一像素约束(选中所需约束，切换到类处，选择此类，运行时就是0.5的约束)

#import <UIKit/UIKit.h>
/**
 *  @author songxing, 16-01-14 15:01:14
 *
 *  一像素约束(选中所需约束，切换到类处，选择此类，运行时就是0.5的约束)
 */
@interface NSLayoutConstraintOnePixelLine : NSLayoutConstraint

@end

#import "NSLayoutConstraintOnePixelLine.h"

@implementation NSLayoutConstraintOnePixelLine

- (void)awakeFromNib {
    [super awakeFromNib];
    
    if (self.constant == 1) {
        self.constant = 1 / [UIScreen mainScreen].scale;
    }
    
}

@end