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https://issues.apache.org/jira/browse/WICKET-6246?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15505737#comment-15505737
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Sven Meier commented on WICKET-6246:
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Thanks!

I'll try to find the difference to 7.x and why it works there.

> 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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