The Observer Pattern
This video presents the Observer Design Pattern, which can be very useful in forming a pattern of communication from the Subject to a group of Observers. The video describes the pattern using a simulated action where the Subject changes its value of a variable and, when the value changes, the Subject notifies the relevant observers that the value has changed so that they can then recompute a value. The Concrete Observers perform (1) Division using the Subject's value, and (2) Modulus using the Subject's value. The important rule to be followed by all Observers is that they may never modify the state of the Subject.