GWT - Fading in/out a background image - image

Question

I have a custom class as follows which works fine, the button grows/shrinks to accomodate the text and the bg image changes on a click.
Probem I want to solve is how to "fadeIN" one or other image when clicked/notClicked is called
Here is my code
public ExpandingOvalButton(String text) {
if (text.length() > 15) {
label.getElement().getStyle().setFontSize(20, Unit.PX);
} else {
label.getElement().getStyle().setFontSize(30, Unit.PX);
}
int width = 120;
initWidget(panel);
label.setText(text);
// width = width + (text.length() * 8);
String widthStr = width + "px";
image.setWidth(widthStr);
image.setHeight("100px");
button = new PushButton(image);
button.setWidth(widthStr);
button.setHeight("50px");
panel.add(button, 0, 0);
panel.add(label, 18, 14);
}
public void isClicked()
{
image.setUrl("images/rectangle_green.png");
}
public void unClicked()
{
image.setUrl("images/rectangle_blue.png");
}
#Override
public HandlerRegistration addClickHandler(ClickHandler handler) {
return addDomHandler(handler, ClickEvent.getType());
}
public void setButtonEnabled(boolean enabled) {
// panel.setVisible(enabled);
// this.label.setVisible(enabled);
this.button.setVisible(enabled);
}

Answer 1

Here's a general utility class to fade any element:
public class ElementFader {
private int stepCount;
public ElementFader() {
this.stepCount = 0;
}
private void incrementStep() {
stepCount++;
}
private int getStepCount() {
return stepCount;
}
public void fade(final Element element, final float startOpacity, final float endOpacity, int totalTimeMillis) {
final int numberOfSteps = 30;
int stepLengthMillis = totalTimeMillis / numberOfSteps;
stepCount = 0;
final float deltaOpacity = (float) (endOpacity - startOpacity) / numberOfSteps;
Timer timer = new Timer() {
#Override
public void run() {
float opacity = startOpacity + (getStepCount() * deltaOpacity);
DOM.setStyleAttribute(element, "opacity", Float.toString(opacity));
incrementStep();
if (getStepCount() == numberOfSteps) {
DOM.setStyleAttribute(element, "opacity", Float.toString(endOpacity));
this.cancel();
}
}
};
timer.scheduleRepeating(stepLengthMillis);
}
}
Calling code for instance:
new ElementFader().fade(image.getElement(), 0, 1, 1000); // one-second fade-in
new ElementFader().fade(image.getElement(), 1, 0, 1000); // one-second fade-out

Answer 2

You could use GwtQuery. It provides fadeIn & fadeOut effects (and many other JQuery goodies), it is cross-browser compatible and seems to be pretty active.