Conditional Evaluation"
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Revision as of 07:37, 8 July 2022
If and Unless
The component creation can be conditional. By specifying the if
, unless
attribute or both, developers can control whether to create the associated component. It is also the most straightforward way.
For example, suppose that we want to use label, if readonly, and textbox, otherwise:
<label value="${customer.label}" if="${param.readonly == 'true'}"/>
<textbox value="${customer.value}" unless="${param.readonly == 'true'}"/>
Besides, if a parent component is ignored (not created), all of its child components are ignored too.
Here is another example:
<window if="${a==1}" unless="${b==2}">
...
</window>
- window is created only if
a
is 1 andb
is not 2.
Switch and Case
With the switch
and case
attributes of the zk
element, you can evaluate a section of a ZUML document only if a variable matches a certain value. It is similar to Java's switch statement.
For example,
<zk switch="${fruit}">
<zk case="apple">
Evaluated only if ${fruit} is apple
</zk>
<zk case="${special}">
Evaluated only if ${fruit} equals ${special}
</zk>
<zk>
Evaluated only if none of the above cases matches.
</zk>
</zk>
ZK Loader will evaluate from the first case to the last case, until it matches the switch condition, which is the value specified in the switch attribute. The evaluation is mutually exclusive conditional. Only the first matched case is evaluated.
The zk element without any case is the default – i.e., it always matches and is evaluated if all the cases above it failed to match.
Multiple Cases
You can specify a list of cases in one case attribute, such that a section of a ZUML document has to be evaluated if one of them matches.
<zk switch="${fruit}">
<zk case="apple, ${special}">
Evaluated if ${fruit} is either apple or ${special}
</zk>
</zk>
Regular Expressions
Regular expressions are allowed in the case
attribute too, as shown below.
<zk switch="${fruit}">
<zk case="/ap*.e/">
Evaluate if the regular expression, ap*.e"., matches the switch condition.
</zk>
</zk>
Used with forEach
Like any other elements, you can use the the forEach attribute (so are if and unless). The forEach
attribute is evaluated first, so the following is the same as multiple cases.
<zk case="${each}" forEach="apple, orange">
is equivalent to
<zk case= "apple, orange">
Choose and When
The choose
and when
attributes of the zk
element is the third approach of conditional evaluation.
As shown below, it is enclosed with a zk
element with the choose
attribute, and the ZK Loader will evaluate its child elements (the zk
elements with the when
attribute) one-by-one until the first one matches:
<zk choose="">
<zk when="${fruit == 'apple'}">
Evaluated if the when condition is true.
</zk>
<zk><!-- default -->
Evaluated if none of above cases matches.
</zk>
</zk>
You don't have to assign any value to the choose
attribute, which is used only to identify the range of the mutually exclusive conditional evaluation.
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