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

NSLog(@"%.0f", [[UIScreen mainScreen] bounds].size.width);
NSLog(@"%.0f", [[UIScreen mainScreen] bounds].size.height);

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];

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)
    
}

如果你想让屏幕宽度/高度与设备方向无关(只适用于从横向方向启动的视图控制器的纵向大小):

CGFloat screenWidthInPoints = [UIScreen mainScreen].nativeBounds.size.width/[UIScreen mainScreen].nativeScale;
CGFloat screenHeightInPoints = [UIScreen mainScreen].nativeBounds.size.height/[UIScreen mainScreen].nativeScale;

[UIScreen mainScreen]。物理屏幕的边界矩形，以像素为单位。这个矩形是基于设备的肖像向上方向。该值不会随着设备的旋转而改变。

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