add hairline to a view

add_hair_line.m

//| ----------------------------------------------------------------------------
//  Called when the view is about to be displayed.  May be called more than
//  once.
//
- (void)willMoveToWindow:(UIWindow *)newWindow {
    // Use the layer shadow to draw a one pixel hairline under this view.
    [self.layer setShadowOffset:CGSizeMake(0, 1.0f/UIScreen.mainScreen.scale)];
    [self.layer setShadowRadius:0];
    
    // UINavigationBar's hairline is adaptive, its properties change with
    // the contents it overlies.  You may need to experiment with these
    // values to best match your content.
    [self.layer setShadowColor:[UIColor blackColor].CGColor];
    [self.layer setShadowOpacity:0.25f];
}

cited from Apple's Demo NavBar