[NSLayoutConstraint ConstraintsWithVisualFormat：options：metrics：views：]的options参数作为NULL值传递的内容

Question

Fairly easy to answer I think: What to pass if I don't want to use the options paramater of [NSLayoutConstraint constraintsWithVisualFormat:options:metrics:views:] Passing nil does work but compiler complains about the obvious wrong type ( void* instead of NSLayoutFormatOptions which is a NSUInteger typedef). Is it safe to pass 0 when I don't know about the value of the enums used in the framework?

Regards!

Answer 1

Use 0.

Here is a sample from Working with Auto Layout Programmatically :

NSArray *constraints =
        [NSLayoutConstraint constraintsWithVisualFormat:@"[button1]-[button2]"
                            options:0 metrics:nil views:viewsDictionary];

Answer 2

From Apple's documentation you can see that passing 0 is actually the same as NSLayoutFormatDirectionLeadingToTrailing :

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;

Which is considered the default state, so passing 0 is safe.