MAUI: Listview cannot be refreshed automatically - maui

Question

There are 2 elements binding data in my test app, one is Button, another is Listview. But Listview cannnot be refreshed automatically. But if I disable ButtonText = $"It took {totalSecs} seconds" in ViewModel, Listview can be refreshed automatically. Any ideas? TIA.
Here is the Xaml file.
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:test"
x:Class="test.MainPage">
<ContentPage.BindingContext>
<local:PurchasesInvoiceMasterViewModel />
</ContentPage.BindingContext>
<ScrollView>
<VerticalStackLayout
Spacing="25"
Padding="30,0"
VerticalOptions="Start">
<Button
x:Name="PurchasesInvoiceList"
Text="{Binding ButtonText}"
Command="{Binding AddListCommand}"
SemanticProperties.Hint="Get Purchases Invoice List"
HorizontalOptions="Center" />
<ListView x:Name="PurchasesInvoiceMasterObjectView" ItemsSource="{Binding PurchasesInvoiceMasterObject}" HeightRequest="500">
<ListView.ItemTemplate>
<DataTemplate>
<TextCell Text="{Binding purchases_invoice_id}"
Detail="{Binding purchases_invoice_date }" />
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</VerticalStackLayout>
</ScrollView>
Here is ViewModel.
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Input;
using System.Xml.Linq;
namespace test
{
class PurchasesInvoiceMasterViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public ICommand AddListCommand { get; set; }
private string _buttonText = "Purchases Invoice List";
public string ButtonText
{
get => _buttonText;
set
{
if (_buttonText != value)
{
_buttonText = value;
OnPropertyChanged("");
}
}
}
private ObservableCollection<PurchasesInvoiceMaster> _purchasesInvoiceMasterObject = new();
public ObservableCollection<PurchasesInvoiceMaster> PurchasesInvoiceMasterObject
{
get => _purchasesInvoiceMasterObject;
set
{
if (_purchasesInvoiceMasterObject != value)
{
_purchasesInvoiceMasterObject = value;
OnPropertyChanged("");
}
}
}
public PurchasesInvoiceMasterViewModel()
{
AddListCommand = new Command(() =>
{
double statrtSecs = DateTime.Now.TimeOfDay.TotalSeconds;
string purchasesInvoiceMaster = "test.PurchasesInvoiceMaster.xml";
using (Stream streamPurchasesInvoiceMaster = this.GetType().Assembly.
GetManifestResourceStream(purchasesInvoiceMaster))
{
using (var readerPurchasesInvoiceMaster = new System.IO.StreamReader(streamPurchasesInvoiceMaster))
{
var xmlpurchasesInvoiceMaster = XDocument.Load(readerPurchasesInvoiceMaster);
foreach (XElement xmmd in xmlpurchasesInvoiceMaster.Descendants("curtemp01"))
{
_purchasesInvoiceMasterObject.Add(new PurchasesInvoiceMaster
{
purchases_invoice_id = xmmd.Element("purchases_invoice_id").Value,
purchases_invoice_date = xmmd.Element("purchases_invoice_date").Value,
});
}
}
}
double endSecs = DateTime.Now.TimeOfDay.TotalSeconds;
float totalSecs = (float)(endSecs - statrtSecs);
ButtonText = $"It took {totalSecs} seconds";
});
}
public void OnPropertyChanged([CallerMemberName] string name = "") =>
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
}
}