NSOperationQueue

Overview 概要

An operation queue invokes its queued NSOperation objects based on their priority and readiness. After you add an operation to a queue, it remains in the queue until the operation finishes its task. You can’t directly remove an operation from a queue after you add it.

For more information about using operation queues, see the Concurrency Programming Guide.

Determine the Execution Order

An operation queue organizes and invokes its operations according to their readiness, priority level, and interoperation dependencies. If all of the queued operations have the same queuePriority and the ready property returns YES, the queue invokes them in the order you added them. Otherwise, the operation queue always invokes the operation with the highest priority relative to the other ready operations.

However, don’t rely on queue semantics to ensure a specific execution order of operations because changes in the readiness of an operation can change the resulting execution order. Interoperation dependencies provide an absolute execution order for operations, even if those operations are located in different operation queues. 相互作用依存関係は、絶対的な遂行順序をオペレーションそれらに対して提供します、たとえそれらオペレーションが異なるオペレーションキューの中に位置してもです。 An operation object isn’t ready to run until all of its dependent operations have finished running.

For details on how to set priority levels and dependencies, see Managing Dependencies in NSOperation. 優先度レベルと依存関係を設定する方法に関する詳細として、Managing DependenciesをNSOperationで見てください。

Respond to Operation Cancelation

Finishing its task doesn’t 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 stop 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. それのタスクを終了することは、必ずしもオペレーションがそのタスクの実行を完了したことを意味しません；オペレーションはまた取り消されることも可能です。あるオペレーションオブジェクトを取り消すことは、そのオブジェクトをキューの中にそのままにしておきます、しかしそのオブジェクトに、それがそれのタスクを可能な限り素早く停止すべきであることを通知します。現在遂行中のオペレーションに対して、これはオペレーションオブジェクトのもつ作業コードがキャンセル状態を確認して、それがしている何かを停止して、そしてそれ自身を終了したと印しなければならないことを意味します。キューに入れられたがまだ遂行状態になっていないオペレーションそれらに対して、キューは依然としてそのオペレーションオブジェクトのもつstartメソッドを呼びださなければなりません、そうすることでそれはキャンセルイベントを処理してそれ自身を終了したと印できます。

For more information about operation cancellation, see Responding to the Cancel Command in NSOperation. オペレーション取り消しについてのさらなる情報として、Responding to the Cancel CommandをNSOperationで見てください。

Observe Operations Using Key-Value Observing

The NSOperationQueue class is key-value coding (KVC) and key-value observing (KVO) compliant. NSOperationQueueクラスは、キー値コーディング（KVC）とキー値監視（KVO）準拠です。 You can observe these properties to control other parts of your application. To observe the properties, use the following key paths: プロパティを監視するには、以下のキーパスを使ってください：

• operations — Read-only

• operationCount — Read-only

• maxConcurrentOperationCount — Readable and writable

• suspended — Readable and writable

• name — Readable and writable

Although you can attach observers to these properties, don’t use Cocoa bindings to bind these properties to elements of your application’s user interface. Code associated with your user interface typically must run only in your app’s main thread. However, KVO notifications associated with an operation queue may occur in any thread. しかしながら、あるオペレーションキューと結びつけられたKVO通知はどんなスレッドに発生してもかまいません。

For more information about KVO and how to attach observers to an object, see the Key-Value Observing Programming Guide.

Plan for Thread Safety

You can safely use a single NSOperationQueue object from multiple threads without creating additional locks to synchronize access to that object.

Operation queues use the Dispatch framework to initiate the execution of their operations. オペレーションキューは、Dispatchフレームワークを使ってそれらのオペレーションの遂行に着手します。 As a result, queues always invoke operations on a separate thread, regardless of whether the operation is synchronous or asynchronous.