What's this?

This is a server-side Vaadin add-on for the HTML5 canvas. You add the Canvas component to your Vaadin UI, and can then draw on it using the Java API.

Downloads

Sources are available here, and you can download the add-on package from the Vaadin add-on Directory.

Compatibility

This add-on is compatible with Vaadin 7 RC2 and up. A canvas add-on for Vaadin 6 is available here.

Installation

To install the add-on to your Vaadin project just copy the JAR file from Vaadin Directory, and compile your widgetset (more details here).

Alternatively you can add the add-on to your project using Maven (see the Maven POM snippet required from Vaadin Directory).

Basic Usage

Here is a trivial Vaadin application that uses the add-on:

public class TestUI extends UI {
    private Canvas canvas;

    @Override
    protected void init(VaadinRequest request) {
        VerticalLayout content = new VerticalLayout();
        setContent(content);

        // Instantiate the component and add it to your UI
        content.addComponent(canvas = new Canvas());

        // Draw a 20x20 filled rectangle with the upper left corner
        // in coordinate 10,10. It will be filled with the default
        // color which is black.
        canvas.fillRect(10,  10,  20,  20);
    }
}

Mouse events

The Canvas component supports listening to mouse clicks within the canvas area. It works simply by adding a CanvasClickListener to the component. Unfortunately the HTML5 Canvas API doesn't support objects, so you can only find out the coordinates where the click happened.

You can find out other details related to the click event using the MouseEventDetails instance you receive from the listener.

Working with images

The HTML5 Canvas image support contains a few pitfalls that complicate drawing images to the canvas. In practice, you should ensure that the image has been preloaded to the browser cache before trying to draw it.

To make sure that your images are preloaded before drawing you should first tell the component to load your images using the loadImages(String[] urls) method, and add a CanvasImageLoadListener to the Canvas. The listener will notify you when the images have been loaded, and you can start issuing drawImage() that work with the preloaded images.