The ghost of NSLayoutConstraint haunts my view hierarchy? - ios

Question

I'm trying to programmatically modify autolayout constraints to move a table view up, but only for an iPhone 6 Plus in landscape mode, because I couldn't achieve the precise visual effect I wanted on all devices using Xcode 6.2 beta 3 Interface Builder's autolayout (got it right from IB for the other supported devices/orientations. Just that iPhone 6 Plus is a bit of an outlier between an iPhone and an iPad, thus a little trickier)
One constraint I remove seems to be deleted after removal (e.g. disappears from the containing view's constraints, as expected), however, the layout manager still seems to finds it and warns that it is a conflict with other constraints and breaks a constraint at runtime with the fortunate result that the app produces the intended visual result, but the unfortunate side-effect of an ugly console warning message, that I want to fix, because it's ugly and Apple documentation blames such warnings user code bug(s).
My code intercepts orientation change (only on iPhone 6 Plus),
and then:
=============
• Iterates over constraints in tableview's owner view
• Prints properties of any constraint with an attribute of .Top
• Removes Center Y constraint referenced via IBOutlet, for the tableview
• Removes constraint with .Top attribute
• Adds new .Top attribute with a different multiplier
============
Here is the swift code in my View Controller:
override func willRotateToInterfaceOrientation(toInterfaceOrientation: UIInterfaceOrientation, duration: NSTimeInterval) {
switch(toInterfaceOrientation) {
case .LandscapeLeft:
fallthrough
case .LandscapeRight:
var deviceType = UIDevice().deviceType
if (deviceType == .iPhone6plus || deviceType == .simulator) {
if centerYconstraint != nil {
self.view.removeConstraint(centerYconstraint)
centerYconstraint = nil
for constraint in self.view.constraints() {
if (constraint.firstItem as NSObject == self.tableView) {
if (constraint.firstAttribute == NSLayoutAttribute.Top) {
println("found item \(constraint)")
let view1 = constraint.firstItem as UIView
let attr1 = constraint.firstAttribute
let view2 = constraint.secondItem as UIView
let attr2 = constraint.secondAttribute
let relation = constraint.relation
let constant = constraint.constant
let newConstraint = NSLayoutConstraint(
item: view1,
attribute: attr1,
relatedBy: relation,
toItem: view2,
attribute: attr2,
multiplier: 0.02,
constant: constant)
self.view.removeConstraint(constraint as NSLayoutConstraint)
self.view.addConstraint(newConstraint)
self.view.layoutIfNeeded()
}
}
}
}
}
default:
break
}
}
Here is Xcode simulator's output. Notice the first line "found item" where I print the constraint I delete.
But you can see the same view1 and view2, multiplier and attribute in the list of potential contflicts layout manager complains about afterward. That's what I'm confused about.
found item <NSLayoutConstraint:0x7f8a05101bb0 UITableView:0x7f8a05853000.top == 0.03*_UILayoutGuide:0x7f8a035517e0.top>
2015-01-03 14:36:35.290 Interphase[46388:74323123] Unable to simultaneously satisfy constraints.
Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints)
(
"<NSLayoutConstraint:0x7f8a0354da40 V:[UITableView:0x7f8a05853000(336)]>",
"<_UILayoutSupportConstraint:0x7f8a0514df70 V:[_UILayoutGuide:0x7f8a035517e0(49)]>",
"<_UILayoutSupportConstraint:0x7f8a051908e0 _UILayoutGuide:0x7f8a035517e0.bottom == UIView:0x7f8a03551480.bottom>",
"<NSLayoutConstraint:0x7f8a051b53d0 'UIView-Encapsulated-Layout-Height' V:[UIView:0x7f8a03551480(414)]>",
"<NSLayoutConstraint:0x7f8a050d9080 UITableView:0x7f8a05853000.top == 0.02*_UILayoutGuide:0x7f8a035517e0.top>",
"<NSLayoutConstraint:0x7f8a0354ef80 UITableView:0x7f8a05853000.centerY == UIView:0x7f8a03551480.centerY>"
)
Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x7f8a0354da40 V:[UITableView:0x7f8a05853000(336)]>
Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.
2015-01-03 14:36:56.720 Interphase[46388:74323123] Unable to simultaneously satisfy constraints.
Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints)
(
"<_UILayoutSupportConstraint:0x7f8a0514df70 V:[_UILayoutGuide:0x7f8a035517e0(49)]>",
"<_UILayoutSupportConstraint:0x7f8a051908e0 _UILayoutGuide:0x7f8a035517e0.bottom == UIView:0x7f8a03551480.bottom>",
"<NSLayoutConstraint:0x7f8a05101bb0 UITableView:0x7f8a05853000.top == 0.03*_UILayoutGuide:0x7f8a035517e0.top>",
"<NSLayoutConstraint:0x7f8a051b53d0 'UIView-Encapsulated-Layout-Height' V:[UIView:0x7f8a03551480(736)]>",
"<NSLayoutConstraint:0x7f8a050d9080 UITableView:0x7f8a05853000.top == 0.02*_UILayoutGuide:0x7f8a035517e0.top>"
)

Answer 1

Adding and removing constraints to a view is a bit flaky. It's never entirely clear which view they should be added to and then it makes it hard to find later when you want to remove it.
A better solution is to keep a reference to the constraint(s) you care about (either as outlets if you're doing it from interface builder, or just store them in properties) and then activate or deactivate them as required.
Activating constraints instead of adding them also prevents you having to decide which is the appropriate view to add them to - the system does this automatically.