How to create a UIView that can be shared/used by multiple UIVIewController?

UIView instances are meant to be used in only one place. They can only have one superview and thus cannot be used in several view hierarchies at the same time.

If you change the superview of a view, you will actually move the view.

There are two possibilities I think:

• either you want to always have this error message on screen. In that case, you should probably create a dedicated UIViewController as you mentioned, and always display it on screen,
• or you want to display different errors. In that way, it makes sense to create an instance of your UIView for every controller in which you display an error.

Yes, use a ViewController, it is better than a simple view. Your message could have actions like buttons.

To avoid too many code duplications, you may use a mixin :

protocol ErrorDisplay {
    func display(_ error: Error)
}

extension ErrorDisplay where Self: UIViewController {
    func display(_ error: Error) {
        let viewController = ErrorViewController(error: error)
        // You could also add it as a child view controller rather than presenting it, 
        // if you want to add its view in the current hierarchy
        present(viewController, animated: true, completion: nil)
    }
}

And implement the protocol in each view controller that wishes to display a error.

extension LoginViewController : ErrorDisplay {}
extension SettingsViewController : ErrorDisplay {}
...

If you want a view which is always present on screen at that time you can use window to display a view.

Window is Root of all view that displaying on screen, Window always present so once you add your view in window that will always be present in screen.

Just use below code to add view to window

let window = UIApplication.shared.keyWindow!
let v = UIView(frame: CGRect(x: window.frame.origin.x, y: window.frame.origin.y, width: window.frame.width, height: window.frame.height))
window.addSubview(v);

For this problem, I prefer you to create a UIView class and you can reuse it in your viewController. For doing this follow these steps,

1. Create a Xib of UIView and design it what ever you want.
2. Create a UIView class
3. Change the fileOwner of Your UIView XIB to the created UIView class
4.  Create an Outlet of ContentView in YourView.h
 @property (strong, nonatomic) IBOutlet UIView *contentView;
5. Add the following codes in YouView.m

 -(instancetype)initWithCoder:(NSCoder *)aDecoder
{
    self = [super initWithCoder:aDecoder];
    if(self)
    {
        [self customInit];
    }
    return self;
}

-(instancetype)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if(self)
    {
        [self customInit];
    }
    return  self;
}

-(void) customInit
{
    [[NSBundle mainBundle]loadNibNamed:@"View" owner:self options:nil];
    [self addSubview:self.contentView];
    self.contentView.frame = self.bounds;
}

6. Use this UIView in your ViewController what ever you want.

YourView *YourViewObj = [[YourView alloc] initWithFrame:CGRectMake(0, 0, 100,100)];
[self.view addSubview:YourViewObj];