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Observer

Note An observer is an object that wishes to be informed about events happening in the system. The entity generating the events is an observable.

Intent

Important Intent Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Explanation

The Observer pattern is a behavioral design pattern that allows an object (the "subject") to notify other objects (the "observers") about changes in its state. Observers subscribe to the subject, and whenever the subject undergoes a change, it broadcasts to all subscribed observers. This pattern promotes loose coupling between the subject and observers.

Motivation to use

  • We need to be informed when certain things happen
    • Object’s property changes
    • Object does something
    • Some external event occurs
  • We want to listen to events and notified when they occur
  • Built into C# with the event keyword
    • But then what is this IObservable<T> / IObserver<T> for?
    • What about INotifyPropertyChanging/ Changed?
    • And what are BindingList<T>/ ObservableCollection<T>?

Summary from the course

  • Observer is an intrusive approach: an observable must provide an event to subscribe to
  • Special care must be taken to prevent issues in multithreaded scenarios
  • .NET comes with observable collections
  • IObserver<T>/ IObservable<T> are used in stream processing (Reactive Extensions)