uikit——Auto Layout——UIView——constraint

constraint

// Installing Constraints

/* A constraint is typically installed on the closest common ancestor of the views involved in the constraint. 
 It is required that a constraint be installed on _a_ common ancestor of every view involved.  The numbers in a constraint are interpreted in the coordinate system of the view it is installed on.  A view is considered to be an ancestor of itself.
 */
@interface UIView (UIConstraintBasedLayoutInstallingConstraints)

@property(nonatomic,readonly) NSArray<__kindof NSLayoutConstraint *> *constraints NS_AVAILABLE_IOS(6_0);

- (void)addConstraint:(NSLayoutConstraint *)constraint NS_AVAILABLE_IOS(6_0); // This method will be deprecated in a future release and should be avoided.  Instead, set NSLayoutConstraint's active property to YES.
- (void)addConstraints:(NSArray<__kindof NSLayoutConstraint *> *)constraints NS_AVAILABLE_IOS(6_0); // This method will be deprecated in a future release and should be avoided.  Instead use +[NSLayoutConstraint activateConstraints:].
- (void)removeConstraint:(NSLayoutConstraint *)constraint NS_AVAILABLE_IOS(6_0); // This method will be deprecated in a future release and should be avoided.  Instead set NSLayoutConstraint's active property to NO.
- (void)removeConstraints:(NSArray<__kindof NSLayoutConstraint *> *)constraints NS_AVAILABLE_IOS(6_0); // This method will be deprecated in a future release and should be avoided.  Instead use +[NSLayoutConstraint deactivateConstraints:].
@end           

解释：

• constraint通常关联着两个UI对象，假设两个UI对象closest common ancestor为ancestor，ancestor就是constraint拥有者
• 如果两个UI对象无common ancestor，add不成功，remove不属于receiver的constraint直接ignored，不会crash
• auto layout计算constraint时以拥有该constraint的view坐标系为参考坐标系
• iOS 8.0及以后，弃用这些方法了，通过设置active属性代替调用view的addConstraint和removeConstraint，设置active属性能自动把constraint add到correct view和自动把constraint从correct view remove