GWT JSNI-Java到Java返回Java导致未定义的参数

Question

I have racked my brain on this for the better part of two days. I've read through the documentation on JSNI here as well as a few different blog posts on JSNI and passing variables like this one and nothing indicates I'm doing anything wrong. Essentially what I'm trying to do is call from my GWT Client Side class to a javascript method which I'm exporting to javascript as my class loads. That method takes params from another JS method and stores them on the instance of the Java class that I passed. That seems to work. But, once I reference those methods back in my java code, they're undefined. I believe that what is happening is that my Java Class instance is getting lost somehow after the JS finishes. Here's some code to help explain the workflow...

I have a Java Class named ProfileWidgee. That class has a method to set local variables for location, lattitude, and longitude. That method name is...

public void handleTargetPicked(String mloc, String mlat, String mlng)  {
    loc = mloc.equalsIgnoreCase("undefined") ? "" : mloc;
    lat = mlat.equalsIgnoreCase("undefined") ? "" : mlat;
    lng = mlng.equalsIgnoreCase("undefined") ? "" : mlng;
    Window.alert("setting on js side" + loc + lat + lng);
}

That method gets exported to the JS as a function using a JSNI method called exportMyFunction...

public static native void exportMyFunction(ProfileWidgee instance)/*-{
   $wnd.handleTargetPicked = $entry(
      instance.@com.n.j.client.widgees.profile.ProfileWidgee::handleTargetPicked(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;));

}-*/;

That all seems to go fine. It exports and I'm able to call the handleTargetPicked in my JS as follows...

handleTargetPicked(encodeURIComponent(place.formatted_address), 
    encodeURIComponent(place.geometry.location.lat()),
    encodeURIComponent(place.geometry.location.lng()));

All of this seems to work and the Window.alert() displays the correct values. That leads me to believe that it has the appropriate instance of my class and that it is setting the variables appropriately. Later on though as I'm back in my Java class, I try and reference those variables and they always come back as 'undefined.'

Window.alert("reading on the java side" + pw.getLoc() + pw.getLat() + pw.getLng());

This results in 'undefined' for all three values. So my big question is ... is it possible to set a value in your Java class from the JS side and then use that value later in your class?

Answer 1

I just ran into a somewhat similar situation like this and happened to see your post. I couldn't see any solutions suggested anywhere, so tried to debug it myself.

What I saw was, 'this' variable was pointing to Window rather than the object instance.

So rather than calling the method directly like eg handleTargetPicked(arg1, arg2), I used method.call() passing the context like eg handleTargetPicked.call(instance, arg1, arg2). That kind of approach solved the issue for me. Hope that helps.