Operation分为并发和非并发两种，也即相对caller线程是同步的还是异步的。对于提交到Operation queue中的operation来说，都是异步的，但是如果手动启动operation的话，就有异步和非异步的区分了。

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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.

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