iOS 13+的答案(找到键窗口的代码来源):

guard let window = UIApplication.shared.windows.first(where: { $0.isKeyWindow }) else {
    return
}

let windowWidth = window.frame.width
let windowHeight = window.frame.height

利用这些struct来了解Swift中当前设备的有用信息。

struct ScreenSize { // Answer to OP's question
    
    static let SCREEN_WIDTH         = UIScreen.main.bounds.size.width
    static let SCREEN_HEIGHT        = UIScreen.main.bounds.size.height
    static let SCREEN_MAX_LENGTH    = max(ScreenSize.SCREEN_WIDTH, ScreenSize.SCREEN_HEIGHT)
    static let SCREEN_MIN_LENGTH    = min(ScreenSize.SCREEN_WIDTH, ScreenSize.SCREEN_HEIGHT)
    
}

struct DeviceType { //Use this to check what is the device kind you're working with
    
    static let IS_IPHONE_4_OR_LESS  = UIDevice.current.userInterfaceIdiom == .phone && ScreenSize.SCREEN_MAX_LENGTH < 568.0
    static let IS_IPHONE_SE         = UIDevice.current.userInterfaceIdiom == .phone && ScreenSize.SCREEN_MAX_LENGTH == 568.0
    static let IS_IPHONE_7          = UIDevice.current.userInterfaceIdiom == .phone && ScreenSize.SCREEN_MAX_LENGTH == 667.0
    static let IS_IPHONE_7PLUS      = UIDevice.current.userInterfaceIdiom == .phone && ScreenSize.SCREEN_MAX_LENGTH == 736.0
    static let IS_IPHONE_X          = UIDevice.current.userInterfaceIdiom == .phone && ScreenSize.SCREEN_MAX_LENGTH == 812.0
    static let IS_IPAD              = UIDevice.current.userInterfaceIdiom == .pad && ScreenSize.SCREEN_MAX_LENGTH == 1024.0
    
}


struct iOSVersion { //Get current device's iOS version
    
    static let SYS_VERSION_FLOAT  = (UIDevice.current.systemVersion as NSString).floatValue
    static let iOS7               = (iOSVersion.SYS_VERSION_FLOAT >= 7.0 && iOSVersion.SYS_VERSION_FLOAT < 8.0)
    static let iOS8               = (iOSVersion.SYS_VERSION_FLOAT >= 8.0 && iOSVersion.SYS_VERSION_FLOAT < 9.0)
    static let iOS9               = (iOSVersion.SYS_VERSION_FLOAT >= 9.0 && iOSVersion.SYS_VERSION_FLOAT < 10.0)
    static let iOS10              = (iOSVersion.SYS_VERSION_FLOAT >= 10.0 && iOSVersion.SYS_VERSION_FLOAT < 11.0)
    static let iOS11              = (iOSVersion.SYS_VERSION_FLOAT >= 11.0 && iOSVersion.SYS_VERSION_FLOAT < 12.0)
    static let iOS12              = (iOSVersion.SYS_VERSION_FLOAT >= 12.0 && iOSVersion.SYS_VERSION_FLOAT < 13.0)
    static let iOS13              = (iOSVersion.SYS_VERSION_FLOAT >= 13.0 && iOSVersion.SYS_VERSION_FLOAT < 14.0)
    static let iOS14              = (iOSVersion.SYS_VERSION_FLOAT >= 14.0 && iOSVersion.SYS_VERSION_FLOAT < 15.0)
    static let iOS15              = (iOSVersion.SYS_VERSION_FLOAT >= 15.0 && iOSVersion.SYS_VERSION_FLOAT < 16.0)
    static let iOS16              = (iOSVersion.SYS_VERSION_FLOAT >= 16.0 && iOSVersion.SYS_VERSION_FLOAT < 17.0)
    
}

我已经把上面的一些Objective-C答案翻译成Swift代码。每一篇译文都引用了原文的答案。

主要解决途径

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

##简单函数回答

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

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

##设备朝向回答

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
}

# #日志回答

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

这是非常非常容易得到你的设备大小，以及考虑方向:

// grab the window frame and adjust it for orientation
UIView *rootView = [[[UIApplication sharedApplication] keyWindow] 
                                   rootViewController].view;
CGRect originalFrame = [[UIScreen mainScreen] bounds];
CGRect adjustedFrame = [rootView convertRect:originalFrame fromView:nil];

下面是一个快速获取屏幕尺寸的方法:

print(screenWidth)
print(screenHeight)

var screenWidth: CGFloat {
    if UIInterfaceOrientationIsPortrait(screenOrientation) {
        return UIScreen.mainScreen().bounds.size.width
    } else {
        return UIScreen.mainScreen().bounds.size.height
    }
}

var screenHeight: CGFloat {
    if UIInterfaceOrientationIsPortrait(screenOrientation) {
        return UIScreen.mainScreen().bounds.size.height
    } else {
        return UIScreen.mainScreen().bounds.size.width
    }
}

var screenOrientation: UIInterfaceOrientation {
    return UIApplication.sharedApplication().statusBarOrientation
}

它们作为标准函数包含在:

https://github.com/goktugyil/EZSwiftExtensions

对于iPad分割窗口，[[UIScreen mainScreen] bounds]不起作用，使用[UIApplication sharedApplication].delegate.window.bounds代替。