Creating a horizontal line

Create a UIView with a black background that is 1 pixel high and 320 pixels wide.

Use a UIView:

UIView * separator = [[UIView alloc] initWithFrame:CGRectMake(x, y, 320, 1)];
separator.backgroundColor = [UIColor colorWithWhite:0.7 alpha:1];
[self.view addSubview:separator];
[separator release];

While Jasarien's solution is nice and simple, it doesn't create an actual 1 pixel hairline, but a 2 pixel wide line on 2X devices.

I found a blog post on how to create the real 1 pixel thin hairline. We need a utility UIView subclass. For Swift that would be:

import UIKit

class HairlineView: UIView {
    override func awakeFromNib() {
        guard let backgroundColor = self.backgroundColor?.CGColor else { return }
        self.layer.borderColor = backgroundColor
        self.layer.borderWidth = (1.0 / UIScreen.mainScreen().scale) / 2;
        self.backgroundColor = UIColor.clearColor()
    }
}