Passing a Java callback in GWT to JSNI method which returns a Promise?

Question

I am new to Promises and need some advice on how to handle a returned Promise in a JSNI method called from Java. Can't use JsInterop due to older version 2.7 of GWT. Any advice is appreciated.

Thanks for your time!

Answer 1

If you just have to read properties on the return object, you may try treating it as JSONObject.

Alternatively you could call a method of your Java object in the resolver function of the promise to check conditions to decide if you want to resolve/reject. p.then() could be called with onFulfilled/onRejected functions just wrapping Java method calls to handle the result.

Example:

public class PromiseWrapper {

    boolean evaluate(String value) {
        return value != null ? true : false;
    }

    void onFulfilled(String value) {
        // use value
    }

    void onRejected(Strin reason) {
        // use reason
    }

    public native void doSomethingAynchronously() /*-{
        var p = new Promise(
            function(resolve, reject) {
                var value = null; // Get value from somewhere
                if([email protected]::evaluate(Ljava/lang/String;)(value)) {
                    resolve(value);
                }
                else {
                    reject("Because");
                }
            }
        );
        p.then(function(value) {
                [email protected]::onFulfilled(Ljava/lang/String;)(value);
            }, 
            function(reason) {
                [email protected]::onRejected(Ljava/lang/String;)(reason);
            }
        );
    }-*/;

}

This is an untested example.