Is it safe to reference an NSOperation instance and call -isFinished?

I create an NSOperation every time my app launches or resigns active. I need to queue them with dependencies such that two never execute at the same time, but one after another.

Is it safe to do this?

• Hold a strong reference to the NSOperation object in the App Delegate.
• When the app res开发者_开发问答igns active, simply check if hat property is not nil.
• If it is not nil, check if the current NSOperation -isFinished.
• If it's finished, just add the new one to the queue.
• If it's not finished yet, create the new one and set a dependency on the running one, then add it to the queue.

I'm concerned a bit with multithreading issues here. The documentation of the -isFinished or -addDependency: methods doesn't say they should not be called from the main thread. So I guess it is ok to do that.

Edit: The NSOperation performs some file system operations in the background.

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If you want to ensure they are not called at the same time, set the maximumConcurrentOperationCount: on your NSOperationQueue to 1.

- (void)setMaxConcurrentOperationCount:(NSInteger)count

This assumes you are putting both of your NSOperations in the same queue.

In response to your other questions. I'm not sure what you are doing - but yes you can hold strong a reference to your NSOperation on the AppDelegate if you want, and you can check isFinished