setting constraints in code doesn't produce the expected result - ios

Question

i'm trying to set a constraint in code and don't get the expected result.
I have a UIView container with 3 buttons (sub views) and i'm trying to set one's leading space to be the average of the other two's leading spaces (so it'll be in the middle, horizontally).
The numbers i get seem to be right when compared to the numbers i see on the storyboard when i place the 3 buttons in the position i want.
I'm getting the leading space by their frame's x value (i've double checked that aligmentRectForFrame: gives the same results), and i average them.
I use:
NSLayoutConstraint *twitterConstraint = [NSLayoutConstraint constraintWithItem:middleButton attribute:NSLayoutAttributeLeading relatedBy:NSLayoutRelationEqual toItem:containerView attribute:NSLayoutAttributeLeading multiplier:1.0 constant:average];
[twitterConstraint setPriority:UILayoutPriorityRequired];
[self.view setTranslatesAutoresizingMaskIntoConstraints:NO];
[NSLayoutConstraint activateConstraints:#[twitterConstraint]];
the basic functionality works i.e. if i put a number instead of the average i see results. I'm getting unexpected results with the current code. specifically, i'm getting the "middle button" on the right hand side of the other 2 buttons.
help!
idan

Answer 1

Expanding on why #Ken Thomases answer in comments works: Auto Layout first goes from subview-up to collect information, and then goes superview-down to set frames. So this code is sampling values of its subview frames, but at the time it is executed (in updateConstraints or updateViewConstraints or somewhere else) those views' frames haven't yet been set to their auto-layout-approved values. Unpredictable results can happen.
Calling -layoutIfNeeded on this view forces the Auto Layout engine to do this for the subviews--actaully do the layout work. So then sampling those subviews can work.
Unfortunately in this method, problems are created both sampling the frames to get information and calling layoutIfNeeded, which duplicates the expensive layout operations. As noted, it requires the buttons to be all the same size. Regardless, it's probably fine for this use case.
The way to set items to be evenly spaced using the native Auto Layout system (and allowing different-sized items) is spacer views. It's inelegant, but necessary. It can be done manually in IB, and here's how to do it with Visual Format Language:
NSMutableDictionary *autoLayoutDict = [[NSMutableDictionary alloc] init];
NSDictionary *setDictionary = #{
#"leftLabel":self.leftLabel,
#"middleLabel":self.middleLabel,
#"rightLabel":self.rightLabel
};
[autoLayoutDict setValuesForKeysWithDictionary:setDictionary];
int numberOfSpacerViewsRequired = 2;
for (int i = 0; i < numberOfSpacerViewsRequired; i++) {
UIView *spacerViewX = [[UIView alloc] init];
spacerViewX.backgroundColor = [UIColor clearColor];
spacerViewX.userInteractionEnabled = NO;
spacerViewX.translatesAutoresizingMaskIntoConstraints = NO;
NSString *spacerViewKey = [NSString stringWithFormat:#"spacerView%i", i];
[autoLayoutDict setObject:spacerViewX forKey:spacerViewKey];
[self.view addSubview:spacerViewX];
}
NSArray *constraintsToAdd = [NSLayoutConstraint constraintsWithVisualFormat:#"H:[leftLabel]-0-[spacerView0(spacerView1)]-0-[middleLabel]-0-[spacerView1(spacerView0)]-0-[rightLabel]"
options:NSLayoutFormatAlignAllCenterY
metrics:0
views:autoLayoutDict];
[self.view addConstraints:constraintsToAdd];