UIAlert after progressView has reached a value

I am kind of lost here. I want to know how to implement an UIAlertView after my progressView has crossed a value of 0.8.

To be more clearer I have a NSTimer running in a ba开发者_开发问答ckground thread which updates a float variable and when the control returns to the main thread I update the value of the progressView.

I would like the alertView to stay in the foreground until user action.

Hope I am clear enough.

thanks

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One way is to use Key-Value-Observing on the variable you are watching, and when it crosses your threshold - present your UIAlertView.