如何在设备竖屏状态下获取横屏模式的size class？

你可以通过以下两种实用方式获取设备不同方向下的Size Class：

1. 临时变更方向读取Trait Collection #

这种方式会临时修改设备的几何方向（仅用于计算，不会改变实际UI显示方向），读取对应方向的traitCollection后再恢复原状态：

import UIKit

func getSizeClass(for orientation: UIInterfaceOrientation) -> (horizontal: UIUserInterfaceSizeClass, vertical: UIUserInterfaceSizeClass) {
    guard let windowScene = UIApplication.shared.connectedScenes.first as? UIWindowScene else {
        return (.unspecified, .unspecified)
    }
    
    // 保存当前方向
    let originalOrientation = windowScene.interfaceOrientation
    // 临时切换到目标方向
    windowScene.requestGeometryUpdate(.iOS(interfaceOrientations: orientation.supportedInterfaceOrientations)) { _ in }
    
    // 获取对应方向的Size Class
    let horizontal = windowScene.traitCollection.horizontalSizeClass
    let vertical = windowScene.traitCollection.verticalSizeClass
    
    // 恢复原方向
    windowScene.requestGeometryUpdate(.iOS(interfaceOrientations: originalOrientation.supportedInterfaceOrientations))
    
    return (horizontal, vertical)
}

调用示例：

// 获取横屏左向的Size Class
let landscapeSizeClass = getSizeClass(for: .landscapeLeft)
print("横屏水平：\(landscapeSizeClass.horizontal)，垂直：\(landscapeSizeClass.vertical)")

2. 基于设备类型和方向手动计算 #

如果不想临时修改设备方向，可根据设备型号和目标方向直接推断Size Class（仅适用于全屏场景，无法处理iPad分屏/Slide Over）：

func calculateSizeClass(for orientation: UIInterfaceOrientation) -> (horizontal: UIUserInterfaceSizeClass, vertical: UIUserInterfaceSizeClass) {
    let screen = UIScreen.main
    let isPad = screen.traitCollection.userInterfaceIdiom == .pad
    let isLandscape = orientation.isLandscape
    
    if isPad {
        // iPad全屏状态下所有方向都是Regular
        return (.regular, .regular)
    } else {
        // iPhone系列按屏幕尺寸区分
        switch screen.nativeBounds.size.height {
        case 1136, 1334, 2436, 2532:
            return isLandscape ? (.compact, .compact) : (.compact, .regular)
        case 2208, 2688, 2778, 2556:
            return isLandscape ? (.regular, .compact) : (.compact, .regular)
        default:
            return (.unspecified, .unspecified)
        }
    }
}

实时监听方向变化的Size Class #

如果需要在方向切换时实时获取更新后的Size Class，可在ViewController中重写以下方法：

override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
    super.viewWillTransition(to: size, with: coordinator)
    coordinator.animate(alongsideTransition: { _ in
        let newHorizontal = self.traitCollection.horizontalSizeClass
        let newVertical = self.traitCollection.verticalSizeClass
        // 处理新的Size Class逻辑
    })
}

内容的提问来源于stack exchange，提问作者Ryan