Question

大家好：我的代码卡在这里了。我的应用程序中有一个地方需要通知用户进行注册和签名，以保存他们的偏好。我一直试图在ViewModel内添加显示警报，但是它不起作用。请帮助我被卡住。

ApproveViewModel

    namespace MyApp.ViewModels
    {
     public class ApproveViewModel
    {
        private DataService dataService = new DataService();
        public Oppotunity SelectedOppotunity { get; set; }


    public ICommand SaveCommand => new Command(async () =>
        { 

        await dataService.PostSaveOppotunity(SelectedOppotunity, Settings.AccessToken);

        });

    public ApproveViewModel()
    {
        SelectedOppotunity = new Oppotunity();
    }

}
}

ApprovePage.xaml

<ScrollView>
<StackLayout Padding ="15">

    <Label Text ="{Binding SelectedOppotunity.Title}"/>
    <Label Text ="{Binding SelectedOppotunity.Description }"/>
    <Label Text ="{Binding SelectedOppotunity.Organisation}"/>
    <Label Text ="{Binding SelectedOppotunity.Venue }"/>
    <Label Text ="{Binding SelectedOppotunity.Eligibility}"/>
    <Label Text ="{Binding SelectedOppotunity.Benefits}"/>
    <Label Text ="{Binding SelectedOppotunity.Province}"/>
    <Label Text ="{Binding SelectedOppotunity.Country}"/>
    <Label Text ="{Binding SelectedOppotunity.OppotunityLink}"/>
    <Label Text ="{Binding SelectedOppotunity.Category}"/>
    <Label Text  ="{Binding SelectedOppotunity.Deadline}"/>
    <!--
    <Switch IsToggled ="{Binding SelectedOppotunity.IsApproved}"></Switch>
    -->

        <Button Text ="Apply" BackgroundColor ="#A91717" TextColor ="White"
            Command ="{Binding SaveCommand }"/>


</StackLayout>

我希望在保存时调用的代码：

 if (!string.IsNullOrEmpty(Settings.AccessToken))
         {
            // Implement the SaveCommand from the ViewModel;
        }
           // Go to Login form to get an access token
        else if (!string.IsNullOrEmpty(Settings.Username) &&
                 !string.IsNullOrEmpty(Settings.Password))
        {
            MainPage = new NavigationPage(new Login());
        }

        else
        {  
            //Register first
            MainPage = new NavigationPage(new NewRegisterPage());
        }

Answer 1

如果您不想从ViewModel中显示警报，则可以：

Application.Current.MainPage.DisplayAlert();

Answer 2

解决方案：

您可以使用MessageCenter订阅来解决此问题。

Xamarin.Forms MessagingCenter 启用视图模型和其他组件   交流而不必彼此了解   除了简单的消息合同。

订阅-收听具有特定签名的邮件并执行   收到通知后采取一些措施。可以有多个订阅者   收听相同的消息。

发送-发布消息，供听众采取行动。如果不   侦听者已订阅，则该消息将被忽略。

视图模型中的代码：

if (!string.IsNullOrEmpty(Settings.AccessToken))
    {
       // Implement the SaveCommand from the ViewModel;
    }
       // Go to Login form to get an access token
    else if (!string.IsNullOrEmpty(Settings.Username) &&
             !string.IsNullOrEmpty(Settings.Password))
    {
        MainPage = new NavigationPage(new Login());
    }
    else
    {  
        //Register first

        //if you want to notify users to register here, use MessageCenter.Send
         MessageCenter.Send(this, "displayAlert")

         MainPage = new NavigationPage(new NewRegisterPage());
    }

视图中的代码：

MessagingCenter.Subscribe<ViewModelName>(this, "displayAlert", (sender) => {
            // do something whenever the "displayAlert" message is sent

            DisplayAlert("notification", "you should register first", "ok");
        });

将消息发送到要在viewModel中显示警报的位置。

有关MessageCenter的更多信息，您可以参考https://docs.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/messaging-center

Xamarin形式条件语句

2 个答案: