MashupPlatform.widget.context   // Widget context   (Only available to widgets)
MashupPlatform.operator.context // Operator context (Only available to operators)
MashupPlatform.mashup.context   // Mashup context
MashupPlatform.context          // Platform context

MashupPlatform.widget.context.get('heightInPixels');
MashupPlatform.mashup.context.get('name');
MashupPlatform.context.get('username');

MashupPlatform.widget.context.registerCallback(function (new_values) {
    ...

    if ('heightInPixels' in new_values) {
        repaint();
    }

    ...
});