Hi ChrisK,
You need to understand how JavaScript works.
Like in many languages, the "this" keyword means the current object.
The JavaScript language allows you to manipulate code as a function.
This function can be affected to any variable or object member, like:
$wnd.js_callback = function() { ... }
Then, you can call your code like this:
$wnd.js_callback()
In the function body, the "this" variable is $wnd. If you assign the
same function to the member of another object, the "this" variable
will be equal to the new object.
>From JSNI, "@mypackage.MyClass::MyMethod()" references the JavaScript
function corresponding to the MyMethod method of mypackage.MyClass.
This method can be static or not, the reference is the same (no need
of "this" before "@..." just to obtain the JavaScript function).
Calling "$entry" just add a function wrapper around it but it stay a
function. When you assign this function to "$wnd.js_callback", the
"this" variable inside of the function will be "$wnd". If MyMethod is
static, it's okay. If MyMethod is not static, GWT will probably "cast"
the $wnd to a MyClass... it's bad...
The "call" method of a JavaScript function allows you to call it with
a differente "this". Then, using "$wnd.js_callback.call(otherObject)",
the "this" variable reference otherObject. So the glue is:
var that = this;
$wnd.js_callback = function() {
$entry(@mypackage.MyClass::MyMethod()).call(that);
}
When "js_callback" will be called, MyMethod is called on "that", the
JavaScript object representing the MyClass instance. Finally, to avoid
memory leaks of closure:
$wnd.js_callback = (function(obj) {
return function() {
$entry(@mypackage.MyClass::MyMethod()).call(obj);
};
})(this);
And remember, for static methods it's just:
$wnd.js_callback = $entry(...);
Good luck,
Olivier
On 22 mar, 20:16, ChrisK <cknow...@gmail.com> wrote:
> Thanks for the reply!
>
> I'm not sure I understand why "this" evaluates to the plain JS
> context. I was going off the documentation here which seems to suggest
> you can use "this" in this
> way:http://code.google.com/webtoolkit/doc/latest/DevGuideCodingBasicsJSNI...
>
> Or perhaps you just can't use it for $entry type JSNI?
>
> Having said that, I've updated to a non-static method - silly me. I
> did actually try that first but only pasted in my last attempt at
> getting some working code. I have tried both of the below now with the
> same stack trace as a result. Note, there are not actually any spaces
> before the .@ in my code I'm just working around Google Groups
> mangling it.
>
> private native void initialiseCallbacks() /*-{
> var that = this;
> $wnd.js_callback =
> $entry(that ....@com.mine.playerimpl::callback());
>
> }-*/;
>
> and
>
> private native void initialiseCallbacks(PlayerImpl pl) /*-{
> $wnd.js_callback = $entry(pl ....@com.mine.playerimpl::callback());
>
> }-*/;
>
> On Mar 22, 5:17 pm, Thomas Broyer <t.bro...@gmail.com> wrote:
>
> > On Mar 22, 4:23 pm, ChrisK <cknow...@gmail.com> wrote:
>
> > > My above code got a little mangled along the way, the $entry part has
> > > "(this", followed by a full stop, followed by
> > > "@com.mine.PlayerImpl::callback());"
>
> > And that's the problem: "this" is a special keyword and will evaluate
> > to the "this" context of the calling "plain JS" code, whereas you'd
> > want it to be some PlayerImpl instance.
>
> > There are several ways to fix this:
> > // evaluates "this" right now
> > var that = this;
> > // then uses "that"
> > $wnd.js_callback = $entry(t...@com.mine.playerimpl::callback());
>
> > or pass the instance as an argument to the method and then use its
> > name.
>
> > But there's another bug in your code: there's no "this" in a "static"
> > method!
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