uikit——Auto Layout——NSLayoutAnchor

NSLayoutAnchor

anchor，即锚，是对UI对象attribute(NSLayoutAttribute)的封装，因此用anchor生成constraint，本质等同用NSLayoutConstraint生成constraint

anchor以factory设计模式实现，NSLayoutAnchor为虚父类

NSLayoutAnchor concrete subclass包括：

• NSLayoutXAxisAnchor：x-axis attribute
• NSLayoutYAxisAnchor：y-axis attribute
• NSLayoutDimension：size attribute

显然两个anchor必须是同一concrete subclass才能生成valid constraint

NSLayoutAnchor生成constraint相较于直接使用NSLayoutConstraint生成constraint有以下优点：

• 代码更加简洁，简明，易读
• NSLayoutAnchor concrete subclass增加类型检查，防止生成invalid constraint

注：尽管NSLayoutAnchor相关concrete subclass增加类型检查，它仅仅是确保使用同一concrete subclass生成constraint，但仍然会生成invalid constraint，比如使用leadingAnchor和leftAnchor生成constraint，因为leadingAnchor和leftAnchor都为NSLayoutXAxisAnchor，因此语法允许，但auto layout逻辑上不允许，这会导致runtime crash

千万别直接使用NSLayoutAnchor，根据constraint生成类型使用合适concrete subclass：

• NSLayoutXAxisAnchor：生成horizontal constraint
• NSLayoutYAxisAnchor：生成vertical constraint
• NSLayoutDimension：生成size constraint

注：view anchor属性自动提供了正确的concrete subclass，但view并没有直接提供margin anchor属性，需通过view layoutMarginsGuide访问margin anchor属性

NSLayoutAnchor接口：

/* These methods return an inactive constraint of the form thisAnchor = otherAnchor.
 */
- (NSLayoutConstraint *)constraintEqualToAnchor:(NSLayoutAnchor *)anchor;
- (NSLayoutConstraint *)constraintGreaterThanOrEqualToAnchor:(NSLayoutAnchor *)anchor;
- (NSLayoutConstraint *)constraintLessThanOrEqualToAnchor:(NSLayoutAnchor *)anchor;

/* These methods return an inactive constraint of the form thisAnchor = otherAnchor + constant.
 */
- (NSLayoutConstraint *)constraintEqualToAnchor:(NSLayoutAnchor *)anchor constant:(CGFloat)c;
- (NSLayoutConstraint *)constraintGreaterThanOrEqualToAnchor:(NSLayoutAnchor *)anchor constant:(CGFloat)c;
- (NSLayoutConstraint *)constraintLessThanOrEqualToAnchor:(NSLayoutAnchor *)anchor constant:(CGFloat)c;

NSLayoutAnchor生成以下relationship constraint：

• anchor1 = anchor2
• anchor1 >= anchor2
• anchor1 <= anchor2
• anchor1 = anchor2 + c
• anchor1 >= anchor2 + c
• anchor1 <= anchor2 + c

注：constant不允许直接赋值给anchor

NSLayoutXAxisAnchor

NSLayoutXAxisAnchor是NSLayoutAnchor concrete subclass，对NSLayoutAnchor接口进行了具体实现，增加了类型检查功能，传入参数必须是NSLayoutXAxisAnchor类型，无新增接口

NSLayoutYAxisAnchor

NSLayoutYAxisAnchor是NSLayoutAnchor的concrete subclass，对NSLayoutAnchor接口进行了具体实现，增加了类型检查功能，传入参数必须是NSLayoutYAxisAnchor类型，无新增接口

NSLayoutDimension

NSLayoutDimension是NSLayoutAnchor的concrete subclass，对NSLayoutAnchor接口进行了具体实现，增加了类型检查功能，传入参数必须是NSLayoutDimension类型，并新增了接口

NSLayoutDimension新增接口：

/* These methods return an inactive constraint of the form 
    thisVariable = constant.
*/
- (NSLayoutConstraint *)constraintEqualToConstant:(CGFloat)c;
- (NSLayoutConstraint *)constraintGreaterThanOrEqualToConstant:(CGFloat)c;
- (NSLayoutConstraint *)constraintLessThanOrEqualToConstant:(CGFloat)c;

/* These methods return an inactive constraint of the form 
    thisAnchor = otherAnchor * multiplier.
*/
- (NSLayoutConstraint *)constraintEqualToAnchor:(NSLayoutDimension *)anchor multiplier:(CGFloat)m;
- (NSLayoutConstraint *)constraintGreaterThanOrEqualToAnchor:(NSLayoutDimension *)anchor multiplier:(CGFloat)m;
- (NSLayoutConstraint *)constraintLessThanOrEqualToAnchor:(NSLayoutDimension *)anchor multiplier:(CGFloat)m;

/* These methods return an inactive constraint of the form 
    thisAnchor = otherAnchor * multiplier + constant.
*/
- (NSLayoutConstraint *)constraintEqualToAnchor:(NSLayoutDimension *)anchor multiplier:(CGFloat)m constant:(CGFloat)c;
- (NSLayoutConstraint *)constraintGreaterThanOrEqualToAnchor:(NSLayoutDimension *)anchor multiplier:(CGFloat)m constant:(CGFloat)c;
- (NSLayoutConstraint *)constraintLessThanOrEqualToAnchor:(NSLayoutDimension *)anchor multiplier:(CGFloat)m constant:(CGFloat)c;

NSLayoutDimension新增接口生成以下relationship constraint：

• anchor1 = c
• anchor1 >= c
• anchor1 <= c
• anchor1 = m * anchor2
• anchor1 >= m * anchor2
• anchor1 <= m * anchor2
• anchor1 = m * anchor2 + c
• anchor1 >= m * anchor2 + c
• anchor1 <= m * anchor2 + c

注：NSLayoutDimension新增relationship只能表示两个UI对象size relationship，constant允许直接赋值给anchor