Question

I am building a GWT app that uses Web SQL Local Storage (
http://dev.w3.org/html5/webdatabase/ ).
The problem is that the Web SQL API uses callback functions as
arguments.

Is it possible to pass “Java” callbacks to JSNI?

Answer 1

Yes, it does:

private static native void doThingWithCallback() /*-{
  var self = this;
  var callbackFn = $entry(function(val) {
    [e-mail](Ljava/lang/String;)(val);
  });
  $wnd.someApiThatTakesACallback(callbackFn);
}-*/;

Two things to remember:

1. $entry() reminds GWT to keep track of the code when using the debugger.
2. var self = this keeps the reference to this inside the function — otherwise this will be the function itself…