NSLayoutAnchor

anchor，即錨，是對UI物件attribute(NSLayoutAttribute)的封裝，因此用anchor生成constraint，本質等同用NSLayoutConstraint生成constraint anchor以factory設計模式實現，NSLayoutAnchor為虛父類 NSLayoutAnchor concrete subclass包括：

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

顯然兩個anchor必須是同一concrete subclass才能生成valid constraint NSLayoutAnchor生成constraint相較於直接使用NSLayoutConstraint生成constraint有以下優點：

• 程式碼更加簡潔，簡明，易讀
• NSLayoutAnchor concrete subclass增加型別檢查，防止生成invalid constraint

注：儘管NSLayoutAnchor相關concrete subclass增加型別檢查，它僅僅是確保使用同一concrete subclass生成constraint，但仍然會生成invalid constraint，比如使用leadingAnchor和leftAnchor生成constraint，因為leadingAnchor和leftAnchor都為NSLayoutXAxisAnchor，因此語法允許，但auto layout邏輯上不允許，這會導致runtime crash 千萬別直接使用NSLayoutAnchor，根據constraint生成型別使用合適concrete subclass：

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

注：view anchor屬性自動提供了正確的concrete subclass，但view並沒有直接提供margin anchor屬性，需通過view layoutMarginsGuide訪問margin anchor屬性 NSLayoutAnchor介面：

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

/* These methods return an inactive constraint of the form thisAnchor = otherAnchor + constant.
 */
- (NSLayoutConstraint *)constraintEqualToAnchor:(NSLayoutAnchor<AnchorType> *)anchor constant:(CGFloat)c;
- (NSLayoutConstraint *)constraintGreaterThanOrEqualToAnchor:(NSLayoutAnchor<AnchorType> *)anchor constant:(CGFloat)c;
- (NSLayoutConstraint *)constraintLessThanOrEqualToAnchor:(NSLayoutAnchor<AnchorType> *)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不允許直接賦值給anchor

NSLayoutXAxisAnchor

NSLayoutXAxisAnchor是NSLayoutAnchor concrete subclass，對NSLayoutAnchor介面進行了具體實現，增加了型別檢查功能，傳入引數必須是NSLayoutXAxisAnchor型別，無新增介面

NSLayoutYAxisAnchor

NSLayoutYAxisAnchor是NSLayoutAnchor的concrete subclass，對NSLayoutAnchor介面進行了具體實現，增加了型別檢查功能，傳入引數必須是NSLayoutYAxisAnchor型別，無新增介面

NSLayoutDimension

NSLayoutDimension是NSLayoutAnchor的concrete subclass，對NSLayoutAnchor介面進行了具體實現，增加了型別檢查功能，傳入引數必須是NSLayoutDimension型別，並新增了介面 NSLayoutDimension新增介面：

/* 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新增介面生成以下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只能表示兩個UI物件size relationship，constant允許直接賦值給anchor