画1像素的线

#define SINGLE_LINE_WIDTH           (1 / [UIScreen mainScreen].scale)
#define SINGLE_LINE_ADJUST_OFFSET   ((1 / [UIScreen mainScreen].scale) /
2)
@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super
viewDidLoad];
    NSLog(@"%f",   [[UIScreen
mainScreen]scale]);
UIView *lineview = [[UIView
alloc] initWithFrame:CGRectMake(10-
SINGLE_LINE_ADJUST_OFFSET,
100, SINGLE_LINE_WIDTH,
200)];
    lineview.backgroundColor=[UIColor
redColor];
    [self.view
addSubview:lineview];
}