anchor

@interface UIView (UIViewLayoutConstraintCreation)
/* Constraint creation conveniences. See NSLayoutAnchor.h for details.
 */
@property(readonly, strong) NSLayoutXAxisAnchor *leadingAnchor NS_AVAILABLE_IOS(9_0);
@property(readonly, strong) NSLayoutXAxisAnchor *trailingAnchor NS_AVAILABLE_IOS(9_0);
@property(readonly, strong) NSLayoutXAxisAnchor *leftAnchor NS_AVAILABLE_IOS(9_0);
@property(readonly, strong) NSLayoutXAxisAnchor *rightAnchor NS_AVAILABLE_IOS(9_0);
@property(readonly, strong) NSLayoutYAxisAnchor *topAnchor NS_AVAILABLE_IOS(9_0);
@property(readonly, strong) NSLayoutYAxisAnchor *bottomAnchor NS_AVAILABLE_IOS(9_0);
@property(readonly, strong) NSLayoutDimension *widthAnchor NS_AVAILABLE_IOS(9_0);
@property(readonly, strong) NSLayoutDimension *heightAnchor NS_AVAILABLE_IOS(9_0);
@property(readonly, strong) NSLayoutXAxisAnchor *centerXAnchor NS_AVAILABLE_IOS(9_0);
@property(readonly, strong) NSLayoutYAxisAnchor *centerYAnchor NS_AVAILABLE_IOS(9_0);
@property(readonly, strong) NSLayoutYAxisAnchor *firstBaselineAnchor NS_AVAILABLE_IOS(9_0);
@property(readonly, strong) NSLayoutYAxisAnchor *lastBaselineAnchor NS_AVAILABLE_IOS(9_0);

@end
 

解釋：

• leadingAnchor：NSLayoutAttributeLeading
• trailingAnchor：NSLayoutAttributeTrailing
• leftAnchor：NSLayoutAttributeLeft
• rightAnchor：NSLayoutAttributeRight
• topAnchor：NSLayoutAttributeTop
• bottomAnchor：NSLayoutAttributeBottom
• widthAnchor：NSLayoutAttributeWidth
• heightAnchor：NSLayoutAttributeHeight
• centerXAnchor：NSLayoutAttributeCenterX
• centerYAnchor：NSLayoutAttributeCenterY
• firstBaselineAnchor：NSLayoutAttributeFirstBaseline
• lastBaselineAnchor：NSLayoutAttributeLastBaseline

view anchor屬性可用來方便建立constraint，各個anchor屬性有各自正確anchor型別，只有同一型別anchor才能生成valid constraint，但不確保生成的constraint一定valid，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 注：NSLayoutXAxisAnchor，NSLayoutYAxisAnchor，NSLayoutDimension會進行引數型別檢查，若不同型別，編譯器warning