Exporting Instance Methods to Hand-Written Javascript

[Geoffrey Wiseman]

I have a piece of GWT code that I wanted to invoke from outside handwritten JavaScript. There's an example shown here under "Calling a Java method from Handwritten JavaScript":

https://developers.google.com/web-toolkit/doc/latest/DevGuideCodingBasicsJSNI

That example uses a static method; I can make that work, but my first instinct was that i'd rather use an instance, so using the syntax shown under JSNI invocations, I tried:

/* package */ native void exportJavascriptMethods( PreviewPane x ) /*-{
       $wnd.imageSelected =
          $entry(this@ca.cpp.spike.gwtapplet.client.PreviewPane::imageSelected(Ljava/lang/String;Ljava/lang/String;));
  }-*/;

That doesn't work. Eclipse (and the GWT compiler) complain:

"JavaScript parsing: Missing ) after argument list PreviewPane.java"

If I replace 'this' with 'x', same problem. If I take 'this' out and leave it binding to the static method:

/* package */ native void exportJavascriptMethods( PreviewPane x ) /*-{
       $wnd.imageSelected =
          $entry(@ca.cpp.spike.gwtapplet.client.PreviewPane::imageSelected(Ljava/lang/String;Ljava/lang/String;));
  }-*/;

All is well in the world.

So -- is that just not an option, exporting an instance method like that?

[Alfredo Quiroga-Villamil]

At first glance it seems like:

"PreviewPane::imageSelected(Ljava/lang/String;Ljava/lang/String;))"

takes two arguments, in this case two Strings. The JSNI error seems to indicate that you are not passing the two arguments as per the method signature. I would think that you would end up with something like this:

PreviewPane::imageSelected(Ljava/lang/String;Ljava/lang/String;))(string1, string2);

Best regards,

Alfredo

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Alfredo Quiroga-Villamil

AOL/Yahoo/Gmail/MSN IM:  lawwton

[Thomas Broyer]

On Monday, April 2, 2012 10:26:27 PM UTC+2, Geoffrey Wiseman wrote:

I have a piece of GWT code that I wanted to invoke from outside handwritten JavaScript. There's an example shown here under "Calling a Java method from Handwritten JavaScript":

https://developers.google.com/web-toolkit/doc/latest/DevGuideCodingBasicsJSNI

That example uses a static method; I can make that work, but my first instinct was that i'd rather use an instance, so using the syntax shown under JSNI invocations, I tried:

/* package */ native void exportJavascriptMethods( PreviewPane x ) /*-{
       $wnd.imageSelected =
          $entry(this@ca.cpp.spike.gwtapplet.client.PreviewPane::imageSelected(Ljava/lang/String;Ljava/lang/String;));
  }-*/;

That doesn't work. Eclipse (and the GWT compiler) complain:

"JavaScript parsing: Missing ) after argument list PreviewPane.java"

You're missing the '.' before the '@', so it might be the issue (the parser chokes on the '@' and says the $entry() call is missing its closing paren, or something like that).

You'll have another issue when you fix this syntax error though: you get a reference on the method but, like in Java with java.lang.reflect.Method, the instance it's called on has to be given at call time.

You have to write:

   var that = this;

   $wnd.imageSelected = $entry(function(s1, s2) {

         return that.@ca.cpp.spike.gwtapplet.client.PreviewPane::imageSelected(Ljava/lang/String;Ljava/lang/String;)(s1, s2);

      });

About the "var that = this;", see http://www.yuiblog.com/blog/2012/03/30/what-is-the-meaning-of-this