弹窗

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UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"错误" message:@"请先安装微信客户端" preferredStyle:UIAlertControllerStyleAlert];

UIAlertAction *actionConfirm = [UIAlertAction actionWithTitle:@"好的" style:UIAlertActionStyleDefault handler:nil];
 [alert addAction:actionConfirm];
 [self presentViewController:alert animated:YES completion:nil];

UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"已经登录" message:@"您已经登录" preferredStyle:UIAlertControllerStyleAlert];
        [alert addAction:[UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
            NSLog(@"点击确认");
            [self.navigationController pushViewController:[UserViewController new] animated:YES];
        }]];
        
        [self presentViewController:alert animated:YES completion:nil];

只显示几秒

    UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"已推出" message:nil preferredStyle:UIAlertControllerStyleAlert];
    [self presentViewController:alert animated:YES completion:nil];
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        [self.presentedViewController dismissViewControllerAnimated:YES completion:nil];
    });