How to pass a GWT method as a parameter into a Javascript function?

Question

Overview

There is a GWT method called: private void handleError()
There is a JSNI Javascript function called: private native void registerErrorHandler()
The native javascript function calls another function from a third party Javascript library: foo.attachEvent("EventName", handlerReference);

Functionality

I need to pass the GWT method as a function parameter into the foo.attachEvent() function, i tried several approaches:

foo.attachEvent("Error", registerErrorHandler); -> TypeMismatchException

foo.attachEvent("Error", [email protected]::registerErrorHandler()); -> TypeMismatchException

var handler = [email protected]::registerErrorHandler()();
foo.attachEvent("Error", handler);

-> TypeMismatchException

Plain Javascript

When I write this in plain Javascript, it's working:

function handleError() {
    alert("Error");
}
function registerErrorHandler() {
    var event = "Error";
    var handler = handleError;

    foo.attachEvent (event, handler);
}

How can I implement that into GWT? I am having a lot of problems completely understanding the Javascript - Java Objects conversion. I understand the handleError function reference as a JavaScriptObject, but I am not able to get it working with that information.

GWT must be executed within a RootLayoutPanel - This a complete page. You cannot mix Javascript with GWT generated functions. Purpose of GWT is not this. — Sreehari Puliyakkot
– Sreehari Puliyakkot, Commented Jul 11, 2012 at 7:58

Daniel Kurka · Accepted Answer · 2012-07-11 07:54:57Z

0

you code needs to look like this: (there are only one time braces, therefore the function is not executed...)

var handler = [email protected]::registerErrorHandler();
foo.attachEvent("Error", handler);

but what you also want to do is wrap your function in the $entry function so that your uncaught exception handler will work:

var handler = [email protected]::registerErrorHandler();
foo.attachEvent("Error", $entry(handler));

What you can always do is build a js function that directly calls your gwt function:

var that = this;
var handler = function(){
    [email protected]::registerErrorHandler()();
};
foo.attachEvent("Error", $entry(handler));

answered Jul 11, 2012 at 7:54

Daniel Kurka

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2 Comments

Sven Plath Over a year ago

Thanks for your insight. Your first exampled worked instantly. However, the third party library is not attaching the event somehow, but when I define a new function like in your third example and provide an concrete implementation there (like $wnd.alert("handled!");) it's working. If i pass a GWT function with the special syntax, it's not working and somewhere in the gwt code (__gwt_tearOffGenerators)an exception is thrown. Do you have an idea for that one, too?

Sven Plath Over a year ago

Ah now i understand the var that stuff. It's working. Thanks again.

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