Open Radar

 
Summary:
Viewing an @IBDesignable view in IB causes Xcode to consume pretty much all available CPU resources. It does this with a relatively low priority as there is minimal lag on a modern MacBook Pro. What’s unexpected is that this CPU consumption continues even if the @IBDesignable view is hidden by another tab.

Steps to Reproduce:
1. Create an iOS app project
2. Add two or more storyboards to the project
3. Create a UIView subclass in swift, mark it @IBDesignable and add a few @IBInspectable properties
4. Use the new @IBDesignable UIView in a storyboard
5. View the storyboard scene that contains the @IBDesignable class in IB
6. Notice the CPU usage peak
7. Open a new Xcode tab in front of the one with IB
8. Navigate to a source code file in the new tab

Expected Results:
Since the tab displaying the @IBDesignable view in IB is hidden behind another tab, the CPU activity is expected to return to normal.

Actual Results:
The CPU stays at near 100% across all cores, even if the Mac’s screen is put to sleep.

Notes:
Note that switching the IB tab to a storyboard that does not contain an @IBDesignable view immediately quiets the CPU, whether this tab is visible or not.