NSOperation各种情况下的状态

invocation operation

1. 添加到队列前，无依赖：
ready
not executing
not finished
not cancelled
not concurrent

2. 添加到队列前，有依赖：
not ready
not executing
not finished
not cancelled
not concurrent

3. 添加到队列，并且有一个依赖，在依赖执行完以前：
not ready
not executing
not finished
not cancelled
not concurrent

4. 添加到队列，并且有一个依赖，在依赖执行完以前，取消掉：
ready
not executing
finished
cancelled
not concurrent

5. 添加到队列，但队列的并行数太小的时候：
ready
not executing
not finished
not cancelled
not concurrent

6. 添加到队列，但队列的并行数太小的时候，取消掉，马上的状态：
ready
not executing
not finished
cancelled
not concurrent

7. 添加到队列，但队列的并行数太小的时候，取消掉，队列完成后的状态：
ready
not executing
finished
cancelled
not concurrent

8. 执行的时候：
ready
executing
not finished
not cancelled
not concurrent

9. 执行的时候，取消掉，马上的状态：
ready
executing
not finished
cancelled
not concurrent

10. 执行的时候，取消掉，队列完成后的状态：
ready
not executing
finished
cancelled
not concurrent

11. 完成：
ready
not executing
finished
not cancelled
not concurrent

12. 完成后取消掉：
ready
not executing
finished
not cancelled
not concurrent