Event echoing is useful for implementing a long operation.
As described in the previous section, HTTP is a request-and-response protocol, so the user won't see any feedback until the request has been served and responsed. Thus, if the processing of a request takes too long to execute, the user has no idea if the request is being process, or he doesn't, say, click the button successfully. The user usually tends to click again to ensure it is really clicked, but it only causes the server much slower to response.
The better approach is to send back some busy message to let the user know what happens during processing the long operation. It can be done easily with event echoing. If you prefer to allow the user to keep accessing other functions, please refer to the Use Event Queues section, which is powerful but more sophisticated to implement.
Event echoing for a long operation basically takes three steps
- Invoke Clients.showBusy(String) to show a busy message and blocking the user from accessing any function
- Invoke Events.echoEvent(String, Component, Object) to echo an event
- Listen to the event being echoed and do the long operation in the listener
- At the end of the event listener, remember to remove the busy message, and update the UI if necessary
For example, assume the long operation is called doLongOperation, then:
<window id="w" width="200px" title="Test echoEvent" border="normal"> <attribute name="onLater"> doLongOperation(); //take long to execute Clients.clearBusy(); //remove the busy message </attribute> <button label="Echo Event"> <attribute name="onClick"> Clients.showBusy("Execute..."); //show a busy message to user Events.echoEvent("onLater", w, null); //echo an event back </attribute> </button> </window>
Better Feedback with Button's autodisable
With event echoing, it might still take hundreds of milliseconds to have the busy message, especially with the slow connection. The feedback to user can be further improved by the use of Button.setAutodisable(String). For example,
<button label="Echo Event" autodisable="self"> ...
Then, the button will be disabled automatically when it is pressed, and enabled automatically when the request has been served.
Last Update : 2012/8/15
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