Tapestry pagelink with dynamic css class

Question 1

It is not quite clear from your code what the difference between currentLinkId en activeId is and where currentId comes from. I'm almost assuming you have some sort of Loop setup you are not sharing here. But given you can obtain these variables from the enclosing component, you are pretty much there in your commented out code, you just need to remove the argument from your getMyDynCss() method. Like so:

Java:

public class TestLink {

    @Property
    @Parameter(required=true)
    private int activeId;

    @Property
    @Parameter(required=true)
    private int currentId;

    public String getMyDynCss() {
        if (currentId == activeId) {
            return "active";
        }
        else {
            return "xxx";
        }
    }
}

Your tml:

<html xmlns:t="http://tapestry.apache.org/schema/tapestry_5_3.xsd">
    <t:pagelink page="mytest" t:id="myLink" class="${myDynCss}">
</html>

Your enclosing component:

<html t:type="layout" title="Test" xmlns:t="http://tapestry.apache.org/schema/tapestry_5_3.xsd"
      xmlns:p="tapestry:parameter">
    <p:app_navigation>
        <t:testLink activeId="1000" currentId="somePropertyFromSomewhere"/>
    </p:app_navigation>
</html>

Question 2

My solution use the the life cycle events. If there is any link that has a id that represents the active id (by convention) mark it as active.

My final component template ...

<html xmlns:t="http://tapestry.apache.org/schema/tapestry_5_3.xsd">
    <!-- convention: id == 'm' + numeric value for active entry --> 
    <t:pagelink page="mytest" id="m1000">
        I'm active
    </t:pagelink>

    <t:pagelink page="mytest2" id="m1001">
        I'm not active
    </t:pagelink>
</html>

The java code of the component ...

public class TestLink {
    @Parameter(required=true)
    private int activeId;

    // ... looking for a link with the active id ...
    void afterRender(final MarkupWriter writer) {
        // works only if the id follows the right convention :-D
        String activeElemId="m"+activeId; // <--
        Element activeLink=writer.getDocument().getElementById(activeElemId);
        if (activeLink!=null)
            activeLink.addClassName("active");
    }    
}

The code that includes the component ...

<html t:type="layout" title="Test" xmlns:t="http://tapestry.apache.org/schema/tapestry_5_3.xsd"
      xmlns:p="tapestry:parameter">
    <p:app_navigation>
        <t:testLink activeId="1000"/>
    </p:app_navigation>
</html>