First you’ll need to load the framework found at /System/Library/PrivateFrameworks/QuickLookUI.framework

if([[NSBundle bundleWithPath:@"/System/…/QuickLookUI.framework"] load])
   NSLog(@"Quick Look loaded!");

Loading the framework with NSBundle is preferable to linking directly for 2 reasons:

• If your app is to support Tiger then we can try to load the framework and only use Quick Look if it succeeds
• Since the framework is in PrivateFrameworks there is the possibility that Apple may move it – e.g. if it becomes public

All your interaction with the QL system will be through the QLPreviewPanel singleton class, accessed using [QLPreviewPanel sharedPreviewPanel] – you’ll need to include this header so that you can use the methods without warnings. I use a define like this:

#define QLPreviewPanel NSClassFromString(@"QLPreviewPanel")

To allow QLPreviewPanel to be used in code.

Quick Look works with URLs (provided as NSURL instances) – to tell it which items to display you pass it an NSArray of NSURLs like so:

[[QLPreviewPanel sharedPreviewPanel] setURLs:URLs currentIndex:0 preservingDisplayState:YES]

The other 2 parameters should be self explanatory.

Then you need to tell the Quick Look panel to display. The simplest method is to have it fade in by using

[[QLPreviewPanel sharedPreviewPanel] makeKeyAndOrderFrontWithEffect:1]

And that’s it! To change the current set of items (e.g. when the user changes the selection) just call setURLs:current:preservingDisplayState: again with the new URLs. Check [[QLPreviewPanel sharedPreviewPanel] isOpen] to determine whether to use makeKeyAndOrderFrontWithEffect: or not. Call closeWithEffect: to close the panel.

To get the zoom effect we need a little more work. First we need to set up a delegate, like this:

[[[QLPreviewPanel sharedPreviewPanel] windowController] setDelegate:theDelegate]

This delegate must implement - (NSRect)previewPanel:(NSPanel*)panel frameForURL:(NSURL*)URL, returning the rect which the item represented by URL originates from.
With a delegate set, simply change your makeKeyAndOrderFrontWithEffect: call to use 2 and Quick Look will automatically request the frame when required.

I’ve put together a simple project which demonstrates using Quick Look, you can download the source code here.