OperationQueue class regulates the execution of a set of
Operation objects. After being added to a queue, an operation remains in that queue until it is explicitly canceled or finishes executing its task. Operations within the queue (but not yet executing) are themselves organized according to priority levels and inter-operation object dependencies and are executed accordingly. An application may create multiple operation queues and submit operations to any of them.
- iOS 2.0+
- macOS 10.5+
- tvOS 2.0+
- watchOS 2.0+
Inter-operation dependencies provide an absolute execution order for operations, even if those operations are located in different operation queues. An operation object is not considered ready to execute until all of its dependent operations have finished executing. For operations that are ready to execute, the operation queue always executes the one with the highest priority relative to the other ready operations. For details on how to set priority levels and dependencies, see
You cannot directly remove an operation from a queue after it has been added. An operation remains in its queue until it reports that it is finished with its task. Finishing its task does not necessarily mean that the operation performed that task to completion. An operation can also be canceled. Canceling an operation object leaves the object in the queue but notifies the object that it should abort its task as quickly as possible. For currently executing operations, this means that the operation object’s work code must check the cancellation state, stop what it is doing, and mark itself as finished. For operations that are queued but not yet executing, the queue must still call the operation object’s
start method so that it can processes the cancellation event and mark itself as finished.
Operation queues usually provide the threads used to run their operations. Operation queues use the
libdispatch library (also known as Grand Central Dispatch) to initiate the execution of their operations. As a result, operations are always executed on a separate thread, regardless of whether they are designated as asynchronous or synchronous operations.
For more information about using operation queues, see Concurrency Programming Guide.
NSOperationQueue class is key-value coding (KVC) and key-value observing (KVO) compliant. You can observe these properties as desired to control other parts of your application. To observe the properties, use the following key paths::
maxConcurrentOperationCount- readable and writable
suspended- readable and writable
name- readable and writable
Although you can attach observers to these properties, you should not use Cocoa bindings to bind them to elements of your application’s user interface. Code associated with your user interface typically must execute only in your application’s main thread. However, KVO notifications associated with an operation queue may occur in any thread.
For more information about key-value observing and how to attach observers to an object, see Key-Value Observing Programming Guide.
It is safe to use a single
NSOperationQueue object from multiple threads without creating additional locks to synchronize access to that object.
Additional Operation Queue Behaviors
An operation queue executes its queued operation objects based on their priority and readiness. If all of the queued operation objects have the same priority and are ready to execute when they are put in the queue—that is, their
isReady method returns
true—they are executed in the order in which they were submitted to the queue. However, you should never rely on queue semantics to ensure a specific execution order of operation objects. Changes in the readiness of an operation can change the resulting execution order. If you need operations to execute in a specific order, use operation-level dependencies as defined by the