Data Binding

Data binding synchronizes data between View and ViewModel according to component definition's annotation. The annotation specifies when to save (or load) which attribute, how to convert, validate and render the data. It can be found in metainfo\zk\lang-addon.xml of zkbind.jar. Please refer to ZK Client-side Reference/Language Definition about how to configure language definition and its addon. If you want data binding can works on your newly-created component, you should define its own annotations.

Annotation Attributes

Here is the attribute list used in lang-addon.xml:

Attribute Name	Description
ACCESS	Access privilege. The value can be "both", "save", or "load"(default value); default value is used if not specify.
CONVERTER	System converter for special properties. (optional) e.g. SelectedItem in listbox. see ListboxSelectedItemConverter
VALIDATOR	System validator for special properties. (optional)
SAVE_EVENT	Save trigger event. It takes effect only when ACCESS attribute is "both" or "save".
LOAD_EVENT	Load trigger event; It takes effect only when ACCESS attribute is "both" or "load".
LOAD_REPLACEMENT	The replacement attribute for loading. It's used when there is a issue to load to original attribute.; e.g. value of textbox, it loads to "rawValue".
LOAD_TYPE	Type of attribute for loading; e.g. rawValue of textbox is java.lang.String.
SAVE_REPLACEMENT	The replacement attribute for saving. It's used when there is a issue to save to original attribute.; e.g. selectedItem of selectbox, it save the value selecteIndex via converter to the bean. (selectedItem is not existed in selectbox).
RENDERER	A special renderer for binding

Example

Let's take a look at some examples.

Textbox's data binding annotation

	<component>
		<component-name>textbox</component-name>
		<extends>textbox</extends>
		<annotation>
			<annotation-name>ZKBIND</annotation-name>
			<property-name>value</property-name>
			<attribute>
				<attribute-name>ACCESS</attribute-name>
				<attribute-value>both</attribute-value>
			</attribute>
			<attribute>
				<attribute-name>SAVE_EVENT</attribute-name>
				<attribute-value>onChange</attribute-value>
			</attribute>
			<attribute>
				<attribute-name>LOAD_REPLACEMENT</attribute-name>
				<attribute-value>rawValue</attribute-value>
			</attribute>
			<attribute>
				<attribute-name>LOAD_TYPE</attribute-name>
				<attribute-value>java.lang.String</attribute-value>
			</attribute>
		</annotation>
	</component>

• We use <extends> to append data binding annotations based on original component definition. If you write these annotation attributes in a component definition file ( metainfo\zk\lang.xml , you don't have to use <extends> .
• ZKBIND is zkbind system's annotation name. It means we use annotation of data binding 2.
• The <property-name> is the target property we want data binding to synchronize.
• Because Textbox is a component for input, its access privilege is "both".
• As the SAVE_EVENT is onChange, you can find when your cursor focus on a Textbox is blured, Textbox's value is saved.
• Here we use LOAD_REPLACEMENT for a "loading empty value" issue related to "constraint" attribute. It is common that when we load textbox first time, its value is empty. If we also define constraint as "not empty", then the first time loading triggers error message to display. This error misleads users, so we use another replacement attribute to load the value for not triggering error message.

Selectbox's data binding annotation

	<component>
		<component-name>selectbox</component-name>
		<extends>selectbox</extends>
		<annotation>
			<annotation-name>ZKBIND</annotation-name>
			<attribute>
				<attribute-name>RENDERER</attribute-name>
				<attribute-value>itemRenderer=org.zkoss.bind.impl.BindSelectboxRenderer</attribute-value>
			</attribute>
		</annotation>
		<annotation>
			<annotation-name>ZKBIND</annotation-name>
			<property-name>selectedItem</property-name>
			<attribute>
				<attribute-name>SAVE_EVENT</attribute-name>
				<attribute-value>onSelect</attribute-value>
			</attribute>
			<attribute>
				<attribute-name>LOAD_EVENT</attribute-name>
				<attribute-value>onAfterRender</attribute-value>
			</attribute>
			<attribute>
				<attribute-name>ACCESS</attribute-name>
				<attribute-value>both</attribute-value>
			</attribute>
			<attribute>
				<attribute-name>SAVE_REPLACEMENT</attribute-name>
				<attribute-value>selectedIndex</attribute-value>
			</attribute>
			<attribute>
				<attribute-name>CONVERTER</attribute-name>
				<attribute-value>org.zkoss.bind.converter.sys.SelectboxSelectedItemConverter</attribute-value>
			</attribute>
		</annotation>

• We set RENDERER to render selectbox's item when model data comes from data binding. (line 7,8)
• Normally users should specify when to reload properties. But if you think the property requires reloading after a specific event, you can set that event's name as value of LOAD_EVENT. (line 19,20)
• The reason we use SAVE_REPLACEMENT is that Selectbox has no "selectedItem" attribute, so we save to another replacement attribute "selectedIndex". (line 27,28) Because those 2 attributes have different type, we have to use a converter, SelectboxSelectedItemConverter to convert selectedItem. (line 31,32)

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