uikit——Auto Layout——UILayoutGuide

UILayoutGuide

传统上，在UILayoutGuide之前，ios使用dummy view实现下述功能：

• 定义内部view间距，dummy view作为内部view sibling
• 封装view，对view进行模块化分组，dummy view作为view parent

注：dummy view是一个自身无可视化元素的empty view

dummy view有以下缺点：

• 创建和维护dummy view自身成本高
• dummy view本质是view，有正常view具有的一切功能，是view hierarchy完全成员，增加了view hierarchy执行每个任务费用
• 不可见的dummy view能拦截本应传递给其它view消息，导致程序出现问题，且这类问题很难发现

因为dummy有这些缺点，apple引入UILayoutGuide代替dummy view，UILayoutGuide无dummy view上述缺点，但有dummy view上述功能：

• 定义内部UI对象间距
• 封装UI对象，对UI对象进行模块化分组

注：引入UILayoutGuide之前，UI对象即UIView，引入UILayoutGuide之后，UILayoutGuide也是UI对象之一

UILayoutGuide执行这些功能更安全，更有效，UILayoutGuide并非view，因此不是view hierarchy成员，UILayoutGuide仅仅在其owning view坐标系统中定义了一块矩形区域，此矩形区域能与auto layout交互，UILayoutGuide作为一个UI对象，具有UI对象具有的通用属性：

• leadingAnchor
• trailingAnchor
• leftAnchor
• rightAnchor
• topAnchor
• bottomAnchor
• widthAnchor
• heightAnchor
• centerXAnchor
• centerYAnchor

不管dummy view封装view，还是UILayoutGuide封装UI对象，并不会封装constraint，auto layout依然会比较分组模块内constraint优先级和分组模块外constraint优先级，即所有constraint是统一进行优先级排序

创建UILayoutGuide，分三步：

• 实例化layout guide
• 通过view的addLayoutGuide添加到view
• 使用auto layout定义layout guide position和size

/* UILayoutGuides will not show up in the view hierarchy, but may be used as items in
 an NSLayoutConstraint and represent a rectangle in the layout engine.
 
 Create a UILayoutGuide with -init, and add to a view with -[UIView addLayoutGuide:]
 before using it in a constraint.
 */

@interface UILayoutGuide : NSObject 

/* The frame of the UILayoutGuide in its owningView's coordinate system.
 Valid by the time the owningView receives -layoutSubviews.
 */
@property(nonatomic,readonly) CGRect layoutFrame;

/* The guide must be added to a view with -[UIView addLayoutGuide:] before being used in a constraint.
 Do not use this property directly to change the owningView of a layout guide. Instead, use 
 -[UIView addLayoutGuide:] and -[UIView removeLayoutGuide:], which will use this property to 
 change the owningView.
 */
@property(nonatomic,weak,nullable) UIView *owningView;

/* For ease of debugging.
 'UI' prefix is reserved for UIKit-created layout guides.
 */
@property(nonatomic,copy) NSString *identifier;


/* Constraint creation conveniences. See NSLayoutAnchor.h for details.
 */
@property(readonly, strong) NSLayoutXAxisAnchor *leadingAnchor;
@property(readonly, strong) NSLayoutXAxisAnchor *trailingAnchor;
@property(readonly, strong) NSLayoutXAxisAnchor *leftAnchor;
@property(readonly, strong) NSLayoutXAxisAnchor *rightAnchor;
@property(readonly, strong) NSLayoutYAxisAnchor *topAnchor;
@property(readonly, strong) NSLayoutYAxisAnchor *bottomAnchor;
@property(readonly, strong) NSLayoutDimension *widthAnchor;
@property(readonly, strong) NSLayoutDimension *heightAnchor;
@property(readonly, strong) NSLayoutXAxisAnchor *centerXAnchor;
@property(readonly, strong) NSLayoutYAxisAnchor *centerYAnchor;

@end

解释：

• identifier方便调试，NS或UI开头identifier为系统所保留
• owningView为UILayoutGuide拥有者，不要直接修改值，调用addLayoutGuide或removeLayoutGuide更新值
• layoutFrame参考坐标系为owningView坐标系，在调用owningView的layoutSubviews时才有valid值