ZK Developer's Reference/Event Handling/Event Firing

Events are usually fired (aka., triggered) by a component (when serving the user at the client). However, applications are allowed to fire events too.

There are three ways to trigger an event: post, send and echo.

Post an Event

Posting is the most common way to trigger an event. By posting, the event is placed at the end of the system event queue. Events stored in the system event queue are processed one-by-one in first-in-first-out order. Each desktop has one system event queue and all events are handled sequentially.

To trigger an event, you could invoke Events.postEvent(String, Component, Object). For example,

Events.postEvent("onClick", button, null); //simulate a click

In addition to posting an event to the end of the system event queue, you could specify a priority with Events.postEvent(int, String, Component, Object). By default, the priority is 0. The higher the priority the earlier an event is processed.

Notice that the invocation returns after placing the event in the system event queue. In other words, the event won't be processed unless all other events posted earlier or with higher priority are processed.

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Send an Event

If you prefer to trigger an event to a component directly and process it immediately, rather than placing it in the system event queue and waiting for execution, you could use Events.sendEvent(String, Component, Object) to trigger the event.

Events.sendEvent("onMyEvent", component, mydata);

Events.sendEvent(String, Component, Object) won't return until all handlers and listeners registered for this event have been processed. You could image it as a method of invocation. Also notice that the event handlers and listeners are invoked directly without starting any event threads (no matter whether the event thread is enabled or not).

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Echo an Event

Echoing is a way to delay event processing until the next AU request (aka., Ajax) is received.

More precisely, the event being echoed won't be queued into the system event queue. Rather, it asks the client to send back an AU request immediately. Furthermore, after the server receives the AU request, the event is then posted to the system event queue for processing.

In other words, the event won't be processed in the current execution. Rather, it is processed in the following request when the event is echoed back from the client. Here is an example of using Events.echoEvent(String, Component, Object):

   Events.echoEvent("onMyEvent", component, mydata);

Event echoing is useful for implementing a long operation. HTTP is a request-and-response protocol, so the user won't receive any feedback until the request has been served and responded. Thus, we could send back some busy messages to let the user know what has happened, and echo back an event to do the long operation. For more information, please refer to the Long Operations: Use Echo Events section.