NSSplitViewController Auto Layout Bug and Workaround

31st March, 2015 — Aral Balkan

Screenshot of the example project. — Setting one of the two split view items in an NSSplitViewController as collapsed in Interface Builder can cause a runtime constraints error.

You can’t constrain this!

NSSplitViewController has an edge-case Auto Layout bug that manifests when a split view item is set as collapsed in Interface Builder (IB) and then shown at some point after viewDidLoad (e.g., in response to a user action).

It actually took me two days to track down this little rascal so I took the time to create a simple isolated example that replicates the issue.

Download the source here. (Test.zip, 68KB)

The exception thrown by the sample is below:

Error console output:2015-03-31 13:46:32.669 Test[7890:1234827] Unable to simultaneously satisfy constraints:
(
    "<NSLayoutConstraint:0x6080000850a0 H:[_NSSplitViewItemViewWrapper:0x6000001411e0(50)]>",
    "<NSLayoutConstraint:0x6000000879e0 H:|-(0)-[NSView:0x6000001221c0]   (Names: '|':_NSSplitViewItemViewWrapper:0x6000001411e0 )>",
    "<NSLayoutConstraint:0x600000087a30 H:[NSView:0x6000001221c0]-(0)-|   (Names: '|':_NSSplitViewItemViewWrapper:0x6000001411e0 )>",
    "<NSLayoutConstraint:0x600000087620 H:[NSBox:0x10052d0a0'Box'(560)]>",
    "<NSLayoutConstraint:0x600000087760 H:|-(20)-[NSBox:0x10052d0a0'Box']   (Names: '|':NSView:0x6000001221c0 )>",
    "<NSLayoutConstraint:0x6000000877b0 H:[NSBox:0x10052d0a0'Box']-(20)-|   (Names: '|':NSView:0x6000001221c0 )>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x600000087620 H:[NSBox:0x10052d0a0'Box'(560)]>

Set the NSUserDefault NSConstraintBasedLayoutVisualizeMutuallyExclusiveConstraints to YES to have -[NSWindow visualizeConstraints:] automatically called when this happens.  And/or, break on objc_exception_throw to catch this in the debugger.

As far as I can tell, the problem appears to be this: When Cocoa sees a collapsed split view item, it automatically creates a width constraint with required priority on the also-automatically-created _NSSplitViewItemViewWrapper. Then, if your split view item’s view also has a required width constraint of some sort, they conflict when you set collapsed = false later on.

Workaround

The isolated example I’ve provided should hopefully make it easy for Apple to fix this but, in the meanwhile, there is an easy workaround: Either,

Leave the Collapsed checkbox unchecked in IB and set the split view item’s collapsed property to true in the viewDidLoad method of your NSSplitViewController subclass.
Check the Collapsed checkbox in IB and then, in your NSSplitViewController subclass’s viewDidLoad method do:
```
Swift: mySplitViewItem.collapsed = false
mySplitViewItem.collapsed = true
			
```
Simply setting the collapsed property to false momentarily and then setting it to true is enough, even though it happens on the same stack frame. I can only assume that doing so results in the width constraint of the view being measured and then correctly and applied to the _NSSplitViewItemViewWrapper’s width constraint.

I hope this helps you work around the problem and makes it easier for the folks at Apple to fix it. (rdar://20432741)