淺談NSOperation的并發與非并發

Operation分為并發和非并發兩種，也即相對caller線程是同步的還是異步的。對于提交到Operation queue中的operation來說，都是異步的，但是如果手動啟動operation的話，就有異步和非異步的區分了。

Although you typically execute operations by adding them to an operation queue, doing so is not required. It is also possible to execute an operation object manually by calling its start method, but doing so does not guarantee that the operation runs concurrency with the rest of your code. The isConcurrent method of the NSOperation class tells you whether an operation runs synchronously or asynchrously with respect to the thread in which its start method was called. By default, this method returns NO, which means the operation runs synchronously in the calling thread.

If you want to implement a concurrent operation - that is, one that runs asynchronously with respect to the calling thread - you must write additional code to start the operation asynchronously. For example, you might spawn a separate thread, call an asynchronous system function, or do anything else to ensure that the start method starts the task and returns immediately and, in all likeihood, before the task is finished.

Most developers should never need to implement concurrent operation objects. If you always ad your operations to an operation queue, you do not need to implement concurrent operations. When you submit a nonconcurrent operation to an operation queue, the queue itself creates a thread on which to run your operation. Thus, adding a nonconcurrent operation to an operation queue still results in the asynchronous execution of your operation object code. The ability to define concurrent operations is only necessary in cases where you need to execute the operation asynchronously without adding it to an operation queue.

For information about how to create a concurrent operation, see "Configuring Operations for Concurrent Execution" and NSOperation Class Reference.