如何检测用户是否在iOS中使用home+power进行屏幕截图?

I'm trying to detect if user take a screenshot in order to warn them this is a video with copyright. I've tried Darwin Notification, but it didn't work... NSMetadataQuery might be a solution, but we want the app to run on iOS4 as well. Can somebody help me? Thanks!:pI'm trying to detect if user take a screenshot