GWT: ‘A widget that has an existing parent widget may not be added to the detach list’
Problem
When wrapping an existing DOM element with a GWT widget (e.g. Button.wrap(…), HTML.wrap(…), etc), the GWT module will not load and report the exception like the following:
java.lang.AssertionError: A widget that has an existing parent widget may not be added to the detach list at com.google.gwt.user.client.ui.RootPanel.detachOnWindowClose(RootPanel.java:136) at com.google.gwt.user.client.ui.Label.wrap(Label.java:130)
Analysis
In GWT it does not seem to be possible, to wrap an element in the document's DOM if any of its parents has been wrapped with a GWT widget before. For instance, see for the following HTML document …
<div style="display: _none_" id=_"outer_element"_>
<form class=_"well"_>
<label><i class=_"icon-exclamation-sign"_></i> An error occurred while requesting your API key.</label>
<div id=_"inner_element"_>No error message.</div>
</form>
</div>
… if we first wrap the element "outer_element" and then the element "inner_element" as in the following …
final HTML errorForm = HTML.wrap(DOM.getElementById("outer_element"));
final Label errorMessage = Label.wrap(DOM.getElementById("inner_element"));
.. an exception will be thrown, since "outer_element" is the parent in the DOM of "inner_element".
Solution
The exception can easily be prevented by wrapping DOM elements as GWT widgets from the inside out. So, the elements nested the deepest in the DOM will be wrapped first. Therefore, the example above will work smoothly, if the order in which the elements are wrapped is reversed, so that the inner element "inner_element" is wrapped before the outer element "outer_element".
// inner element must be wrapped BEFORE outer element
final Label errorMessage = Label.wrap(DOM.getElementById("inner_element"));
final HTML errorForm = HTML.wrap(DOM.getElementById("outer_element"));
References gwt - How to add a custom widget to an element - Stack Overflow
How to wrap an existing div into an HTML widget ? - Google Web Toolkit | Google
