java-design-patterns/half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/AsyncTask.java

package com.iluwatar.halfsynchalfasync;

import java.util.concurrent.Callable;

/**
 * Represents some computation that is performed asynchronously and its result. The computation is
 * typically done is background threads and the result is posted back in form of callback. The
 * callback does not implement {@code isComplete}, {@code cancel} as it is out of scope of this
 * pattern.
 * 
 * @param <O> type of result
 */
public interface AsyncTask<O> extends Callable<O> {
  /**
   * Is called in context of caller thread before call to {@link #call()}. Large tasks should not be
   * performed in this method as it will block the caller thread. Small tasks such as validations
   * can be performed here so that the performance penalty of context switching is not incurred in
   * case of invalid requests.
   */
  void onPreCall();

  /**
   * A callback called after the result is successfully computed by {@link #call()}. In our
   * implementation this method is called in context of background thread but in some variants, such
   * as Android where only UI thread can change the state of UI widgets, this method is called in
   * context of UI thread.
   */
  void onPostCall(O result);

  /**
   * A callback called if computing the task resulted in some exception. This method is called when
   * either of {@link #call()} or {@link #onPreCall()} throw any exception.
   * 
   * @param throwable error cause
   */
  void onError(Throwable throwable);

  /**
   * This is where the computation of task should reside. This method is called in context of
   * background thread.
   */
  @Override
  O call() throws Exception;
}