使用ARC在其自己的完成块中引用NSOperation对象

I'm having difficulty converting some NSOperation code to ARC. My operation object uses a completion block, which in turn contains a GCD block that updates the UI on the main thread. Because I reference my operation object from inside its own completion block, I'm using a __weak pointer to avoid a memory leak. However, the pointer is already set to nil by the time my code runs.I'm having difficulty converting some NSOperati