ios - [NSLayoutConstraint constraintsWithVisualFormat :options:metrics:views:] 的选项参数作为 NULL 值传递的内容

Question

我认为很容易回答:如果我不想使用 [NSLayoutConstraint constraintsWithVisualFormat:options:metrics:views:] 的选项参数，应该传递什么 传递 nil 确实有效，但编译器提示明显的错误类型(void* 而不是 NSLayoutFormatOptions，它是 NSUInteger typedef)。当我不知道框架中使用的枚举的值时传递 0 是否安全？

问候!

Answer 1

来自 Apple's documentation可以看到传0其实和NSLayoutFormatDirectionLeadingToTrailing是一样的:

enum { /* choose only one of these */ NSLayoutFormatAlignAllLeft = NSLayoutAttributeLeft,
NSLayoutFormatAlignAllRight = NSLayoutAttributeRight,
NSLayoutFormatAlignAllTop = NSLayoutAttributeTop,
NSLayoutFormatAlignAllBottom = NSLayoutAttributeBottom,
NSLayoutFormatAlignAllLeading = NSLayoutAttributeLeading,
NSLayoutFormatAlignAllTrailing = NSLayoutAttributeTrailing,
NSLayoutFormatAlignAllCenterX = NSLayoutAttributeCenterX,
NSLayoutFormatAlignAllCenterY = NSLayoutAttributeCenterY,
NSLayoutFormatAlignAllBaseline = NSLayoutAttributeBaseline,

NSLayoutFormatAlignmentMask = 0xFF, /* choose only one of these three */
NSLayoutFormatDirectionLeadingToTrailing = 0 << 8, // default
NSLayoutFormatDirectionLeftToRight = 1 << 8,
NSLayoutFormatDirectionRightToLeft = 2 << 8,
NSLayoutFormatDirectionMask = 0x3 << 8,
};

typedef NSUInteger NSLayoutFormatOptions;

这被认为是默认状态，因此传递 0 是安全的。