NSLayoutConstraintSearchable.swift

import UIKit

protocol NSLayoutConstraintSearchable: class {}

extension NSLayoutConstraintSearchable {
	
	// this function does add support for UIView and UILayoutGuide
	private func getConstraints() -> [NSLayoutConstraint]? {
		
		// only UIView's hold constraints
		if let owningView = (self as? UILayoutGuide)?.owningView {
			
			return owningView.constraints
			
		} else if let view = self as? UIView {
			
			return view.constraints
		}
		
		return nil
	}
	
	func searchConstraintWith(attribute attribute1: NSLayoutAttribute, relatedBy relation: NSLayoutRelation? = nil, toItem item: AnyObject? = nil, attribute attribute2: NSLayoutAttribute? = nil) -> NSLayoutConstraint? {
		
		// self is used as `firstItem` in this function
		// constraints are hold by `firstItem` OR `secondItem`
		
		guard let constraints = self.getConstraints() else {
			
			// there is something wrong when our private function can't return any constraints
			return nil
		}
		
		// convert to a set so we can use `union` function if the second item is defined and returns any constraints
		var constraintsSet = Set(constraints)
		
		if let constraints = (item as? NSLayoutConstraintSearchable)?.getConstraints() {
			
			// union constraints from second item if possible
			constraintsSet = constraintsSet.union(constraints)
		}
		
		// find all constraints with the same relation (if specified)
		let filteredConstraints = constraintsSet.filter {
			
			if let unwrappedRelation = relation {
				
				return $0.relation == unwrappedRelation
			}
			
			return true
		}
		
		// iterate constraints where `firstItem` is `self` and `firstAttribute` equals given `attribute1`
		for constraint in filteredConstraints.filter({ $0.firstItem === self && $0.firstAttribute == attribute1}) {
			
			// check if the `secondAttribute` is specified (nil means we will ignore the value)
			if let secondAttribute = attribute2 {
				
				// skip until `secondAttribute` does equal given attribute
				guard constraint.secondAttribute == secondAttribute else { continue }
				
				// skip any constraint where the `secondItem` does not equal specified `item`
				if let unwrappedItem = item where constraint.secondItem !== unwrappedItem { continue }
				
				// return constraint when `item` equals `secondItem` or swallow if `item` is nil
				return constraint
				
			} else /* ignore `secondAttribute` in all constraints */ {
				
				// skip any constraint where the `secondItem` does not equal specified `item`
				if let unwrappedItem = item where constraint.secondItem !== unwrappedItem { continue }
				
				// return constraint when `item` equals `secondItem` or swallow if `item` is nil
				return constraint
			}
		}
		
		// do the same as above but with reversed item and attribute order
		// item and attribute order does not matter for constraints (except: firstItem & firstAttribute vs. secondItem & secondAttribute)
		for constraint in filteredConstraints.filter({ $0.secondItem === self && $0.secondAttribute == attribute1}) {
			
			if let secondAttribute = attribute2 {
				
				guard constraint.firstAttribute == secondAttribute else { continue }
				
				if let unwrappedItem = item where constraint.firstItem !== unwrappedItem { continue }
				
				return constraint
				
			} else {
				
				if let unwrappedItem = item where constraint.firstItem !== unwrappedItem { continue }
				
				return constraint
			}
		}
		
		// no constraint found with given parameter
		return nil
	}
}

extension UIView: NSLayoutConstraintSearchable {}
extension UILayoutGuide: NSLayoutConstraintSearchable {}