Thanks Jens.  Now switched it to use *$doc*.  And, yes, I just register 
once, and fire a custom event on the event bus.  Although, I just did it 
using my own event bus like this:

final EventBus EVENT_BUS = GWT.create(SimpleEventBus.class);

listenForVisibilityChange(new Command() {
  @Override
  public void execute() {
    boolean visible = "visible".equals(getVisibilityState());
    EVENT_BUS.fireEvent(new WindowVisibilityChangedEvent(visible));
  }
});


I suspect your way is better, although, I don't really understand what 
sinking events onto the DOM is doing.

elemental2 looks cool!  Will check it out.

My custom event if anyone wants to reuse it:

public class WindowVisibilityChangedEvent extends GwtEvent<
WindowVisibilityChangedEventHandler> {
  public static Type<WindowVisibilityChangedEventHandler> TYPE = new Type<
WindowVisibilityChangedEventHandler>();
  private boolean visibile;
 
  public WindowVisibilityChangedEvent(boolean visible) {
    this.visibile = visible;
  }
 
  @Override
  public Type<WindowVisibilityChangedEventHandler> getAssociatedType() {
    return TYPE;
  }

  @Override
  protected void dispatch(WindowVisibilityChangedEventHandler handler) {
    handler.visibilityChanged(this);
  }
 
  public boolean isVisibile() {
    return visibile;
  }
}


And the handler:
public interface WindowVisibilityChangedEventHandler extends EventHandler {
  void visibilityChanged(WindowVisibilityChangedEvent 
windowVisibilityChangedEvent);
}


Cheers.


On Monday, August 7, 2017 at 7:21:33 PM UTC+10, Jens wrote:
>
> Generally ok, given it is a global listener and you probably just install 
> it once on app initialization. However you should use *$doc* instead of 
> *document* to make sure you listen on the global document. Depending on 
> your browser support you might want a utility method to check if visibility 
> change events are actually supported by the browser.
>
> static native boolean isSupported() /*-{
>   return $doc.hidden !== 'undefined' && $doc.visibilityState !== 
> 'undefined';
> }-*/;
>
> In our app we created a PageVisibilityChangedEvent which extends GwtEvent 
> and then fire it on the app wide EventBus so everyone can easily listen 
> for it. To register the DOM handler we used 
>
> Element doc = Document.get().cast();
> DOM.sinkBitlessEvent(doc, VISIBILITY_CHANGE_EVENT);
> DOM.setEventListener(doc, new EventListener() {
>   @Override
>   public void onBrowserEvent(final Event event) {
>     if (VISIBILITY_CHANGE_EVENT.equals(event.getType())) {
>      firePageVisibilityChangedEvent();
>    }
>  }
> });
>
> You could of course also use elemental2 (requires newest GWT) or JsInterop 
> to reduce the amount of JSNI if that is important for you.
>
> -- J.
>

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