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trueBLOCK

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.{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:

xmlSwitchMe.tml<html xmlns:t="http://tapestry.apache.org/schema/tapestry_5_1_0.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.

javaSwitchMe.javapublic 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; } } }

Happy switching!

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