Observers

Observers are attached to a single Operation. They receive callbacks when operation events occur. There are currently four events

Protocol	Event
`OperationDidStartObserver`	Triggered directly before the operation's `execute` method is invoked.
`OperationDidCancelObserver`	Triggered if and when the operation is cancelled.
`OperationDidProduceOperationObserver`	Triggered if and when the operation produces another operation.
`OperationDidFinishObserver`	Triggered directly before the operation moves to the finished state.

Adding an observer

Add an observer before the operation becomes ready. If possible setup observers before adding the operation to a queue.

operation.addObserver(StartedObserver { op in 
    print("Lets go!")
})

StartedObserver, CancelledObserver, ProducedObserver and FinishedObserver are all initialized with a block which is executed when the observer is trigger for its event. In the example above, the block would be executed just before operation has execute called.

If it is necessary to perform side effects when an operation starts and finishes, add a StartedObserver and a FinishedObserver there is no restriction on the number of observers.

The framework also includes BackgroundObserver, TimeoutObserver, and NetworkObserver.