NotifyingAsyncCallback.java

/*
 * Copyright 2012 Cedric Hauber
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *        http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */

package org.jboss.errai.mvp.client.events;

import com.google.gwt.user.client.rpc.AsyncCallback;
import com.google.web.bindery.event.shared.EventBus;

/**
 * An implementation of {@link com.google.gwt.user.client.rpc.AsyncCallback} that sends events on the {@link com.google.web.bindery.event.shared.EventBus} whenever
 * an async call starts, succeeds, or fails. The events fired are {@link AsyncCallStartEvent},
 * {@link AsyncCallSucceedEvent}, {@link AsyncCallFailEvent}.
 *
 * So the way to use {@link NotifyingAsyncCallback} is the following:
 * <pre>
 *   NotifyingAsyncCallback callback = new NotifyingAsyncCallback(eventBus) { ... };
 *   callback.prepare();
 *   someRpcMethod(callback);
 *   callback.checkLoading();
 * </pre>
 *
 * @param <T> The type of the return value. See {@link com.google.gwt.user.client.rpc.AsyncCallback}.
 *
 * @author Philippe Beaudoin
 */
public abstract class NotifyingAsyncCallback<T> implements AsyncCallback<T> {

  /**
   * This enum indicates the state of the notifying async callback. It starts in
   * the {@code UNKNOWN} state and moves to {@code INITIALIZED} when {@link #prepare}
   * is called. If {@link #checkLoading} is called before the async call has succeeded
   * or failed, then it moves to {@code LOADING}. When the call terminates it moves to
   * {@code SUCCEEDED} or {@code FAILED}.
   */
  public enum State {
    UNKNOWN,
    INITIALIZED,
    LOADING,
    SUCCEEDED,
    FAILED
  }

  // Using a static counter to ensure we only fire once, otherwise we risk running into an
  // infinite loop if the handler requires an async call to fetch the presenter (which is the
  // case when using @ProxyEvent).
  // Using an external injected resource would be better, but it's really cumbersome here because
  // this class is created inside generators
  static int counter;

  private final EventBus eventBus;

  private State state;

  public NotifyingAsyncCallback(EventBus eventBus) {
    this.eventBus = eventBus;
    state = State.UNKNOWN;
  }

  @Override
  public final void onFailure(final Throwable caught) {
    assert state == State.INITIALIZED || state == State.LOADING;

    if (state == State.LOADING) {
      counter--;
      if (counter == 0) {
        AsyncCallFailEvent.fire(eventBus, caught);
      }
    }
    state = State.FAILED;
    failure(caught);
  }

  @Override
  public final void onSuccess(T result) {
    assert state == State.INITIALIZED || state == State.LOADING;
    if (state == State.LOADING) {
      counter--;
      if (counter == 0) {
        AsyncCallSucceedEvent.fire(eventBus);
      }
    }
    state = State.SUCCEEDED;
    success(result);
  }

  /**
   * This method is used to place that the async callback in its {@code INITIALIZED} stated. It
   * should be call prior to invoking any RPC that takes this callback as a parameter. See
   * {@link NotifyingAsyncCallback} for usage.
   */
  public void prepare() {
    assert state != State.LOADING;
    state = State.INITIALIZED;
  }

  /**
   * This method is used to indicate that the async callback is currently loading. It will not
   * do anything if the call is already finished (i.e. in the case of an immediate failure or
   * success). See {@link NotifyingAsyncCallback} for usage.
   */
  public void checkLoading() {
    assert state != State.UNKNOWN;
    if (state == State.INITIALIZED) {
      counter++;
      if (counter == 1) {
        AsyncCallStartEvent.fire(eventBus);
      }
      state = State.LOADING;
    }
  }

  /**
   * This method is called whenever the asynchronous call returns successfully.
   *
   * @param result the return value of the remote produced call.
   */
  protected abstract void success(T result);

  /**
   * This method is called whenever the asynchronous call returns with a failure. Override this
   * method if you want to perform clean-up operation when an async call fails. The default
   * implementation is empty.
   *
   * @param caught failure encountered while executing a remote procedure call.
   */
  protected void failure(Throwable caught) {
  }
}