internal lazy var constraint: Constraint? = {
guard let relation = self.relation,
let related = self.related,
let sourceLocation = self.sourceLocation else {
return nil
}
let from = ConstraintItem(target: self.item, attributes: self.attributes)
return Constraint(
from: from,
to: related,
relation: relation,
sourceLocation: sourceLocation,
label: self.label,
multiplier: self.multiplier,
constant: self.constant,
priority: self.priority
)
}()
先判断relation，related，sourceLocation的值是否为nil，若为nil则返回nil，否则就根据description属性创建一个Constraint对象并返回。
internal init(from: ConstraintItem,
to: ConstraintItem,
relation: ConstraintRelation,
sourceLocation: (String, UInt),
label: String?,
multiplier: ConstraintMultiplierTarget,
constant: ConstraintConstantTarget,
priority: ConstraintPriorityTarget) {
self.from = from
self.to = to
self.relation = relation
self.sourceLocation = sourceLocation
self.label = label
self.multiplier = multiplier
self.constant = constant
self.priority = priority
self.layoutConstraints = []
// get attributes
let layoutFromAttributes = self.from.attributes.layoutAttributes
let layoutToAttributes = self.to.attributes.layoutAttributes
// get layout from
let layoutFrom = self.from.layoutConstraintItem!
// get relation
let layoutRelation = self.relation.layoutRelation
for layoutFromAttribute in layoutFromAttributes {
// get layout to attribute
let layoutToAttribute: LayoutAttribute
#if os(iOS) || os(tvOS)
if layoutToAttributes.count > 0 {
if self.from.attributes == .edges && self.to.attributes == .margins {
switch layoutFromAttribute {
case .left:
layoutToAttribute = .leftMargin
case .right:
layoutToAttribute = .rightMargin
case .top:
layoutToAttribute = .topMargin
case .bottom:
layoutToAttribute = .bottomMargin
default:
fatalError()
}
} else if self.from.attributes == .margins && self.to.attributes == .edges {
switch layoutFromAttribute {
case .leftMargin:
layoutToAttribute = .left
case .rightMargin:
layoutToAttribute = .right
case .topMargin:
layoutToAttribute = .top
case .bottomMargin:
layoutToAttribute = .bottom
default:
fatalError()
}
} else if self.from.attributes == self.to.attributes {
layoutToAttribute = layoutFromAttribute
} else {
layoutToAttribute = layoutToAttributes[0]
}
} else {
if self.to.target == nil && (layoutFromAttribute == .centerX || layoutFromAttribute == .centerY) {
layoutToAttribute = layoutFromAttribute == .centerX ? .left : .top
} else {
layoutToAttribute = layoutFromAttribute
}
}
#else
if self.from.attributes == self.to.attributes {
layoutToAttribute = layoutFromAttribute
} else if layoutToAttributes.count > 0 {
layoutToAttribute = layoutToAttributes[0]
} else {
layoutToAttribute = layoutFromAttribute
}
#endif
// get layout constant
let layoutConstant: CGFloat = self.constant.constraintConstantTargetValueFor(layoutAttribute: layoutToAttribute)
// get layout to
var layoutTo: AnyObject? = self.to.target
// use superview if possible
if layoutTo == nil && layoutToAttribu