A domain class fulfills the M in the Model View Controller (MVC) pattern and represents a persistent entity that is mapped onto an underlying database table. In Grails a domain is a class that lives in the grails-app/domain directory.

Compose beautifully formatted text in your web application. Trix is an editor for writing messages, comments, articles, and lists—the simple documents most web apps are made of. It features a sophisticated document model, support for embedded attachments, and outputs terse and consistent HTML.

The Asset-Pipeline is a plugin used for managing and processing static assets in JVM applications primarily via Gradle (however not mandatory). Asset-Pipeline functions include processing and minification of both CSS and JavaScript files

A tag library fulfills role of "view helper" in the Model View Controller (MVC) pattern and helps with GSP rendering. In Grails a tag library is a class with a name that ends in the convention "TagLib" and lives in the grails-app/taglib directory. Use the create-tag-lib command create a tag library _

./grailsw create-taglib Trix

The TagLib integrates Trix with forms as described in their documentation:

/grails-app/taglib/demo/TrixTagLib.groovy

package demo

class TrixTagLib {
    static namespace = 'trix' (1)
    static defaultEncodeAs = [taglib: 'text'] (2)

    def editor = { attrs, body ->
        def id = attrs.id ?: attrs.name

        out << "<input id=\"${id}\" type=\"hidden\" name=\"${attrs.name}\""
        if ( attrs.value ) {
            out << " value=\"${attrs.value.encodeAsHTML()}\""  (2)
        }
        out << ' />'
        out << "<trix-editor input=\"${id}\"></trix-editor>"
    }
}

1	We use a custom name space; trix
2	By default we encode as Text but the value, if present, is encoded as Html

And we test the TagLib:

/src/test/groovy/demo/TrixTagLibSpec.groovy

package demo

import grails.testing.web.taglib.TagLibUnitTest
import spock.lang.Specification

/**
 * See the API for {@link grails.test.mixin.web.GroovyPageUnitTestMixin} for usage instructions
 */
@SuppressWarnings(['LineLength', 'MethodName'])
class TrixTagLibSpec extends Specification implements TagLibUnitTest<TrixTagLib> {

    void 'test trix editor markup is created'() {
        when:
        def expected = '''<input id="x" type="hidden" name="content" value="Editor content goes here" /><trix-editor input="x"></trix-editor>'''
        def output = applyTemplate('<trix:editor name="content" id="x" value="Editor content goes here"/>') (1)

        then:
        output == expected
    }

    void 'trix editor value and id parameters are optional'() {
        given:
        def expected = '<input id="messsage" type="hidden" name="messsage" /><trix-editor input="messsage"></trix-editor>'

        expect:
        tagLib.editor(name: 'messsage') == expected (2)
    }
}

1	applyTemplate gets as an argument anything that would be valid in a GSP. That code gets evaluated as if it were in a GSP and what’s get returned is the result of evaluating that.
2	Because this test is annotated with @TestFor(TrixTagLib) the compiler will add a property called tagLib. You can interact with that tagLib property and invoke editor tag as it were a method. You could pass a map which will be passed to the args or a closure as the last method parameter which will be used as the body of the tag.