How can I know when the Magnifying Glass comes up in UIWebView?

How I can detect when the magnifying 开发者_开发技巧glass comes up in a UIWebView?

Assuming you're writing an app to release on the app store, you can't - the structure of a UIWebView is opaque to you, and you can't drill down into its component parts. Whilst I suspect it would be possible to descend into its structure and figure out exactly what was getting triggered when, you run a very large risk that Apple will change UIWebView's structure in a subsequent OS update and break your app.

So unless you're going outside of the public API (and thus making available through channels other than the app store) I'm afraid you may be out of luck.