbstractAjaxResponse.writeTo(Response, String) to header is
> again replaced with a default one.
> But this default header is removed on MarkupContainer.detachChildren() method.
> Now the ajax request is "continued" and writeHeaderContribution is called for
> the second component.
> header is not null in writeHeaderContribution because it was already created
> when processing the first component, so the header is not added to the page
> again (remember it was replaced while websocket request).
> Now at the end of writeTo method, wicket tries again to replace the header
> with a default one (comment there says "restore normal header").
> This leads to an exception: Cannot replace a component which has not been
> added: id='_header_'....
> A little notice:
> The error would not happen, if second component would be processed before
> first component, because then the HeaderContainerItem of the ajax request
> isn't craeted before the websocket request is finished and so the header is
> added to the page on creation. So the problem is the removal of the
> HeaderContainerItem in detachChildren() after websocket request.
> In Wicket 7.4 (the only one i tested, this works without problems)
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