NSScrollView with auto layout not resizing until first manual window resize

Question

I've got an OSX Cocoa app that has been built programatically (i.e., not with a NIB/XIB), which I'm trying to lay out using auto layout - but I'm getting some odd behaviour when the window first displays.

My main content is an NSView that holds has a collection of 100 NSButtons as subviews, laid out vertically. The buttons are all constrained relative to the NSView and each other; both the NSView and all the NSButtons have translatesAutoresizingMaskIntoConstraints=NO set. I believe the code for the content view is good (i.e., no ambiguous layouts, etc), because if I set the main window's contentView to the NSView, the buttons display as expected.

However, if I set the main window's contentView to be an NSScrollView, and set the documentView of the NSScrollView to be the NSView, I get display problems.

On first display, I get a blank window - no scroll bars, nothing:

The NSScrollView has translatesAutoresizingMaskIntoConstraints=NO. For debug purposes, I've also set the background colour of the NSScrollView to blue so that I can confirm what is being laid out where - but there's no blue shown anywhere.

But, as soon as I resize the window, the layout kicks in, and I get an NSScrollView the full size of the main window, with blue background, and scrollbars as expected:

I've read some references that suggest the problem is the lack of constraints on the clipView that is part of the NSScrollView. On that basis, I've tried setting up constraints binding [NSScrollView contentView] to [NSScrollView documentView] in the vertical and horizontal directions (with constant 0, multiplier 1, on the left, right, top and bottom). When I do this, the NSScrollView is now visible on first display, but it's the wrong size. The scroll doesn't scroll the full height of the internal content - the scrollable content scrolls as if it is the same size as the visible window. Lastly, the content overlaps the titlebar of the window:

Again, as soon as I resize the window, the constraints kick in, and the window displays as I'd expect (see the previous screenshot). So, I take it the extra constraints don't hurt, but they don't seem to be adding anything, either.

Further confusing matters - if I leave the buttons off altogether, and just use an empty NSView with no subviews as the content view, I get a full window of blue on startup, as I'd expect.

So - what's going on here? It feels like I'm missing a call to force the evaluation of constraints on the buttons; is that the case, or is something else going on here?

For those interested - here's my sample code. It's not Objective C - it's Python - but the language binding can convert Python method names into Objective C messages; the mapping to native ObjectiveC API should be obvious:

app = NSApplication.sharedApplication()
app.setActivationPolicy_(NSApplicationActivationPolicyRegular)

main_window = NSWindow.alloc().initWithContentRect_styleMask_backing_defer_(
    NSMakeRect(100, 100, 640, 480),
    NSTitledWindowMask | NSClosableWindowMask | NSResizableWindowMask | NSMiniaturizableWindowMask,
    NSBackingStoreBuffered,
    False)

scrollview = NSScrollView.alloc().init()
scrollview.setHasVerticalScroller_(True)
scrollview.setHasHorizontalScroller_(True)
scrollview.setAutohidesScrollers_(True)
scrollview.setBorderType_(NSNoBorder)
scrollview.setTranslatesAutoresizingMaskIntoConstraints_(False)

scrollview.backgroundColor = NSColor.blueColor()

container = NSView.alloc().init()
container.setTranslatesAutoresizingMaskIntoConstraints_(False)

buttons = [
    NSButton.alloc().init()
    for b in range(0, 100)
]

for i, button in enumerate(buttons):
    button.setBezelStyle_(NSRoundedBezelStyle)
    button.setButtonType_(NSMomentaryPushInButton)
    button.setTitle_(get_NSString('Button %s' % i))
    button.setTranslatesAutoresizingMaskIntoConstraints_(False)

    container.addSubview_(button)

    if i == 0:
        container.addConstraint_(NSLayoutConstraint.constraintWithItem_attribute_relatedBy_toItem_attribute_multiplier_constant_(
            button, NSLayoutAttributeTop,
            NSLayoutRelationEqual,
            container, NSLayoutAttributeTop,
            1, 50,
        ))
    else:
        container.addConstraint_(NSLayoutConstraint.constraintWithItem_attribute_relatedBy_toItem_attribute_multiplier_constant_(
            button, NSLayoutAttributeBottom,
            NSLayoutRelationEqual,
            buttons[i-1], NSLayoutAttributeBottom,
            1, 50,
        ))

    container.addConstraint_(NSLayoutConstraint.constraintWithItem_attribute_relatedBy_toItem_attribute_multiplier_constant_(
        button, NSLayoutAttributeLeft,
        NSLayoutRelationEqual,
        container, NSLayoutAttributeLeft,
        1, 50,
    ))

    container.addConstraint_(NSLayoutConstraint.constraintWithItem_attribute_relatedBy_toItem_attribute_multiplier_constant_(
        button, NSLayoutAttributeRight,
        NSLayoutRelationEqual,
        container, NSLayoutAttributeRight,
        1, -50,
    ))

container.addConstraint_(NSLayoutConstraint.constraintWithItem_attribute_relatedBy_toItem_attribute_multiplier_constant_(
    buttons[-1], NSLayoutAttributeBottom,
    NSLayoutRelationEqual,
    container, NSLayoutAttributeBottom,
    1, -50,
))

scrollview.setDocumentView_(container)

main_window.setContentView_(scrollview)

main_window.makeKeyAndOrderFront_(None)

app.activateIgnoringOtherApps_(True)
app.run()

Answer 1

I've worked out an answer - I'm not entirely happy with it, but it seems to work.

There are certain Cocoa widgets that don't deal well with autolayout - in particular, I've found problems with top level NSWindows and NSTabViewItems; I'm guessing other widgets might also be affected. Essentially, these are "container" widgets that have a top level "view" that must be set. If the "contained" widget is an NSScrollView (which itself will contain other widgets), the "container" widget has difficulty establishing a size for the "contained" scroll view.

The fix is to re-enable translatesAutoresizingMaskIntoConstraints for the view that will be used as the "contained" widget. In the example provided, the object scroll_view created on line 10 is the "contained" widget; the boolean value of the call to setTranslatesAutoresizingMaskIntoConstraints on line 15 should be True, not False.

These problems get better with more recent versions of OS/X - Mavericks doesn't have a problem with NSWindow, but it still has a problem with NSTabViewItem. However, it doesn't seem to do any damage to turn on translatesAutoresizingMaskIntoConstraints on newer versions of OS X; all you're losing it the theoretical purity of a 100% autolayout solution.