iOS something is trying to start the receiver simultaneously from more than one thread

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Trapped uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSOperationInternal _start:]: something is trying to start the receiver simultaneously from more than one thread'

自定义operation和NSInvocationOperation一样，
如果直接调用start方法，不把operation添加到操作队列中，任务直接在当前线程同步执行。

如果把自定义operation添加到操作队列，那么任务会在新线程中异步执行。

警告：不要即把操作添加到操作队列中，又调用操作的start方法，这样是不允许的！否则运行时直接报错。

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参考

https://blog.csdn.net/chuange6363/article/details/100754692/

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