Operation Queues

An operation queue is the Cocoa equivalent of a concurrent dispatch queue and is implemented by the NSOperationQueue class. Whereas dispatch queues always execute tasks in first-in, first-out order, operation queues take other factors into account when determining the execution order of tasks. Primary among these factors is whether a given task depends on the completion of other tasks. You configure dependencies when defining your tasks and can use them to create complex execution-order graphs for your tasks.

The tasks you submit to an operation queue must be instances of the NSOperation class. An operation object is an Objective-C object that encapsulates the work you want to perform and any data needed to perform it. Because the NSOperation class is essentially an abstract base class, you typically define custom subclasses to perform your tasks. However, the Foundation framework does include some concrete subclasses that you can create and use as is to perform tasks.

Operation objects generate key-value observing (KVO) notifications, which can be a useful way of monitoring the progress of your task. Although operation queues always execute operations concurrently, you can use dependencies to ensure they are executed serially when needed.

For more information about how to use operation queues, and how to define custom operation objects, see Operation Queues.

https://developer.apple.com/library/content/documentation/General/Conceptual/ConcurrencyProgrammingGuide/ConcurrencyandApplicationDesign/ConcurrencyandApplicationDesign.html#//apple_ref/doc/uid/TP40008091-CH100-SW8

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