GWT 2.7: Warn users they are using an unsupported browser - gwt

Question

I recently upgraded my application to GWT 2.7 from GWT 2.5. This has caused me to drop support for IE6 and IE7.
I would like to provide users with IE6 or IE7 with a warning that their browser is outdated and will not work. At the moment if you go to the app with one of those browsers, you get a blank screen.
I know there are a couple ways that I could hack something together but I would rather use the GWT way, rather than some hack. Is there a GWT hook for unsupported browsers?
Option (hack) One
Drop this into my main.html:
if(document.documentMode === 6 || document.documentMode === 7){
myUnsupportedBrowserWarningFunction();
}
Potential problem with this is that if someone is using a browser that GWT doesn't recognise and I don't recognise (mobile opera? Some other browser), they will still get a blank page.
Option (hack) Two
GWT looks for the compiled JS here:
gwt/myApp/ASDFKLSDJFLSFDJSLDFJLSJDFSDES.cache.js
When someone is using an unsupported browser the following is requested (and is not found):
gwt/myApp/undefined.cache.js
It would be possible to create undefined.cache.js and put your unsupported browser code there. This is obviously a brittle solution and will break with future GWT updates.
Option Three
A recent patch (available in GWT 2.7) allows you to provide a default
permutation (e.g. safari) if GWT can not detect the browser and with
deferred binding you can display a warning that the provided app might not
work correctly as the browser is generally unsupported by GWT.
-- J.
Source
I don't want to set a default permutation for unsupported browsers. I want the site to not work and to display a warning. So this solution doesn't really provide what I am looking for.
Similar Questions & Posts
The same question was asked for an eariler version of GWT in 2009. I hope that GWT has added some kind of hook or best practice in the last 6 years.
More info on setting a default (fallback) permutation

Answer 1

You should be able to use onLoadErrorFn for that: https://code.google.com/p/google-web-toolkit/issues/detail?id=8135
<script>
function gwtLoadError(errMsg) {
// GWT app couldn't load, reason in errorMsg
}
</script>
<meta name="gwt:onLoadErrorFn" content="gwtLoadError">
or possibly onPropertyErrorFn:
<script>
function gwtPropError(propName, allowedValues, actualValue) {
if (propName == 'user.agent') {
// unsupported browser
}
}
</script>
<meta name="gwt:onPropertyErrorFn" content="gwtPropError">
(I don't think user.agent.runtimeWarning would help in this case, but maybe have a look)

Answer 2

There is an easy way:
Conditional Comments
<!--[if lt IE 8]>
<p>You are using an unsupportet browser. Please perform an update</p>
<![endif]-->
I think Option 3 may be the best one, but there is a problem: This will start the actual application (which still may be incompatible).
If this is an issue and you want a clear warning, you can rewrite the permutation selection script (You would need to update the script with the upcoming GWT releases)
You will need to copy this source:
https://gwt.googlesource.com/gwt/+/2.7.0/user/src/com/google/gwt/useragent/rebind/UserAgentPropertyGenerator.java
You could add something like:
$wnd.Location.replace('nosupported.html');
between line 90 and 91