opacity property
      
      set
      opacity
      (double newValue) 
      
    
    
    
Implementation
set opacity(double newValue) {
  if (_opacity != newValue) {
    final bool canChange = _onPropertyChange?.call(this, 'opacity') ?? true;
    if (canChange) {
      final double oldValue = _opacity;
      _opacity = newValue;
      _onPropertyChanged?.call(this, 'opacity', oldValue, newValue);
      _notify();
    }
  }
}