flow_control.md

Flow Control

To use flow control in shadow elements, we can use the following shadow element tags:

• <if>: allows the conditional execution of its body according to the value of the test attribute.
• <choose>/<when>/<otherwise>: they are used for logic and flow control, such as Java's switch/case/default statement.

Work with UI components

Continuing with the example from the previous chapter, we will render a menu node upon its sub-menu existence. First we specify whether or not to show Home icon. And if a menu node has a sub-menu, we render it by <nav>; otherwise render it by <navitem>. To achieve this, we have to use shadow components for flow control, <if>, <choose>, <when>, and <otherwise>.

• The <if> is just like Java if statement.
• The <choose> is just like Java switch statement.
• The <when> is like case statement, and you should specify a data binding expression on test attribute to be evaluated.
• The <otherwise> works like default case in a switch statement to specify a default action. Now, we can test whether a menu node has a subMenus or not by test="@load(empty each.subMenus)". If it does, create a <nav>, otherwise create a <navitem>.

<navbar id="navbar" orient="horizontal" collapsed="false">
    <if test="@load(showHome)">
        <navitem label="Home" iconSclass="z-icon-home" />
    </if>
    <forEach items="@load(menuItems)">
        <choose>
            <when test="@load(empty each.subMenus)">
                <navitem label="@load(each.label)" />
            </when>
            <otherwise>
                <nav label="@load(each.label)" iconSclass="@load(each.iconSclass)"/>
            </otherwise>
        </choose>
    </forEach>
</navbar>

But this way we only create a <nav> without its <navitem>. We need to traverse each node in its sub-menu and create corresponding components (<nav> or <navitem>) which is just like what we would do now for each menu-node. It means we need to reuse this <forEach> and its child elements.