Advanced Operations

Some time ago, at WWDC 2015, Apple released a sample application called Advanced NSOperations. You can find the accompanying video presentation on-line here. The code itself appears under Resources.

The presentation along with its sample-code contains some really useful ideas for using operations to structure iOS applications (and Mac apps too). Although Apple's tools and frameworks make it pretty easy to construct toy apps, serious applications require some serious thought and take a considerable amount of effort to accomplish effectively. iOS applications are really embedded applications with restricted compute, memory and networking resources. The under-utilised NSOperation class has been part of Apple's Foundation framework for some time. Yet, it has not been clear exactly when and how to best use it; and more importantly, when and how to use NSOperation instances together in concert. The sample code includes some great ideas.

Trouble is, the Advanced NSOperations sample code can introduce some complications and snags that can be very difficult to track down and debug. The sample interweaves concepts such as conditions, producing operations, and observing operation state changes. This AdvancedOperations framework decouples the concepts as much as possible in order to allow applications to take what they need, and not take what they do not need. You might call this AdvancedOperations decoupled, or AdvancedOperations-lite. Rather than one-size-fits-all approach (actually, fits none) this framework provides a ‘toolbox’ of operations-related constructs. Take what you need, leave what you do not.

What Can It Do?

This framework introduces two new complementary classes, called Operation and OperationQueue. Both inherit from their Foundation-framework counterparts. See basic class diagram below.

Notice that AdvancedOperations.Operation is a special kind of Foundation.Operation, where Foundation.Operation is the traditionally named NSOperation; and similarly for AdvancedOperations.OperationQueue and Foundation.OperationQueue. You add operations to an operation queue in order to execute them. Foundation operations run their main() method whereas Advanced operations run their execute() method. This is only in order that the Advanced operation can signal will-and-did start, execute and cancel events. Observers can watch Advanced operations transition between running states: from not started to started; from started to executing or cancelled; and finally from executing to finished or cancelled. Advanced operations only execute if not cancelled, and usually stop executing when cancelled.

Observe Operations

You can observe operation events. You add an observer to an operation. All operations support observers, and that includes Foundation Operations.