- Quality of Service Inference and Promotion
- Simple and Reliable Threading with NSOperation
- wwdc2015 - 706
The default implementation of this method updates the execution state of the operation and calls the receiver’s main() method.
Calling super.start() causes some KVO issues (the doc says "Your (concurrent) custom implementation must not call super at any time"); for instance the KVO on isFinishied is called multiple times (moslty due to the fact that super.start() changes internally super.isFinished value).
super.start() method also performs several checks to ensure that the operation can actually run:
- For example, if the receiver was cancelled or is already finished, this method simply returns without calling
main(). - If the operation is currently executing or is not ready to execute, this method throws an NSInvalidArgumentException exception.
Investigation on how super.start() works:
- If
start()is called on a not yet ready operation,super.start()will throw an exception. - If
start()is called multiple times from different threads,super.start()will throw an exception (if the operation wasn't already cancelled). - If
start()is called on an already cancelled but noy yet executed operation,super.start()will change its state to finished. - The
isReadyvalue is kept to true once the Operation is finished.
The operation readiness is evaluated after checking if the operation is already finished. (In fact, if a dependency is added once the operation is already finished no exceptions are thrown if we attempt to start the operation again (silly test, I know):
let op1 = BlockOperation()
let op2 = BlockOperation()
print(op2.isReady) // true
op2.start()
print(op2.isFinished) // true
op2.addDependency(op1)
print(op2.isReady) // false
op2.start() // Nothing happens (no exceptions either)If an operation is finished, calling cancel() won't change its isCancelled value to true.
- Calling
cancel()on the OperationQueue progress doesn't cancel all the running operations. - NSProgress Documentation Conflicts