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Arez Browser based Extras and Addons
There is a newer version: 0.76
package arez.browser.extras;

import arez.annotations.Action;
import arez.annotations.ArezComponent;
import arez.annotations.Computed;
import arez.annotations.Observable;
import arez.annotations.OnActivate;
import arez.annotations.OnDeactivate;
import elemental2.dom.DomGlobal;
import elemental2.dom.EventListener;
import java.util.Date;
import javax.annotation.Nonnull;
import javax.annotation.Nullable;
import org.realityforge.anodoc.Unsupported;

/**
 * An observable model that declares state that tracks when the user is "online".
 * The online state is essentially a reflection of the browsers "navigator.onLine"
 * value. If an observer is observing the model, the model listens for changes from
 * the browser and updates the online state as appropriate. However if there is no
 * observer for the state, the model will not listen to to the browser events so as
 * not to have any significant performance impact.
 *
 * A very simple example
 * {@code
 * import com.google.gwt.core.client.EntryPoint;
 * import elemental2.dom.DomGlobal;
 * import arez.Arez;
 * import arez.browser.extras.NetworkStatus;
 *
 * public class NetworkStatusExample
 *   implements EntryPoint
 * {
 *   public void onModuleLoad()
 *   {
 *     final NetworkStatus networkStatus = NetworkStatus.create();
 *     Arez.context().autorun( () -> {
 *       final String message = "Network Status: " + ( networkStatus.isOnLine() ? "Online" : "Offline" );
 *       DomGlobal.console.log( message );
 *       if ( !networkStatus.isOnLine() )
 *       {
 *         DomGlobal.console.log( "Offline since: " + networkStatus.getLastChangedAt() );
 *       }
 *     } );
 *   }
 * }
 * }
 */
@Unsupported( "This is still considered experimental and will likely evolve over time" )
@ArezComponent( nameIncludesId = false )
public abstract class NetworkStatus
{
  private final EventListener _listener;
  private boolean _rawOnLine;
  @Nullable
  private Date _lastChangedAt;

  /**
   * Create an instance of NetworkStatus.
   *
   * @return the NetworkStatus instance.
   */
  @Nonnull
  public static NetworkStatus create()
  {
    return new Arez_NetworkStatus();
  }

  NetworkStatus()
  {
    _listener = e -> updateOnlineStatus();
    _rawOnLine = DomGlobal.navigator.onLine;
    _lastChangedAt = new Date();
  }

  /**
   * Return true if the browser is online, false otherwise.
   *
   * @return true if the browser is online, false otherwise.
   */
  @Computed
  public boolean isOnLine()
  {
    return isRawOnLine();
  }

  /**
   * Return the last time at which online status changed.
   * This will default to the time the component was created, otherwise
   * the time at which the online status was changed.
   *
   * @return the last time at which online status changed.
   */
  @Observable
  @Nullable
  public Date getLastChangedAt()
  {
    return _lastChangedAt;
  }

  @OnActivate
  final void onOnLineActivate()
  {
    DomGlobal.window.addEventListener( "online", _listener );
    DomGlobal.window.addEventListener( "offline", _listener );
  }

  @OnDeactivate
  final void onOnLineDeactivate()
  {
    DomGlobal.window.removeEventListener( "online", _listener );
    DomGlobal.window.removeEventListener( "offline", _listener );
  }

  @Observable
  boolean isRawOnLine()
  {
    return _rawOnLine;
  }

  void setRawOnLine( final boolean rawOnLine )
  {
    _rawOnLine = rawOnLine;
  }

  void setLastChangedAt( @Nullable final Date lastChangedAt )
  {
    _lastChangedAt = lastChangedAt;
  }

  @Action
  void updateOnlineStatus()
  {
    setRawOnLine( DomGlobal.navigator.onLine );
    setLastChangedAt( new Date() );
  }
}