当用户选择 UI 选项卡项时，我需要执行一些 UI 任务。以下代表可用，

虽然，为了回答我的特定问题，内部 UI 转换问题在这里并不重要，但我仍然分享一个概述代码片段。

-(void)tabBar:(UITabBar *)tabBar didSelectItem:(UITabBarItem *)item
{
    AppDelegate *delegate = (AppDelegate*)[[UIApplication sharedApplication] delegate];
    [delegate.rootTabBarController showConsentErrorPage];
}

但是，我在此委托中的 UI 任务在转换时显示了一个小故障，因为它在选项卡已经显示后才开始作业。我想在 UI 可见之前先执行 UI 任务。有没有这样的技巧代表来解决这个问题？

uj5u.com热心网友回复：

这可能会对您有所帮助（没有其他信息，我真的不能说）。

• 子类UITabBarController符合<UITabBarControllerDelegate>
• 在新的自定义UITabBarController ，一定要设定self.delegate = self;在viewDidLoad
• 实施 shouldSelectViewController
• 如果该控制器是您的“需要同意查看”视图控制器，
  • 检查用户是否已经同意
  • 如果是，则回传YES（即允许选择选项卡）
  • 如果没有，请出示您的“征求同意”控制器并回传 NO
  • 如果用户同意，请导航到该选项卡

这是一些示例代码...

使用此选项，我们会显示“询问同意”控制器，并且仅在用户选择“是”时导航到“需要同意才能查看”选项卡：

- (BOOL)tabBarController:(UITabBarController *)tabBarController shouldSelectViewController:(UIViewController *)viewController {
    
    if ([viewController isKindOfClass:NeedsConsentViewController.class]) {
        
        NeedsConsentViewController *vc = (NeedsConsentViewController *)viewController;
        
        // whatever you're using to track the user's consent
        if (vc.hasConsent) {
            // allow the tab to be selected
            return YES;
        }
        
        // configure / instantiate your "Consent" view controller
        UIAlertController * alert = [UIAlertController
                                     alertControllerWithTitle:@"Yes/No"
                                     message:@"Need your consent..."
                                     preferredStyle:UIAlertControllerStyleAlert];
        
        UIAlertAction* okButton = [UIAlertAction
                                   actionWithTitle:@"Yes"
                                   style:UIAlertActionStyleDefault
                                   handler:^(UIAlertAction * action) {
            
            // however you're setting your user consent tracking
            vc.hasConsent = YES;
            
            // show that tab
            [self setSelectedViewController:vc];
            
        }];
        
        [alert addAction:okButton];
        
        UIAlertAction* noButton = [UIAlertAction
                                   actionWithTitle:@"No"
                                   style:UIAlertActionStyleDefault
                                   handler:^(UIAlertAction * action) {
            
            // user said NO... nothing else to do

        }];
        
        [alert addAction:noButton];
        
        [self presentViewController:alert animated:YES completion:nil];
        
        // don't show the tab
        return NO;
    }
    
    // all other tabs
    return YES;
}

使用此选项，我们会显示“征求同意”控制器并导航到其后面的“需要同意才能查看”选项卡。如果用户回答“否”，我们将导航回之前选择的选项卡：

- (BOOL)tabBarController:(UITabBarController *)tabBarController shouldSelectViewController:(UIViewController *)viewController {
    
    NSInteger curTabIDX = self.selectedIndex;
    
    if ([viewController isKindOfClass:NeedsConsentViewController.class]) {

        NeedsConsentViewController *vc = (NeedsConsentViewController *)viewController;

        // whatever you're using to track the user's consent
        if (vc.hasConsent) {
            // allow the tab to be selected
            return YES;
        }

        // configure / instantiate your "Consent" view controller
        UIAlertController * alert = [UIAlertController
                                     alertControllerWithTitle:@"Yes/No"
                                     message:@"Need your consent..."
                                     preferredStyle:UIAlertControllerStyleAlert];
        
        UIAlertAction* okButton = [UIAlertAction
                                   actionWithTitle:@"Yes"
                                   style:UIAlertActionStyleDefault
                                   handler:^(UIAlertAction * action) {
            
            // however you're setting your user consent tracking
            vc.hasConsent = YES;
            
            // we've already navigated to the tab, with the Consent VC presented on top of it
            //  so nothing else to do
            
        }];
        
        [alert addAction:okButton];
        
        UIAlertAction* noButton = [UIAlertAction
                                   actionWithTitle:@"No"
                                   style:UIAlertActionStyleDefault
                                   handler:^(UIAlertAction * action) {
            
            // user said NO, so return to the previous tab
            [self setSelectedIndex:curTabIDX];
            
        }];
        
        [alert addAction:noButton];
        
        [self presentViewController:alert animated:YES completion:nil];

        // show the tab behind the Consent VC
        return YES;
    }
    
    // all other tabs
    return YES;
}

注意：这只是示例代码，不打算也不应被视为“生产就绪”。