How to launch my app programmatically, if an other app opens?

I want, that m开发者_运维知识库y app opens when (for example) the user opens iTunes. How Can I do this?

Have a program always running in the background that is subscribed to NSWorkspace's NSWorkspaceDidLaunchApplicationNotification and have it do its thing when it sees iTunes as the NSWorkspaceApplicationKey object.

If you are talking iOS; you can't.