Fluent class for AsyncTask

Fluent.groovy

package me.champeau.gr8confagenda.app;

import android.os.AsyncTask;

import groovy.lang.Closure;

/**
 * An implementation of {@link android.os.AsyncTask} which makes it easy to deal with
 * requests/callbacks using Groovy closures
 */
public class Fluent<Result, Progress> extends AsyncTask<Void, Progress, Result> {
    private final Closure<Result> request;
    private final ResultConsumer<Result> then;
    private final Closure progress;

    private Fluent(Closure<Result> request, ResultConsumer<Result> then, Closure progress) {
        this.request = request;
        this.then = then;
        this.progress = progress;
    }


    @Override
    protected Result doInBackground(Void... params) {
        return request.call();
    }

    @Override
    protected void onPostExecute(Result result) {
        then.consume(result);
    }

    @Override
    protected void onProgressUpdate(Progress... values) {
        if (progress!=null) {
            progress.call(values);
        }
    }

    public static interface ResultConsumer<T> {
        void consume(T result);
    }

    public static class FluentAsyncTaskBuilder<Result,Progress> {
        Closure<Result> request;
        Closure progress;

        private FluentAsyncTaskBuilder<Result,Progress> from(Closure<Result> request) {
            this.request = request;
            return this;
        }

        FluentAsyncTaskBuilder<Result,?> onProgress(Closure progress) {
            this.progress = progress;
            return this;
        }

        void then(ResultConsumer<Result> then) {
            Fluent<Result,Progress> resultFluent = new Fluent<Result,Progress>(request, then, progress);
            resultFluent.execute();
        }
    }

    public static <Result,Progress> FluentAsyncTaskBuilder<Result,Progress> async(Closure<Result> request) {
        return new FluentAsyncTaskBuilder<Result,Progress>().from(request);
    }
}

Why is ResultConsumer the only functional interface?

Shouldn't the Fluent class set itself as delegate to the closures? How else can you check isCancelled and invoke progressUpdate? https://developer.android.com/reference/android/os/AsyncTask.html

See https://gist.github.com/karfunkel/6eba3c237890f90c2779 for an extended version.