Easy Progressive-Enhancement with GWT

Progressive-enhancement is a technique for delivering a basic website suitable for search-engines and older web-browsers, then layering on polish, effects, interactivity and general goodness using javascript - the idea being to have a graceful degradation of service, not an all-or-nothing experience.

Traditionally GWT works the other way around - it builds all of the DOM structure of your web-app in script, so search-engines (and browsers with script disabled) only see an inscrutable mass of script.

GWT.Progressive helps to bring your ninja GWT skills to bear on web-sites that need progressive enhancement.

You can get GWT.Progressive from the git repository at github.

What can GWT.Progressive do for me?

  • Bind GWT widgets to html elements on static (or server-generated) html pages, using the html as a template and GWT magic to "enhance" the user-experience
  • Automatically bind nested html elements to properties of widget classes (child widgets, grand-child widgets, etc)
  • Bind repeated html elements to List<MyWidgetClass> properties of widget classes
  • Automatically bind attributes of html elements to properties of widget classes - with automatic type coercion
  • Make partial page-updates a breeze (replacing parts of the DOM with new server-generated content)

How GWT.Progressive works

Element Binding

You can bind a GWT widget to any html element on your website - just by adding an annotation to the widget class:

// bind by id
class MyWidget extends Composite {

// bind by tag name
class MyWidget extends Composite {

// bind by css class
class MyWidget extends Composite {

// bind by tag name and css class
@RootBinding(tag="div", cssClass="bind-me")
class MyWidget extends Composite {

Bind nested html to child widgets:

// bind the first &lt;h1> tag we find by breadth first
// search of the DOM within our widget's root element
@RuntimeUiWidget(tag="h1") MyHeaderWidget header;

// bind the first element we find by breadth first
// search that has the css class "foo"
@RuntimeUiWidget(cssClass="foo") MyFooWidget foo;

Bind repeated elements to a List of child widgets:

// bind all the &lt;img> elements we find inside our
// widget's root element to the list of Image widgets
@RuntimeUiWidget(tag="img") List<Image> images;

Attribute Binding

Bind html attributes as properties of your widgets, with automatic type coercion:

// the following attributes will have their default values (set below)
// overridden by the values of attributes in the html, if they exist
@RuntimeUiAttribute("text-attr") String textAttr = "default";
@RuntimeUiAttribute("int-attr") Integer integerAttr = 22;
@RuntimeUiAttribute("long-attr") Long longAttr = 9L;
@RuntimeUiAttribute("float-attr") Float floatAttr = 2.3f;
@RuntimeUiAttribute("double-attr") Double doubleAttr = 2222222.222d;
@RuntimeUiAttribute("boolean-attr") Boolean booleanAttr = false;

Bind attributes to custom types by supplying converters - given html like: <div coords="200x400"> ... </div>, automatically bind the coords property of our widget as an instance of Coords, converted by CoordsAttributeConverter...

In the Widget we're binding:

// automatically bind our custom Coords class 
@RuntimeUiAttribute("coords") Coords coords;

The Coords class:

class Coords {
    public Integer x, y;
    public Coords(Integer anX, Integer aY) {
        x = anX;
        y = aY;

The AttributeConverter:

public class CoordsAttributeConverter 
extends AttributeConverter<Coords> {
    public Coords convert(String aString) {
        String[] _parts = aString.split("x");
        return new Coords(

Partial Page Updates

Replace chunks of html with new html from the server. Your server can provide this html as it pleases - for example you can call servlets, jsp's, PHP or whatever you like to get the html for the replacement. Your widgets will re-bind to the new html after adding it to the DOM.

// UiBinder afficionados will be familiar with this plumbing
// which we'll use to invoke our page update...
interface MyActivator extends ElementActivator<Partial>{}
MyActivator activator = GWT.create(MyActivator.class);

// here's the widget we're going to update
@RuntimeUiWidget(tag="div") UpdateMe updateMe;


// call this method to trigger the update
private void someMethod() {
        updateMe, "/update-me.jsp", 
        new CallMeWhenUpdated()

Partial Page Update callbacks

You can register callbacks to trigger when a partial update completes - successfully or otherwise. Here's a more complete partial update example:

public class Partial extends BoundRootPanel
    // UiBinder afficionados will be familiar with this
    interface MyActivator extends ElementActivator<Partial>{}
    MyActivator activator = GWT.create(MyActivator.class);

    @RuntimeUiWidget(tag="div") UpdateMe updateMe;

    public Partial(Element anElement)
        setElement(activator.activate(this, anElement));

        // Update the page when we're clicked...
        addDomHandler(new ClickHandler() {
            public void onClick(ClickEvent aEvent) {
                    Partial.this, "/update-me.jsp", 
                    new CallMeWhenUpdated()
    }, ClickEvent.getType());

    class CallMeWhenUpdated 
    implements PageUpdateCallback<Partial>() {
        public void onSuccess(Partial anUpdated) {

        public void onError(Throwable anExc) {
            GWT.log("oops, something bad happened", anExc);

Complete example - Progressive Enhancement with GWT

Here's the HTML we're going to enhance:

    <div class="widget">
      <img class="first" src="/image-1.png">
      <img class="second" src="/image-2.png">
    <!-- our script loads last, and progressive 
         enhancement begins -->
        type="text/javascript" language="javascript" 

Here's the widget we're going to bind to the <div> of css-class "widget":

class MyWidget extends BoundRootPanel {
    interface MyActivator extends ElementActivator<MyWidget>{}
    MyActivator activator = GWT.create(MyWidget.class);

    @RuntimeUiWidget(cssClass="first") Image first;
    @RuntimeUiWidget(cssClass="second") Image second;

    public MyWidget(Element anElement) {
        setElement(activator.activate(this, anElement);

        // ... now we do some enhancements, e.g. add
        // click-handlers to our image's or something

We'll need an EntryPoint - this is still a GWT application:

class Example1 implements EntryPoint {
    public void onModuleLoad() {
        PageActivator _activator = GWT.create(PageActivator.class);

... and a GWT module descriptor (.gwt.xml) of course:

<module rename-to="example">
  <inherits name="com.google.gwt.user.User"/>
  <inherits name="com.sjl.gwt.progressive.Progressive"/>

  <entry-point class="com.sjl.example.client.Example" />

  <!-- for maximum compatibility, use the xs linker -->
  <add-linker name="xs" />

Rules of engagement

GWT.Progressive requires that you follow a few simple rules to ensure that your widgets get "activated" correctly, integrated into the GWT/browser eventing system, etc.

Your widgets must:

  • Have a public Constructor that takes a com.google.gwt.dom.client.Element argument, OR
  • Have a public static <T> wrap(Element anElement) method that returns an instance of your widget class (this is how native GWT widgets like Image and Label are bound by GWT.Progressive).
  • GWT.create() a special class that allows GWT.Progressive's generator to work its magic on your widgets (either com.sjl.gwt.progressive.client.ElementActivator or com.sjl.gwt.progressive.client.WidgetActivator).
  • Set your Widget's Element as the return value from activation (setElement(activator.activate(this, anElement))) or init your Composite with it (initWidget(activator.activate(this, anElement))).

If you've ever used UiBinder, this pattern will be very familiar to you.

Also, make sure to use the "xs" linker by specifying <add-linker name="xs"/> in your gwt module descriptor for maximum compatibility.

Where can I find more examples of GWT.progressive?

In the github repository, in the test source tree.

Are there any examples in the wild?

There are two examples on this page! - The Twitter feed in the sidebar, and the code syntax-highlighting are both implemented using gwt.progressive.

You can also try my mortgage/loan overpayment calculator which is built with gwt.progressive.

Can I use this in my products?

Sure, its available under the Apache-2.0 license, and is a github fork away, but ... disclaimer: use at your own risk and discretion, I cannot accept responsibility for any issues that may arise from using this library.

Can I modify the code, fix bugs, etc?

Knock yourself out :) ... I'd love to hear about it if you do.

Final Words

I just want to say - props to my employer (KnowledgeView ltd.) for allowing me to release the code, some of which was developed on their dime. Mucho thanks KV :)

You can find some more background on GWT.Progressive in some of my earlier blog posts: part-1, part-2, and part-3

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