How to update decorator on file save? - eclipse

Question

I have created a plugin with decorator which takes current file as input and show appropriate decorator.
This is working fine and decorator is shown properly when the eclipse loads for the first time.
However, when file changes, decorator is not updated.
How can I update decorator every time file is modified? i.e. How can I update decorator on file save event?
This is the sample code
public class Decorator implements ILightweightLabelDecorator {
private final ImageDescriptor OVERLAY1 = AbstractUIPlugin.imageDescriptorFromPlugin(Activator.PLUGIN_ID, "icons/1.png");
private final ImageDescriptor OVERLAY2 = AbstractUIPlugin.imageDescriptorFromPlugin(Activator.PLUGIN_ID, "icons/2.png");
/*
* (non-Javadoc)
*
* #see org.eclipse.jface.viewers.IBaseLabelProvider#addListener(org.eclipse.jface.viewers.ILabelProviderListener)
*/
#Override
public void addListener(ILabelProviderListener listener) {
// TODO Auto-generated method stub
// IResourceChangeEvent.POST_CHANGE
}
/*
* (non-Javadoc)
*
* #see org.eclipse.jface.viewers.IBaseLabelProvider#dispose()
*/
#Override
public void dispose() {
// TODO Auto-generated method stub
}
/*
* (non-Javadoc)
*
* #see org.eclipse.jface.viewers.IBaseLabelProvider#isLabelProperty(java.lang.Object, java.lang.String)
*/
#Override
public boolean isLabelProperty(Object element, String property) {
// TODO Auto-generated method stub
return false;
}
/*
* (non-Javadoc)
*
* #see org.eclipse.jface.viewers.IBaseLabelProvider#removeListener(org.eclipse.jface.viewers.ILabelProviderListener)
*/
#Override
public void removeListener(ILabelProviderListener listener) {
// TODO Auto-generated method stub
}
/*
* (non-Javadoc)
*
* #see org.eclipse.jface.viewers.ILightweightLabelDecorator#decorate(java.lang.Object, org.eclipse.jface.viewers.IDecoration)
*/
#Override
public void decorate(Object element, IDecoration decoration) {
if (some_condition)
decoration.addOverlay(OVERLAY1);
else
decoration.addOverlay(OVERLAY2);
}
}

Answer 1

You can ask the IDecoratorManager to update decorations with a specific id using:
IDecoratorManager decoratorManager = PlatformUI.getWorkbench().getDecoratorManager();
decoratorManager.update("decorator id");
To do this on file save you will have to use an IResourceChangeListener to listen to workspace resource changes and react when you see your file being changed.
Set up a listener with:
ResourcesPlugin.getWorkspace().addResourceChangeListener(listener);
In the IResourceChangeListener you can do something like:
public void resourceChanged(IResourceChangeEvent event) {
IResourceDelta delta = event.getDelta();
IResourceDelta fileDelta = delta.findMember(IPath of file you are interested in);
if (fileDelta != null) {
// TODO handle the delta
}
}