te != .width && layoutToAttribute != .height {
layoutTo = layoutFrom.superview
}
// create layout constraint
let layoutConstraint = LayoutConstraint(
item: layoutFrom,
attribute: layoutFromAttribute,
relatedBy: layoutRelation,
toItem: layoutTo,
attribute: layoutToAttribute,
multiplier: self.multiplier.constraintMultiplierTargetValue,
constant: layoutConstant
)
// set label
layoutConstraint.label = self.label
// set priority
layoutConstraint.priority = LayoutPriority(rawValue: self.priority.constraintPriorityTargetValue)
// set constraint
layoutConstraint.constraint = self
// append
self.layoutConstraints.append(layoutConstraint) }
}
重点看红色部分，遍历layoutAttributes，并根据layoutAttribute的值生成一个LayoutConstraint对象添加到layoutConstraints数组中。LayoutConstraint继承自系统类NSLayoutConstraint。
4.最后再看3中的第二个for循环，使用activeIfNeeded()方法激活约束：
internal func activateIfNeeded(updatingExisting: Bool = false) {
guard let item = self.from.layoutConstraintItem else {
print("WARNING: SnapKit failed to get from item from constraint. Activate will be a no-op.")
return
}
let layoutConstraints = self.layoutConstraints
if updatingExisting {
var existingLayoutConstraints: [LayoutConstraint] = []
for constraint in item.constraints {
existingLayoutConstraints += constraint.layoutConstraints
}
for layoutConstraint in layoutConstraints {
let existingLayoutConstraint = existingLayoutConstraints.first { $0 == layoutConstraint }
guard let updateLayoutConstraint = existingLayoutConstraint else {
fatalError("Updated constraint could not find existing matching constraint to update: \(layoutConstraint)")
}
let updateLayoutAttribute = (updateLayoutConstraint.secondAttribute == .notAnAttribute) ? updateLayoutConstraint.firstAttribute : updateLayoutConstraint.secondAttribute
updateLayoutConstraint.constant = self.constant.constraintConstantTargetValueFor(layoutAttribute: updateLayoutAttribute)
}
} else {
NSLayoutConstraint.activate(layoutConstraints)
item.add(constraints: [self])
}
}
当updatingExisting为false时，进入else语句，使用的系统类NSLayoutConstraint的方法激活约束：
/* Convenience method that activates each constraint in the contained array, in the same manner as setting active=YES. This is often more efficient than activating each constraint individually. */
@available(iOS 8.0, *)
open class func activate(_ constraints: [NSLayoutConstraint])
并将设置过的约束添加到item的constraintSet这个私有属性中：
internal var constraints: [Constraint] {
return self.constraintsSet.allObjects as! [Constraint]
}
internal func add(constraints: [Constraint]) {
let constraintsSet = self.constraintsSet
for constraint in constraints {
constraintsSet.add(constraint)
}
}
internal func remove(constraints: [Constraint]) {
let constraintsSet = self.constraintsSet
for constraint in constraints {
constraintsSet.remove(constraint)
}
}
private var constraintsSet: NSMutableSet {
let constraintsSet: NSMutableSet
if let existing = objc_getAssociatedObject(self, &constraintsKey) as? NSMutableSet {
constraintsSet = existing
} else {
constraintsSet = NSMutableSet()
objc_setAssociatedObject(self, &constraintsKey, constraintsSet, .OBJC_ASSOCIATION_RETAIN_