Question

How would I go about locking a screen like Keychain does, meaning preventing all access to Dock, menubar, desktop, etc. Basically just a black screen that I can add a password field to, for the user to return to the desktop? I am well aware of the Carbon method, but I want the NSApplication method because this is an all Cocoa application.

Thanks~

Answer 1

If you can get away with not writing this code yourself, all for the better. It is usually a terrible idea to write your own code to lock the screen, considering the number of vulnerabilities that have been found in screen locking code over the years. If you have a Carbon call that can do it, go ahead and use that… don’t worry about the “purity” of your Cocoa code.

However, if you decide to write this yourself, here’s what you do:

First, capture all the screens using CoreGraphics. See: http://developer.apple.com/mac/library/documentation/GraphicsImaging/Conceptual/QuartzDisplayServicesConceptual/Articles/DisplayCapture.html

Next, create a new NSWindow and put it in front of the window that’s used for capturing the screens. You’ll have to call a CG function to get the “order” of the black window covering each screen, and order the new window in front of that. Normally, the black window has an order so far forward that everything is behind it. Put a password field in the window. Do NOT use an ordinary text field or write your own code for password input. The password input field has a ton of special code in it so you can’t copy text out of it, and other programs can’t listen to keystrokes while you’re typing into a password field. So use the one that Apple provides.

Last, put the computer in “kiosk mode”. This mode allows you to disable alt-tab, user switching, the menubar and dock, and even the ability to force quit. See: http://developer.apple.com/mac/library/technotes/KioskMode/Introduction/Introduction.html

It’s not a lot of code, it just uses a few different APIs so you’ll spend most of your time bouncing between API docs. I suggest writing the screen lock code as its own application (just add a new application target to your Xcode project) and then put the screen locker inside your application bundle. This used to be (as of 10.4) how Apple Remote Desktop implemented the “Lock Screen” functionality, but I can’t find the app anymore.