GWT: How to execute native js function passed as param (and thus not accessible globally)

Question

Here's the minimal snippet of code to reproduce the problem I'm struggling with:

import java.util.function.Consumer;

public class NaivePromise<T> {

  public NaivePromise(Consumer<Consumer<T>> resolve) {
    super();
    create(resolve);
  }

  public native NaivePromise<T> create(Consumer<Consumer<T>> handler) /*-{
    return new Promise(function (resolve) {
      console.log("DBG NATIVE RESOLVE");
      [email protected]::accept(*)(resolve)
    });
  }-*/;


  public static void pong() {
    new NaivePromise<String>(resolve -> {
      resolve.accept("HERE WE'LL GET AN ERROR, SINCE RESOLVE IS ACTUALLY A NATIVE FUNCTION");
    });
  }

}

My question would be - how can I execute native functions passed as lambdas to GWT Consumer (or any other functional interface)?

Answer 1

You'd have to make resolve a JavaScriptObject rather than a Consumer<String>, and use JSNI to call it:

private native void call(JavaScriptObject resolve, String arg) /*-{
  resolve(arg);
}-*/;

Though you actually really should use JsInterop here, with a @JsFunction interface; and probably actually just use Elemental 2's mapping of Promise.