uikit——Auto Layout——UIView——anchor

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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

解释：

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属性可用来方便建立constraint，各个anchor属性有各自正确anchor类型，只有同一类型anchor才能生成valid constraint，但不确保生成的constraint一定valid，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

注：NSLayoutXAxisAnchor，NSLayoutYAxisAnchor，NSLayoutDimension会进行参数类型检查，若不同类型，编译器warning

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