Dynamic UIScrollView content: Unable to simultaneously satisfy constraints - ios

Question

I'm attempting to set up autolayout for a paging scroll view with a dynamic number of pages (one primary subview per page). My view hierarchy is set up as follows:
Main view
Scroll view
UIView (fits content)
TutorialSubview
...
After adding all the views to an array, I have the following code to dynamically generate constraints:
self.iphoneSVContentWConstr.constant = (self.subVWidth * self.contentSV.frame.size.width);
NSMutableDictionary *views = [NSMutableDictionary new];
NSLayoutFormatOptions formatForVert = (NSLayoutFormatAlignAllTop | NSLayoutFormatAlignAllBottom);
NSMutableString *format = [NSMutableString stringWithString:#"|"];
int idx = 0; for (UIView *v in self.viewArr) {
NSString *key = [NSString stringWithFormat:#"View%i", idx];
[views setObject:v forKey:key];
[format appendFormat:#"[%#(%.f)]", key, self.subVWidth];
idx++;
}
[format appendString:#"|"];
NSLog(#"%#", format);
//Update the content view
[self.iphoneSVContent addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:format options:formatForVert metrics:nil views:views]];
[self.contentSV layoutIfNeeded];
This ends up outputting:
|[View0(320)][View1(320)][View2(320)][View3(320)]|
Which seems correct. However, I'm getting the following errors thrown:
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:0x17409f1d0 H:[UIView:0x174197c40(102400)]>",
"<NSLayoutConstraint:0x1742811d0 H:|-(0)-[TutorialSubview:0x1743a1960] (Names: '|':UIView:0x174197c40 )>",
"<NSLayoutConstraint:0x170287490 H:[TutorialSubview:0x1743a1960(320)]>",
"<NSLayoutConstraint:0x1702873f0 H:[TutorialSubview:0x1743a1960]-(0)-[TutorialSubview:0x1743a3100]>",
"<NSLayoutConstraint:0x1702871c0 H:[TutorialSubview:0x1743a3100(320)]>",
"<NSLayoutConstraint:0x1702872b0 H:[TutorialSubview:0x1743a3100]-(0)-[TutorialSubview:0x1743a3480]>",
"<NSLayoutConstraint:0x170287210 H:[TutorialSubview:0x1743a3480(320)]>",
"<NSLayoutConstraint:0x170286fe0 H:[TutorialSubview:0x1743a3480]-(0)-[TutorialSubview:0x1703a31e0]>",
"<NSLayoutConstraint:0x170287080 H:[TutorialSubview:0x1703a31e0(320)]>",
"<NSLayoutConstraint:0x170286f40 H:[TutorialSubview:0x1703a31e0]-(0)-| (Names: '|':UIView:0x174197c40 )>"
)
Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x170286fe0 H:[TutorialSubview:0x1743a3480]-(0)-[TutorialSubview:0x1703a31e0]>
I have the following constraints set up on self.iphoneSVContent (the view they're all being added to within the scroll view):
At this point, I'm just unsure as to what's causing the issue. Any insight is much appreciated!

Answer 1

The problem seems to be the first constraint you are changing the constant
self.iphoneSVContentWConstr.constant = (self.subVWidth * self.contentSV.frame.size.width);
The value for the view's width you set with this constraint (which is 102400, from the conflicting constraints output) is not the same as 4* 320 (which is the width of the views inside it).
If the width of the UIView inside the scrollview is defined completely by the number of subviews it has inside (which already have defined width), you should not need this constraint self.iphoneSVContentWConstr, because you have enough constraints for the width to be calculated unambiguously.
If you still want to keep this width constraint, make sure you set the constant to the correct value, with something like:
self.iphoneSVContentWConstr.constant = (self.subVWidth * self.viewArr.count);
I suppose the width/height of the scrollview you set in the storyboard. That should be enough for it to work and for the constraints to not crash anymore.
I really liked the way you constructed the visual format string! Cool :)
Let me know how it went. Good luck!