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Excerpt
hiddentrue

Performing the equivalent of a "switch" statement in your component template using blocks and a delegate

Switching Cases

With Tapestry's If component you can only test one condition at a time. In order to distinguish multiple cases, you'd have to write complex nested if/else constructs in your page template and have a checker method for each test inside your page class.

 

Div
stylewidth: 20em; float:right
Panel
borderColor#eee
titleBGColor#eee
titleJumpStart Demo

 If, Not, Negate, Switch, Else, Unless

Wiki Markup
{float:right|background=#eee|padding=0 1em} *JumpStart Demo:* [If, Not, Negate, Switch, Else, Unless|http://jumpstart.doublenegative.com.au/jumpstart/examples/lang/ifnotnegateswitchelseunless] {float}

In cases where you have to distinguish multiple cases, the Delegate component comes in. It delegates rendering to some other component, for example a Block. For each case you have, you basically wrap the content inside a Block that doesn't get rendered by default. You then place a Delegate component on your page and point it to a method inside your page class that will decide which of your Blocks should be rendered.

Imagine for example a use case, where you want to distinguish between 4 cases and you have an int property called whichCase that should be tested against. Your page template would look as follows:

Code Block
xml
languagexml
titleSwitchMe.tml

<html xmlns:t="http://tapestry.apache.org/schema/tapestry_5_1_04.xsd">
    <body>
        <h1>Switch</h1>

        <t:delegate to="case"/>

        <t:block t:id="case1">
            Here is the content for case1.
        </t:block>

        <t:block t:id="case2">
            Here is the content for case2.
        </t:block>
        
        <t:block t:id="case3">
            Here is the content for case3.
        </t:block>
        
        <t:block t:id="case4">
            Here is the content for case4.
        </t:block>
    </body>
</html>

You can see, that the Delegate component's to parameter is bound to the case property of your page class. In your page class you therefore have a getCase() method that is responsible for telling the Delegate component which component should be rendered. For that we are injecting references to the Block}}s defined in your page template into the page class and return the according {{Block in the getCase() method.

Code Block
java
languagejava
titleSwitchMe.java

public class SwitchMe
{
    @Persist
    private int whichCase;

    @Inject
    private Block case1, case2, case3, case4;

    public Object getCase()
    {
        switch (whichCase)
        {
            case 1:
                return case1;
            case 2:
                return case2;
            case 3:
                return case3;
            case 4:
                return case4;
            default:
                return null;
        }
    }
}

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