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.


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:

<html xmlns:t="http://tapestry.apache.org/schema/tapestry_5_4.xsd">

        <t:delegate to="case"/>

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

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

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.

public class SwitchMe
    private int whichCase;

    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;
                return null;

Happy switching!

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