ActivityAsyncProxy

ActivityAsyncProxy.java

/*
 *    Copyright 2012 Thomas Broyer <t.broyer@ltgt.net>
 *
 * 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 net.ltgt.gwt.activity;

import com.google.gwt.activity.shared.Activity;
import com.google.gwt.core.client.GWT;
import com.google.gwt.core.client.RunAsyncCallback;
import com.google.gwt.event.shared.EventBus;
import com.google.gwt.user.client.ui.AcceptsOneWidget;

/**
 * Makes it easy to put any {@link Activity} behind a split-point.
 * <p>
 * For simple activities, that could be done using an
 * {@link com.google.gwt.user.client.AsyncProxy AsyncProxy}, but it's not
 * possible for activities created through factories or via dependency
 * injection.
 * <p>
 * As a bonus, the {@link Activity} won't be instantiated at all in case
 * the {@link ActivityAsyncProxy} is {@link #onCancel cancelled} before
 * the {@link GWT.runAsync async call} returns.
 */
public abstract class ActivityAsyncProxy implements Activity {

  private boolean hasAsyncBeenIssued;
  private boolean hasAsyncBeenCancelled;
  private boolean hasAsyncFailed;
  private AcceptsOneWidget display;
  private EventBus eventBus;
  private Activity instance;

  @Override
  public String mayStop() {
    checkHasAsyncFailed();
    assert this.instance != null;
    return this.instance.mayStop();
  }

  @Override
  public void onCancel() {
    if (this.instance != null) {
      this.instance.onCancel();
    }
    checkHasAsyncFailed();
    this.hasAsyncBeenCancelled = true;
  }

  @Override
  public void onStop() {
    checkHasAsyncFailed();
    assert this.instance != null;
    this.instance.onStop();
  }

  @Override
  public void start(AcceptsOneWidget panel, EventBus eventBus) {
    if (this.instance != null) {
      this.instance.start(panel, eventBus);
      return;
    }
    checkHasAsyncFailed();
    assert !this.hasAsyncBeenIssued || this.hasAsyncBeenCancelled;
    this.display = panel;
    this.eventBus = eventBus;
    this.hasAsyncBeenCancelled = false;
    if (!this.hasAsyncBeenIssued) {
      this.hasAsyncBeenIssued = true;
      doAsync(new RunAsyncCallback() {

        @Override
        public void onSuccess() {
          if (!ActivityAsyncProxy.this.hasAsyncBeenCancelled) {
            assert ActivityAsyncProxy.this.instance == null;
            ActivityAsyncProxy.this.instance = createInstance();
            ActivityAsyncProxy.this.instance.start(ActivityAsyncProxy.this.display, ActivityAsyncProxy.this.eventBus);
          }
        }

        @Override
        public void onFailure(Throwable reason) {
          ActivityAsyncProxy.this.hasAsyncFailed = true;
          if (GWT.getUncaughtExceptionHandler() != null) {
            GWT.getUncaughtExceptionHandler().onUncaughtException(reason);
          }
        }
      });
    }
  }

  /**
   * Implementors should simply call {@link GWT#runAsync} here, and nothing else.
   * <p>
   * This is required to have a different split-point generated for each
   * {@link ActivityAsyncProxy} sub-class.
   */
  protected abstract void doAsync(RunAsyncCallback callback);

  protected abstract Activity createInstance();

  private void checkHasAsyncFailed() {
    if (this.hasAsyncFailed) {
      throw new IllegalStateException("runAsync load previously failed");
    }
  }
}

Example using a GIN Assisted-Inject factory; assuming a factory field in the class (typically an ActivityMapper), and a final String id local variable to pass as argument to the factory (typically extracted from the Place)

// typically found inside your ActivityMapper's getActivity method
return new ActivityAsyncProxy() {
   @Override
   protected void doAsync(RunAsyncCallback callback) {
      GWT.runAsync(callback);
   }
   @Override
   protected Activity createInstance() {
      return factory.employeeActivity(id);
   }
}

Hi Thomas, in your posts that date a couple of years back, you state that GWT Activities don't support nesting. Has anything changed in this regard; do they support nesting now?