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When a component is attached to a page for the first time, the renderProperties(ContentRenderer) method is called to render all properties that will be sent to the client for creating a peer widget. All the required properties have to be rendered such that the client can create a peer widget with the same set of properties.

Implementing renderProperties(ContentRenderer) is straightforward. As shown below, all you need to do is to invoke super.renderProperties to render all inherited properties (such as width and height), and then the render method to render the property defined in this class.

protected void renderProperties(org.zkoss.zk.ui.sys.ContentRenderer renderer)
throws {
 render(renderer, "value", _value);

Why renderProperties and smartUpdate?

A common question is why renderProperties(ContentRenderer) (and redraw(Writer)) is required to render value again, when we notify the client with smartUpdate(String, boolean) in setValue.

The simple answer is that renderProperties is used to send all properties at once, when a component is going to be attached to the page at the client. On the other hand, smartUpdate(String, boolean) is used to send a property that was modified after the peer widget is attached at the client. ZK will minimize the traffic by removing unnecessary smartUpdate(String, boolean) (if renderProperties is called).

Once a widget is attached to the page at the client, renderProperties won’t be called. If you want to regenerate all properties and rerender it at the client, you can invoke the invalidate method. Note that if you update a property multiple times in the same execution (with smartUpdate(String, boolean)), only the last value will be sent to the client. If you prefer to send them all (rather than overwriting), then you can set the 3rd argument append to true (smartUpdate(String, Object, boolean)). For example you may use this when adding a listener for a certain event and you do not want to overwrite the present listener this is achieved by passing true as the third argument of smartUpdate(String, boolean). The table below outlines the differences.

smartUpdate smartUpdate (append, 3rd argument true)
Sent to client if invalidated or new created No No
Sent to client if the component is removed No No
Sent to client if name is the same Only the last value is sent Yes

Copyright © Potix Corporation. This article is licensed under GNU Free Documentation License.