ABSTRACT
Often systems have objects that are dependent on some underlying core information: if this information changes then the objects must react accordingly. One such classic example is a spreadsheet where the data in the sheet may be simultaneously displayed as bar charts and pie charts in their own windows. Of course I want these windows to be synchronized i.e. if I change a value in a cell in the spreadsheet then the pie chart and bar chart should immediately redraw to reflect the change. Many other examples exist as for instance
• A central computer to monitor a parking lot may receive information about bought parking time from the lot’s set of parking pay stations in order to calculate the number of free parking spaces and display it at the entrance.
