shouldRepaint abstract method

Override this method to return true when the children need to be repainted. This should compare the fields of the current delegate and the given oldDelegate and return true if the fields are such that paintChildren would act differently.

The delegate can also trigger a repaint if the delegate provides the repaint animation argument to this object's constructor and that animation ticks. Triggering a repaint using this animation-based mechanism is more efficient than rebuilding the Flow widget to change its delegate.

The flow container might repaint even if this function returns false, for example if layout triggers painting (e.g., if shouldRelayout returns true).