By use of the echoEvent method in the org.zkoss.zk.ui.event.Events class, the application could ask the client to echo back the event for processing later. This method returned immediately after queuing the response asking the client to echo back the event.

Notice that, unlike sendEvent and postEvent, the event won't be processed in the current execution. Rather, it is processed later after the client echoes back the event. In other words, the event is processed later after the client has updated its user interfaces. Thus, it is useful to prompt the user before starting a long operation.

For example, you can open a highlighted window and then invoke echoEvent to do the long operation after the client shows the window (and echoes back the event).

For example, we can use the org.zkoss.zk.ui.util.Clients.showBusy method to show the busy message such that the user knows the system is busy. So, the end user will see "Execute..." first and then, after two seconds, "Done." in the following example. If you use postEvent, the end user will see "Execute..." and "Done" at the same time after two seconds.

<window id="w" title="Test echoEvent">
    <attribute name="onLater">    
    Thread.sleep(2000);    
    Clients.showBusy(null, false);    
    new Label("Done.").setParent(w);    
    </attribute>    

    <button label="echo">    
    <attribute name="onClick">    
    Clients.showBusy("Execute...", true);    
    Events.echoEvent("onLater", w, null);    
    </attribute>    
    </button>    
</window>