Operation-Test-Swift

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Swift Operation Test

There seems to be much confusion about concurrent/asynchronous NSOperation. Now that iOS 8 (and Mac OS 10.10) exposes the finished and executing properties, there is a common reaction that the standard KVO keys would be, logically, finished and executing. That is the standard pattern in writing KVO-compliant properties.

But NSOperation alway has (and as of this point in time, continues to) require KVO of isFinished and isExecuting. See the Configuring Operations for Concurrent Execution section of the Operation Queue section of the Concurrency Programming Guide.

Note: This requirement, to post isFinished and isExecuting KVO, is only applicable if writing asynchronous/concurrent operations, namely:

• Set asynchronous (and if supporting iOS 7.1 and earlier, concurrent) to true.

• When the operation starts, it completes asynchronously (i.e. does doesn't complete until after it returns from the start/main function).

If you write a non-concurrent operation (or if it's a concurrent operation that happens to finish before start/main finishes), the problems arising from the failure to do the appropriate KVO will not manifest themselves. But if it's truly a asynchronous/concurrent operation, you will experience the following problems:

1. Dependencies between operations will never fire (because it awaits isFinished of the first operation before starting the second operation that depends upon it);

2. Use of maxConcurrentOperationCount in the NSOperationQueue will fail to be informed of operations that finish, thus using up worker threads, and once you hit the maximum concurrent operation count, the entire queue will deadlock.

By way of demonstration, see the OperationTestSwiftTests unit test. In this, I test both GoodAsynchronousOperation (which does the isFinished/isExecuting notifications) and BadAsynchronousOperation (which doesn't). You'll see that with the appropriate notifications the operations complete successfully, but if you don't, the dependent operation will never fire (resulting in the expectation to never being fulfilled and the test will fail).

Rob Ryan
22 September 2014