Creating a Simple Component with Polymer Templates

In this section, we demonstrate how to create a simple component using only the PolymerTemplate API.

Our example:

• Creates a simple view that allows the user to input their name and click a button for a response.

• Uses standard Polymer 3.0 features. See Polymer Library 3.0 to learn more.

Creating the Template File on the Client Side

The first step is to create the Polymer JavaScript template file on the client side in frontend/src/hello-world.js. This file contains the view structure.

Example: Creating the hello-world.js JavaScript Polymer template file.

js

import {PolymerElement,html} from '@polymer/polymer/polymer-element.js';
import '@polymer/paper-input/paper-input.js';
class HelloWorld extends PolymerElement {
    static get template() {
        return html`
            <div>
                <paper-input id="inputId" value="{{userInput}}"></paper-input>
                <button id="helloButton" on-click="sayHello">Say hello</button>
                <div id="greeting">[[greeting]]</div>
            </div>`;
    }
    static get is() {
          return 'hello-world';
    }
}
customElements.define(HelloWorld.is, HelloWorld);

• This is the JavaScript ES6 module that defines a Polymer template.

• The is() function defines the name of the HTML tag that is used to reference this module. The tag name should contain at least one dash (-). For example, hello-world is a valid tag name, but helloworld is not.

• The template contains two bindings: [[greeting]] is a one-way binding and {{userInput}} is a two-way binding.

• An event handler, sayHello, is triggered on a helloButton click.

• No model is specified: it will be propagated automatically from the server side.

• The imported dependencies are:

  • PolymerElement (from the Polymer library): This is the required superclass of all Polymer templates.

  • paper-input Polymer component: You can use any component you like, for example the Vaadin TextField.

  • html for inline DOM templating.

Working on the Server Side

Creating the Java Template Class

To use the client-side JavaScript template on the server side, you need to create an associated Java class that extends the PolymerTemplate class.

Example: Creating the HelloWorld Java template class.

Java

@Tag("hello-world")
@NpmPackage(value = "@polymer/paper-input", version = "3.0.2")
@JsModule("./src/hello-world.js")
public class HelloWorld extends PolymerTemplate<HelloWorldModel> {
    private static final String EMPTY_NAME_GREETING = "Please enter your name";
    /**
     * Creates the hello world template.
     */
    public HelloWorld() {
        setId("template");
        getModel().setGreeting(EMPTY_NAME_GREETING);
    }
    @EventHandler
    private void sayHello() {
        // Called from the template click handler
        String userInput = getModel().getUserInput();
        if (userInput == null || userInput.isEmpty()) {
            getModel().setGreeting(EMPTY_NAME_GREETING);
        } else {
            getModel().setGreeting(String.format("Hello %s!", userInput));
        }
    }
}

• The @Tag annotation name matches the return value of the is() function (static getter) in the JavaScript template. This ensures that the tag name is the same on the server and the client.

• The @JsModule annotation binds the Java class to the hello-world.js template class by specifying the relative path to the JavaScript module in the frontend folder in the project root. You can import multiple JavaScript resources using the @JsModule annotation, if needed.

• The @NpmPackage annotation declares a dependency to the paper-input npm package: @polymer/paper-input 3.0.2. This annotation can be used to declare a dependency to any npm package.

• The @EventHandler annotation defines the event handler called by the template click hander. Specifically, it defines the sayHello method that is called from the client side and triggered by a helloButton button click. The method name is used to map function calls between the JavaScript template and the Java class.

Creating a Model Class

Our example also needs a model class.

Example: Creating the HelloWorldModel model class.

Java

/**
 * Model for the template.
 */
public interface HelloWorldModel extends TemplateModel {
    /**
     * Gets user input from corresponding template page.
     *
     * @return user input string
     */
    String getUserInput();
    /**
     * Sets greeting that is displayed in corresponding template page.
     *
     * @param greeting
     *            greeting string
     */
    void setGreeting(String greeting);
}

• The model class describes all properties passed to the html template and used on the client side.

• It is a simple Java interface that:

  • Extends the TemplateModel interface.

  • Defines getter and/or setter methods for properties.

• To use a model, you need to specify it when extending the PolymerTemplate class. It is not necessary to create a model object yourself, because Vaadin handles this for you. All you need to do is create the interface.

• You can access the model instance using the getModel() method inside the template Java class.

Using the Polymer Template

You can now use the HelloWorld component in the same way as any other component.

Example: Using the HelloWorld component in a Java class.

Java

HelloWorld hello = new HelloWorld();
Div layout = new Div();
layout.add(hello);