How to trigger event in the end of async procedure

Question

Hello,

I have an async method. If the method do its work it should trigger an event, but not before.

    public async Task HandleProjectItem(Project clickedItem)
    {

//some code

//if code did all his work

        this.DataLoaded?.Invoke(this, EventArgs.Empty);

}

Answer 1

Hi, not sure what you need to do in this task. Can you use something like this:

await taskA.ContinueWith( antecedent => Console.WriteLine("Today is {0}.", antecedent.Result) );  

You can get the task result from antecedent and the exceptions from antecedent.Exception if there are any exceptions in the task.

FYI: https://learn.microsoft.com/en-us/dotnet/api/system.threading.tasks.task.continuewith?view=netcore-3.1

By the way, I think you can just use await keyword here and use try-catch block to wrapper the task. Because if there are any errors before the event trigger, the event trigger won't be executed. eg:

try  
{  
    //do some tasks  
    await taskA();  
    await taskB();  
    this.DataLoaded?.Invoke(this, EventArgs.Empty);  
}  
catch(Exception ex)  
{  
    //do something  
}  

Please correct me if I missed anything.

Answer 2

Hi, you can build class with event and in this class include HandleProjectItem. Try following demo.

XAML:

<Page  
    x:Class="App1.Page08"  
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"  
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"  
    xmlns:local="using:App08"  
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"  
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"  
    mc:Ignorable="d"  
    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">  
  <Page.DataContext>  
    <local:ViewModel/>  
  </Page.DataContext>  
  <StackPanel>  
    <Button Content="Start read" Command="{Binding Cmd}"/>  
    <TextBox/>  
    <ListBox ItemsSource="{Binding View}">  
      <ListBox.ItemTemplate>  
        <DataTemplate>  
          <TextBlock Text="{Binding Info}"/>  
        </DataTemplate>  
      </ListBox.ItemTemplate>  
    </ListBox>  
  </StackPanel>  
</Page>  

And classes:

using System;  
using System.Collections.ObjectModel;  
using System.ComponentModel;  
using System.Runtime.CompilerServices;  
using System.Threading;  
using System.Threading.Tasks;  
using System.Windows.Input;  
using Windows.UI.Xaml.Controls;  
  
namespace App08  
{  
  public class ViewModel : INotifyPropertyChanged  
  {  
    ObservableCollection<Project> col;  
    public ObservableCollection<Project> View { get => col; }  
  
    DataModel m;  
  
    public ICommand Cmd  
    {  
      get => new RelayCommand(async (state) =>  
      {  
        if (m == null)  
        {  
          m = new DataModel();  
          m.DataLoaded += M_DataLoaded;  
        }  
        await m.HandleProjectItem(null);  
      });  
    }  
  
    private void M_DataLoaded(object sender, EventArgs e)  
    {  
      col = m.Collection;  
      OnPropertyChanged(nameof(View));  
    }  
  
    public event PropertyChangedEventHandler PropertyChanged;  
    internal void OnPropertyChanged([CallerMemberName] string propName = "") =>  
      PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propName));  
  }  

  public class Project  
  {  
    public string Info { get; set; }  
  }