How to get the screen width and height in iOS?

How can one get the dimensions of the screen in iOS?

Currently, I use:

lCurrentWidth = self.view.frame.size.width;
lCurrentHeight = self.view.frame.size.height;

in viewWillAppear: and willAnimateRotationToInterfaceOrientation:duration:

The first time I get the entire screen size. The second time i get the screen minus the nav bar.

How can one get the dimensions of the screen in iOS?

The problem with the code that you posted is that you're counting on the view size to match that of the screen, and as you've seen that's not always the case. If you need the screen size, you should look at the object that represents the screen itself, like this:

CGRect screenRect = [[UIScreen mainScreen] bounds];
CGFloat screenWidth = screenRect.size.width;
CGFloat screenHeight = screenRect.size.height;

Update for split view: In comments, Dmitry asked:

How can I get the size of the screen in the split view?

The code given above reports the size of the screen, even in split screen mode. When you use split screen mode, your app's window changes. If the code above doesn't give you the information you expect, then like the OP, you're looking at the wrong object. In this case, though, you should look at the window instead of the screen, like this:

CGRect windowRect = self.view.window.frame;
CGFloat windowWidth = windowRect.size.width;
CGFloat windowHeight = windowRect.size.height;

Swift 4.2

let screenRect = UIScreen.main.bounds
let screenWidth = screenRect.size.width
let screenHeight = screenRect.size.height

// split screen            
let windowRect = self.view.window?.frame
let windowWidth = windowRect?.size.width
let windowHeight = windowRect?.size.height

Careful, [UIScreen mainScreen] contains status bar as well, if you want to retrieve the frame for your application (excluding status bar) you should use

+ (CGFloat) window_height   {
    return [UIScreen mainScreen].applicationFrame.size.height;
}

+ (CGFloat) window_width   {
    return [UIScreen mainScreen].applicationFrame.size.width;
}

I've translated some of the above Objective-C answers into Swift code. Each translation is proceeded with a reference to the original answer.

Main Answer

let screen = UIScreen.main.bounds
let screenWidth = screen.size.width
let screenHeight = screen.size.height

##Simple Function Answer

func windowHeight() -> CGFloat {
    return UIScreen.main.bounds.size.height
}

func windowWidth() -> CGFloat {
    return UIScreen.main.bounds.size.width
}

##Device Orientation Answer

let screenHeight: CGFloat
let statusBarOrientation = UIApplication.shared.statusBarOrientation
// it is important to do this after presentModalViewController:animated:
if (statusBarOrientation != .portrait
    && statusBarOrientation != .portraitUpsideDown) {
    screenHeight = UIScreen.main.bounds.size.width
} else {
    screenHeight = UIScreen.main.bounds.size.height
}

##Log Answer

let screenWidth = UIScreen.main.bounds.size.width
let screenHeight = UIScreen.main.bounds.size.height
println("width: \(screenWidth)")
println("height: \(screenHeight)")

I have used these convenience methods before:

- (CGRect)getScreenFrameForCurrentOrientation {
    return [self getScreenFrameForOrientation:[UIApplication sharedApplication].statusBarOrientation];
}

- (CGRect)getScreenFrameForOrientation:(UIInterfaceOrientation)orientation {

    CGRect fullScreenRect = [[UIScreen mainScreen] bounds];

    // implicitly in Portrait orientation.
    if (UIInterfaceOrientationIsLandscape(orientation)) {
      CGRect temp = CGRectZero;
      temp.size.width = fullScreenRect.size.height;
      temp.size.height = fullScreenRect.size.width;
      fullScreenRect = temp;
    }

    if (![[UIApplication sharedApplication] statusBarHidden]) {
      CGFloat statusBarHeight = 20; // Needs a better solution, FYI statusBarFrame reports wrong in some cases..
      fullScreenRect.size.height -= statusBarHeight;
    }

    return fullScreenRect;
}