无法从 Xamarin c# 中的 Firebase 检索记录

Question

在我的应用程序中，我已连接 Firebase 并尝试填充列表视图，但在开始时，我没有看到任何记录。它只是显示一个空白页面，列表视图中没有填充任何内容。

这是我的 firebasehelper.cs

{
class FirebaseHelper
{
    FirebaseClient firebase = new FirebaseClient("https://mylimo-b2029.firebaseio.com/");


    public async Task<List<users>> GetAllUsers()
    {

        try
        {
          return (await firebase
         .Child("Users")
         .OnceAsync<users>()).Select(item => new users
         {
             //,

             first_name = item.Object.first_name,
             last_name = item.Object.last_name,
             password = item.Object.password,
             user_id = item.Object.user_id,
         }).ToList();
        }
        catch (Exception ex)
        {
            throw new Exception("GetUsers  Additional information..." + ex, ex);
        }
    }    


    public async Task<users> GetUsers(string usr_id)
    {
        try
        {
            var allUsers = await GetAllUsers();
            await firebase
              .Child("users")
              .OnceAsync<users>();
            return allUsers.Where(a => a.user_id == usr_id).FirstOrDefault();
        }


        catch (Exception ex)
        {
            throw new Exception("GetUsers  Additional information..." + ex, ex);
        }
    }
}

下面是 MainPage.xaml.cs

 public partial class MainPage : ContentPage
{
    FirebaseHelper firebaseHelper = new FirebaseHelper();
    public MainPage()
    {
        try
        {
            InitializeComponent();
        }
        catch (Exception ex)
        {
            throw new Exception("InitializeComponent  Additional information..." + ex, ex);
        }
    }

    protected async override void OnAppearing()
    {
        try
        {
            base.OnAppearing();
            var allUsers = await firebaseHelper.GetAllUsers();
            lstPersons.ItemsSource = allUsers;
        }

        catch (Exception ex)
        {
            throw new Exception("OnAppearing  Additional information..." + ex, ex);
        }




    }

在应用启动时，它只会显示一个没有任何记录的空白列表视图控件，

.

我的firebase数据库如下]1

Answer 1

你的代码是正常的。但是你可能会少一些步骤。

Firebase 数据库的使用请参考以下方法。

首先，请打开firebase数据库控制台，将读/写身份验证规则设置为null，如下图所示。

然后创建一个模块user。

public class User
{
    public string first_name { get; set; }
    public string last_name { get; set; }

    public string password { get; set; }

    public int userid { get; set; }
}

有我的 firebasehelper。

public class Firebasehelper
{
    FirebaseClient firebase;

    public Firebasehelper()
    {
        firebase = new FirebaseClient("https://fir-databasedemo-62a72.firebaseio.com/");
    }
   
    public async Task AddUser(int userid, string first_name, string last_name, string password)
    {
        await firebase
          .Child("Users")
          .PostAsync(new User() { userid = userid, first_name = first_name, last_name= last_name, password= password });
    }

    public async Task<List<User>> GetAllUsers()
    {
        return (await firebase
          .Child("Users")
          .OnceAsync<User>()).Select(item => new User
          {

              userid = item.Object.userid,
              first_name = item.Object.first_name,
              last_name = item.Object.last_name,
              password = item.Object.password
          }).ToList();
    }
}

请不要直接在 firebase 中创建记录。

我们应该通过代码添加这些记录。成功添加这些记录后，您可以在firebase控制台中看到这些记录。

与以下代码一起使用。

  private async void BtnAdd_Clicked(object sender, EventArgs e)
  {
        // AddUser(int userid, string first_name, string last_name, string password)
        await firebaseHelper.AddUser(Convert.ToInt32(1), "Jon","hard","123.com");
        await firebaseHelper.AddUser(Convert.ToInt32(1), "Leon", "esay", "1234.com");
        await firebaseHelper.AddUser(Convert.ToInt32(1), "Rebecca", "middlue", "12345.com");
        await DisplayAlert("Success", "Person Added Successfully", "OK");
       var allPersons = await firebaseHelper.GetAllUsers();
       lstPersons.ItemsSource = allPersons;
    }

这是我的MainPage.xaml。

 <ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:local="clr-namespace:RetrieveRecordDemo"
         x:Class="RetrieveRecordDemo.MainPage">
  <StackLayout>
    <Button x:Name="BtnAdd" Text="add" />

    <ListView x:Name="lstPersons">
        <ListView.ItemTemplate>
            <DataTemplate>
                <ViewCell>
                    <StackLayout>

                        <Label Text="{Binding first_name}"  > 
                 </Label>
                        <Label Text="{Binding last_name}" > 
                 </Label>
                     
                    </StackLayout>
                </ViewCell>
            </DataTemplate>
        </ListView.ItemTemplate>
    </ListView>
</StackLayout>
</ContentPage>

MainPage.xml.cs

   public partial class MainPage : ContentPage
{
    Firebasehelper firebaseHelper;
    public MainPage()
    {
        InitializeComponent();
        firebaseHelper = new Firebasehelper();

        BtnAdd.Clicked += BtnAdd_Clicked;
    }

    private async void BtnAdd_Clicked(object sender, EventArgs e)
    {
        // AddUser(int userid, string first_name, string last_name, string password)
        await firebaseHelper.AddUser(Convert.ToInt32(1), "Jon","hard","123.com");
        await firebaseHelper.AddUser(Convert.ToInt32(1), "Leon", "esay", "1234.com");
        await firebaseHelper.AddUser(Convert.ToInt32(1), "Rebecca", "middlue", "12345.com");
        await DisplayAlert("Success", "Person Added Successfully", "OK");
       var allPersons = await firebaseHelper.GetAllUsers();
       lstPersons.ItemsSource = allPersons;
    }

    protected async override void OnAppearing()
    {

        base.OnAppearing();


        try
        {
            base.OnAppearing();
            var allUsers = await firebaseHelper.GetAllUsers();
            lstPersons.ItemsSource = allUsers;
        }

        catch (Exception ex)
        {
            throw new Exception("OnAppearing  Additional information..." + ex, ex);
        }
    }
}

有运行截图。

这是我的演示：https://github.com/851265601/RetrieveRecordDemo

firebase 数据库中有一篇关于 CRUD 的有用文章。

https://www.c-sharpcorner.com/article/xamarin-forms-working-with-firebase-realtime-database-crud-operations/