如何先以表单页展示ViewController，再点击按钮切换为全屏？

这个需求我之前做项目时也碰到过，核心思路就是修改ViewController的modalPresentationStyle，再重新触发present逻辑，下面给你一步步拆解实现：

第一步：保存presentedViewController的引用 #

首先得在当前VC里存好要展示的ViewController（或它的容器控制器）的引用，后面切换时才能精准拿到它。比如在你的VC头文件里加个属性：

@property (nonatomic, strong) UIViewController *presentedSheetVC;

然后修改你现有的present代码：

NewViewController *vc = [[NewViewController alloc] init];
vc.delegate = self;
UIViewController *presentViewController;

if (IPAD) {
    // 如果你用了自定义的表单容器（比如你的formSheetControllerWithVC），要拿到容器实例
    presentViewController = [self formSheetControllerWithVC:vc];
    self.presentedSheetVC = presentViewController;
} else {
    presentViewController = vc;
    self.presentedSheetVC = vc;
}

[self presentViewController:presentViewController animated:YES completion:nil];

第二步：实现切换逻辑（两种可选方案） #

方案一：直接在NewViewController里处理（简单直接） #

给NewViewController里的按钮加点击事件，直接修改自身的modal样式，通过dismiss再present来应用变更：

- (IBAction)toggleFullScreenBtnTapped:(UIButton *)sender {
    // 切换modal展示样式
    if (self.modalPresentationStyle == UIModalPresentationFormSheet) {
        self.modalPresentationStyle = UIModalPresentationFullScreen;
    } else {
        self.modalPresentationStyle = UIModalPresentationFormSheet;
    }

    // 先dismiss再重新present，让系统应用新样式
    [self dismissViewControllerAnimated:YES completion:^{
        [[self presentingViewController] presentViewController:self animated:YES completion:nil];
    }];
}

方案二：通过代理让原VC处理（更解耦） #

如果想让逻辑分层更清晰，可以用代理通知原VC处理切换：

1. 先在NewViewController的头文件定义代理协议：

@protocol NewViewControllerDelegate <NSObject>
- (void)newViewControllerDidRequestToggleFullScreen:(NewViewController *)vc;
@end

@interface NewViewController : UIViewController
@property (nonatomic, weak) id<NewViewControllerDelegate> delegate;
@end

2. 在NewViewController的按钮点击事件里调用代理：

- (IBAction)toggleFullScreenBtnTapped:(UIButton *)sender {
    if ([self.delegate respondsToSelector:@selector(newViewControllerDidRequestToggleFullScreen:)]) {
        [self.delegate newViewControllerDidRequestToggleFullScreen:self];
    }
}

3. 在原VC里实现代理方法：

- (void)newViewControllerDidRequestToggleFullScreen:(NewViewController *)vc {
    // 修改保存的presentedSheetVC的展示样式
    if (self.presentedSheetVC.modalPresentationStyle == UIModalPresentationFormSheet) {
        self.presentedSheetVC.modalPresentationStyle = UIModalPresentationFullScreen;
    } else {
        self.presentedSheetVC.modalPresentationStyle = UIModalPresentationFormSheet;
    }

    // 重新present使样式生效
    [self dismissViewControllerAnimated:YES completion:^{
        [self presentViewController:self.presentedSheetVC animated:YES completion:nil];
    }];
}

注意事项 #

• iOS 13及以后，默认的modalPresentationStyle是UIModalPresentationAutomatic，一定要在present前明确设置为UIModalPresentationFormSheet或UIModalPresentationFullScreen
• 如果你的formSheetControllerWithVC是自定义容器控制器（比如带导航栏的表单页），要确保修改的是容器控制器的modalPresentationStyle，而非内部的NewViewController

内容的提问来源于stack exchange，提问作者SriTeja Chilakamarri