setFlex method
Implementation
setFlex(int v) {
// create the _flex observable if it
// hasn't already been created
if (_flex == null) {
_flex = IntegerObservable(Binding.toKey(id, 'flex'), null, scope: scope);
_flex!.registerListener(onPropertyChange);
}
// set the width
_flex?.set(v, notify: false);
}