iOS - Find top constraint for a view?
Instead of iterating through self.constraints
, you should iterate through self.superview.constraints
.
The self.constraints
only contain constraints related to just the view (e.g. height and width constraints).
Here's a code example of what this might look like:
- (void)awakeFromNib
{
[super awakeFromNib];
if (!self.topConstraint) {
[self findTopConstraint];
}
}
- (void)findTopConstraint
{
for (NSLayoutConstraint *constraint in self.superview.constraints) {
if ([self isTopConstraint:constraint]) {
self.topConstraint = constraint;
break;
}
}
}
- (BOOL)isTopConstraint:(NSLayoutConstraint *)constraint
{
return [self firstItemMatchesTopConstraint:constraint] ||
[self secondItemMatchesTopConstraint:constraint];
}
- (BOOL)firstItemMatchesTopConstraint:(NSLayoutConstraint *)constraint
{
return constraint.firstItem == self && constraint.firstAttribute == NSLayoutAttributeTop;
}
- (BOOL)secondItemMatchesTopConstraint:(NSLayoutConstraint *)constraint
{
return constraint.secondItem == self && constraint.secondAttribute == NSLayoutAttributeTop;
}
Using swift and UIView extension
extension UIView {
func findConstraint(layoutAttribute: NSLayoutAttribute) -> NSLayoutConstraint? {
if let constraints = superview?.constraints {
for constraint in constraints where itemMatch(constraint: constraint, layoutAttribute: layoutAttribute) {
return constraint
}
}
return nil
}
func itemMatch(constraint: NSLayoutConstraint, layoutAttribute: NSLayoutAttribute) -> Bool {
if let firstItem = constraint.firstItem as? UIView, let secondItem = constraint.secondItem as? UIView {
let firstItemMatch = firstItem == self && constraint.firstAttribute == layoutAttribute
let secondItemMatch = secondItem == self && constraint.secondAttribute == layoutAttribute
return firstItemMatch || secondItemMatch
}
return false
}
}
Based on @Igor answer, I changed a bit itemMatch method to consider when first item or second item is not a UIView. For example when constraint a UIView top to safe area top.
extension UIView {
func findConstraint(layoutAttribute: NSLayoutConstraint.Attribute) -> NSLayoutConstraint? {
if let constraints = superview?.constraints {
for constraint in constraints where itemMatch(constraint: constraint, layoutAttribute: layoutAttribute) {
return constraint
}
}
return nil
}
func itemMatch(constraint: NSLayoutConstraint, layoutAttribute: NSLayoutConstraint.Attribute) -> Bool {
let firstItemMatch = constraint.firstItem as? UIView == self && constraint.firstAttribute == layoutAttribute
let secondItemMatch = constraint.secondItem as? UIView == self && constraint.secondAttribute == layoutAttribute
return firstItemMatch || secondItemMatch
}
}
I usually set an identifier
of a required constraint in the IB and then find it in the code like this (Swift):
if let index = constraints.index(where: { $0.identifier == "checkmarkLeftMargin" }) {
checkmarkImageViewLeftMargin = constraints[index]
}
OR by @Tim Vermeulen
checkmarkImageViewLeftMargin = constraints.first { $0.identifier == "checkmarkLeftMargin" }