Add a UIView above all, even the navigation bar

Here is a simple elegant solution that is working for me. You can set the z position of the navigation bar to below the view:

self.navigationController.navigationBar.layer.zPosition = -1;

Just remember to set it back to 0 when done.

**Note if you want add view in Full screen then only use below code**

Add these **extension** of **UIViewController**

public extension UIViewController {
internal func makeViewAsFullScreen() {
var viewFrame:CGRect = self.view.frame
if viewFrame.origin.y > 0 || viewFrame.origin.x > 0 {
self.view.frame = UIScreen.main.bounds
}
}
}

Continue as normal adding process of subview

Now use in adding **UIViewController's viewDidAppear**

override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)

self.makeViewAsFullScreen()
}

UIApplication.shared.keyWindow?.insertSubview(yourView, at: 1)

This method works with xcode 9.4 , iOS 11.4

@Nam's answer works great if you just want to display your custom view **but if your custom view needs user interaction you need to disable interaction for the `navigationBar`**.

self.navigationController.navigationBar.layer.zPosition = -1
self.navigationController.navigationBar.isUserInteractionEnabled = false

Like said in Nam's answer don't forget to reverse these changes:

self.navigationController.navigationBar.layer.zPosition = 0
self.navigationController.navigationBar.isUserInteractionEnabled = true

<hr>
You can do this in a better way with an extension:

extension UINavigationBar {
func toggle() {
if self.layer.zPosition == -1 {
self.layer.zPosition = 0
self.isUserInteractionEnabled = true
} else {
self.layer.zPosition = -1
self.isUserInteractionEnabled = false
}
}
}

And simply use it like this:

self.navigationController.navigationBar.toggle()

Swift versions for the checked response :

**Swift 4 :**

let view = UIView()
view.frame = UIApplication.shared.keyWindow!.frame
UIApplication.shared.keyWindow!.addSubview(view)

**Swift 3.1 :**

let view = UIView()
view.frame = UIApplication.sharedApplication().keyWindow!.frame
UIApplication.sharedApplication().keyWindow!.addSubview(view)

You need to add a subview to the first window with the UITextEffectsWindow type. To the first, because custom keyboards add their UITextEffectsWindow, and if you add a subview to it this won't work correctly.
Also, you cannot add a subview to the last window because the keyboard, for example, is also a window and you can`t present from the keyboard window.
So the best solution I found is to add subview (or even push view controller) to the first window with UITextEffectsWindow type, this window covers accessory view, navbar - everything.

```
let myView = MyView()
myView.frame = UIScreen.main.bounds

guard let textEffectsWindow = NSClassFromString("UITextEffectsWindow") else { return }
let window = UIApplication.shared.windows.first { window in
window.isKind(of: textEffectsWindow)
}
window?.rootViewController?.view.addSubview(myView)
```

You can do that by adding your view directly to the keyWindow:

UIView *myView = /* <- Your custom view */;
UIWindow *currentWindow = [UIApplication sharedApplication].keyWindow;
[currentWindow addSubview:myView];

**UPDATE -- For Swift 4.1 and above**

let currentWindow: UIWindow? = UIApplication.shared.keyWindow
currentWindow?.addSubview(myView)

**UPDATE for iOS13 and above**

`keyWindow` is deprecated. You should use the following:

```swift
UIApplication.shared.windows.first(where: { $0.isKeyWindow })?.addSubview(myView)
```

In Swift 4.2 and Xcode 10

var spinnerView: UIView? //This is your view

spinnerView = UIView(frame: CGRect(x: 0, y: 0, width: UIScreen.main.bounds.size.width, height: UIScreen.main.bounds.size.height))
//Based on your requirement change width and height like self.view.bounds.size.width
spinnerView?.backgroundColor = UIColor.black.withAlphaComponent(0.6)
// self.view.addSubview(spinnerView)
let currentWindow: UIWindow? = UIApplication.shared.keyWindow
currentWindow?.addSubview(spinnerView!)

In **Objective C**

UIView *spinnerView;//This is your view

self.spinnerView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, UIScreen.mainScreen.bounds.size.width, UIScreen.mainScreen.bounds.size.height)];
//Based on your requirement change width and height like self.view.bounds.size.width
self.spinnerView.backgroundColor = [[UIColor blackColor] colorWithAlphaComponent:0.6];
// [self.view addSubview:self.spinnerView];
UIWindow *currentWindow = [UIApplication sharedApplication].keyWindow;
[currentWindow addSubview:self.spinnerView];


This can work either Portrait OR Landscape mode.

One more simple code is:

yourViewName.layer.zPosition = 1//Change your view name