SpreadsheetSumSheet:

I tell the CellSheets in my collection that I am dependent on their value when I add them to my collection. I have a method to receive their messages(update:). In this way I will be able to know about changes in their value.

CellAverageSheet:

I tell the Cells in my collection that I am dependent on their value when I add them to my collection. I have a method to receive their messages(update:). In this way I will be able to know about changes in their value.