SQLite: Show sum of values of a column in the label-CodePudding

I am new to Xamarin - and I'm encountering a problem. How can I display the sum of the values of a column in a label from SQLite?

Here is my code.

Model Budget

 public class Budget
 {
    [PrimaryKey, AutoIncrement]
    public int Id { get; set; }
    public int Money { get; set; }
 }

SQLite Database Method GetBudgets()

public class StatisticsService
{
    static SQLiteAsyncConnection db;
    static async Task Init()
    {
        if (db != null)
            return;
        // Get an absolute path to the database file
        var databasePath = Path.Combine(FileSystem.AppDataDirectory, "MyApp.db");

        db = new SQLiteAsyncConnection(databasePath);
        await db.CreateTableAsync<Budget>();
    }

    public static async Task AddBudget(int money)
    {
        await Init();
        var budget = new Budget
        {
            Money = money,
        };

        await db.InsertAsync(budget);
    }

    public static async Task<int> GetBudgets()
    {
        await Init();

        int sumBudgets = await db.ExecuteScalarAsync<int>("SELECT SUM(Money) FROM Budget");
        return sumBudgets;
    }

}

ViewModel Code

    int budgetMoney;
    public int  BudgetMoney { get => budgetMoney; set => SetProperty(ref budgetMoney, value); }
    public AsyncCommand OpenAddBudget { get; }
    public AsyncCommand ListBudget { get; }

    public StatisticsViewModel()
    {

        OpenAddBudget = new AsyncCommand(Open);
        ListBudget = new AsyncCommand(ListGetBudget);

        
    }
    async Task Open()
    {
        var route = "addBudgetPage";
        await Shell.Current.GoToAsync(route);
    }
    async Task ListGetBudget()
    {
        budgetMoney = await StatisticsService.GetBudgets();
    }

View Xaml

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         x:Class="MyApp.Views.Statistics"
         xmlns:viewmodels="clr-namespace:MyApp.ViewModels"
         xmlns:model="clr-namespace:MyApp.Models">
<ContentPage.BindingContext>
    <viewmodels:StatisticsViewModel/>
</ContentPage.BindingContext>
<ContentPage.ToolbarItems>
    <ToolbarItem Text="Add" Command="{Binding OpenAddBudget}"/>
</ContentPage.ToolbarItems>
<ContentPage.Content>
    <StackLayout>
        <Button Text="Reload" Command="{Binding ListBudget}"/>
        <Label Text="{Binding BudgetMoney}"/>
    </StackLayout>
</ContentPage.Content>

I don't get any errors, but when debugging I noticed that the variable sumBudget is always 0. Do I have something wrong in the syntax of SQLite?

public static async Task<int> GetBudgets()
    {
        await Init();

        int sumBudgets = await db.ExecuteScalarAsync<int>("SELECT SUM(Money) FROM Budget");
        return sumBudgets;
    }

Unfortunately, I somehow do not get further. The goal should be that when I click on the button "Reload" the sum of the individual budgets are displayed in the label.

Thanks for your help!

Edit: Call AddButton

public class AddBudgetViewModel : ViewModelBase
{
    int money;
    public int Money { get => money; set => SetProperty(ref money, value); }
    public AsyncCommand SaveCommand { get; }

    public AddBudgetViewModel()
    {
        SaveCommand = new AsyncCommand(Save);

    }
    async Task Save()
    {
        if (money == 0)
            return;

        await StatisticsService.AddBudget(money);

        await Shell.Current.GoToAsync("..");
    }
}

CodePudding user response：

this is setting the private field budgetMoney, which does NOT call PropertyChanged

budgetMoney = await StatisticsService.GetBudgets();

instead, you should set the public property, which will call PropertyChanged

BudgetMoney = await StatisticsService.GetBudgets();