didChange method
Check whether the cell's value has changed.
This method is called when calling update on the observers of the cell.
If false is returned, false is provided as the value of the didChange
argument, otherwise the value of didChange
received in update is
forwarded to the observer.
This class's implementation simply returns true. Override this method in subclasses to actually compare the cell values.
Implementation
@protected
bool didChange() => true;