# Load On Demand in WPF Scheduler (SfScheduler)
The [WPF Scheduler](https://www.syncfusion.com/wpf-controls/scheduler) supports to load appointment on demand with loading indicator and it improves the loading performance when there are appointments range for multiple years.

![Load on-demand in WPF Scheduler](LoadOnDemand_Images/LoadOnDemand.gif)

## QueryAppointments event
The [QueryAppointments](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.SfScheduler.html#Syncfusion_UI_Xaml_Scheduler_SfScheduler_QueryAppointments) event is used to load appointments in on-demand for the visible date range. Start and stop the loading indicator animation before and after the appointments loaded using the [ShowBusyIndicator](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.SfScheduler.html#Syncfusion_UI_Xaml_Scheduler_SfScheduler_ShowBusyIndicator).
The [QueryAppointmentsEventArgs](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.QueryAppointmentsEventArgs.html) has the following members which provides information for the `QueryAppointments` event.

[VisibleDateRange](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.DateRange.html) - Gets the current visible date range of scheduler that is used to load the appointments.

{% tabs %}
{% highlight xaml %}
<syncfusion:SfScheduler x:Name="Scheduler"
                        ViewType="Month" QueryAppointments="Scheduler_QueryAppointments">
</syncfusion:SfScheduler>
{% endhighlight %}
{% highlight c#%}
Scheduler.QueryAppointments += Scheduler_QueryAppointments;
private void Scheduler_QueryAppointments(object sender, QueryAppointmentsEventArgs e)
{
 Scheduler.ShowBusyIndicator = true;
 Scheduler.ItemsSource = this.GenerateSchedulerAppointments(e.VisibleDateRange);
 Scheduler.ShowBusyIndicator = false;
}

private IEnumerable GenerateSchedulerAppointments(DateRange dateRange)
{
  Random ran = new Random();
  int daysCount = (dateRange.ActualEndDate - dateRange.ActualStartDate).Days;
  var appointments = new ObservableCollection<SchedulerModel>();
      
  for (int i = 0; i < 50; i++)
  {
    var startTime = dateRange.ActualStartDate.AddDays(ran.Next(0, daysCount + 1)).AddHours(ran.Next(0, 24));
    appointments.Add(new SchedulerModel
    {
    From = startTime,
    To = startTime.AddHours(1),
    EventName = subjectCollection[ran.Next(0, subjectCollection.Count)],
    Color = brush[ran.Next(0, brush.Count)],
    });
  }
  return appointments;
}
{% endhighlight %}
{% endtabs %}

The [QueryAppointments](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.SfScheduler.html#Syncfusion_UI_Xaml_Scheduler_SfScheduler_QueryAppointments) will be raised once any one of the following action is taken.

 * Once the [ViewChanged](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.SfScheduler.html#Syncfusion_UI_Xaml_Scheduler_SfScheduler_ViewChanged) event is raised, the `QueryAppointments` will be raised.

* If the appointment has been added, removed, or changed (Resize, drag, and drop) in the current time visible date range, then the `QueryAppointments` event will not be triggered. Since the appointments for that visible date range have been loaded already.

* The `QueryAppointments` event is triggered when the Schedule [ResourceCollection](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.SfScheduler.html#Syncfusion_UI_Xaml_Scheduler_SfScheduler_ResourceCollection) is updated to load appointments based on the changed resource collection.

* The `QueryAppointments` event will be triggered when the [ResourceGroupType](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.SfScheduler.html#Syncfusion_UI_Xaml_Scheduler_SfScheduler_ResourceGroupType) is changed.

## Load On Demand command
The Scheduler notifies by [LoadOnDemandCommand](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.SfScheduler.html#Syncfusion_UI_Xaml_Scheduler_SfScheduler_LoadOnDemandCommand) when the user changes the visible date range. Get a visible date range from the [QueryAppointmentsEventArgs](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.QueryAppointmentsEventArgs.html). The default value for this `ICommand` is null. The `QueryAppointmentsEventArgs` passed as a command parameter.

Define a ViewModel class that implements command and handle it by the CanExecute and Execute methods to check and execute on-demand loading. In the execute method, perform the following operations,
* Start and stop the loading indicator animation before and after the appointments loaded using the [ShowBusyIndicator](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.SfScheduler.html#Syncfusion_UI_Xaml_Scheduler_SfScheduler_ShowBusyIndicator).
* Once the appointment collection is got, load into the scheduler itemsource.

{% tabs %}
{% highlight xaml %}
<syncfusion:SfScheduler x:Name="Scheduler"
                        ViewType="Month" 
                        ShowBusyIndicator="{Binding ShowBusyIndicator}"
                        LoadOnDemandCommand="{Binding LoadOnDemandCommand}"
                        ItemsSource="{Binding Events}">
</syncfusion:SfScheduler>
{% endhighlight %}
{% highlight c#%}

public class LoadOnDemandViewModel : NotificationObject
  {
    public ICommand LoadOnDemandCommand { get; set; }
    private IEnumerable events;
    public IEnumerable Events
      {
        get { return events; }
        set
          {
            events = value;
            this.RaisePropertyChanged("Events");
          }
      }

    private bool showBusyIndicator;
    public bool ShowBusyIndicator
    {
      get { return showBusyIndicator; }
      set
          {
            showBusyIndicator = value;
            this.RaisePropertyChanged("ShowBusyIndicator");
          }
    }
    public SchedulerViewModel()
    {
       this.LoadOnDemandCommand = new DelegateCommand(ExecuteOnDemandLoading, CanExecuteOnDemandLoading);
    }
       
    public event PropertyChangedEventHandler PropertyChanged;
    public async void ExecuteOnDemandLoading(object parameter)
    {
      this.ShowBusyIndicator = true;
      await Task.Delay(1000);
      await Application.Current.MainWindow.Dispatcher.BeginInvoke(DispatcherPriority.ApplicationIdle, new Action(() =>
            {
                this.Events = this.GenerateSchedulerAppointments((parameter as QueryAppointmentsEventArgs).VisibleDateRange);
            }));
      this.ShowBusyIndicator = false;
    }

    private bool CanExecuteOnDemandLoading(object sender)
    {
      return true;
    }

    private IEnumerable GenerateSchedulerAppointments(DateRange dateRange)
    {
      Random ran = new Random();
      int daysCount = (dateRange.ActualEndDate - dateRange.ActualStartDate).Days;
      var appointments = new ObservableCollection<SchedulerModel>();
      for (int i = 0; i < 50; i++)
      {
        var startTime = dateRange.ActualStartDate.AddDays(ran.Next(0, daysCount + 1)).AddHours(ran.Next(0, 24));appointments.Add(new SchedulerModel
        {
          From = startTime,
          To = startTime.AddHours(1),
          EventName = subjectCollection[ran.Next(0, subjectCollection.Count)],
          Color = brush[ran.Next(0, brush.Count)],
        });
      }
      return appointments;
    }
  }
{% endhighlight %}
{% endtabs %}

The [LoadOnDemandCommand](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.SfScheduler.html#Syncfusion_UI_Xaml_Scheduler_SfScheduler_LoadOnDemandCommand) will be invoked once any one of the following actions is taken.

* Once the [ViewChanged](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.SfScheduler.html#Syncfusion_UI_Xaml_Scheduler_SfScheduler_ViewChanged) event is raised, the `LoadOnDemandCommand` will also be raised.

* If the appointment has been added, removed, or changed (Resize, drag, and drop) in the current time visible date range, then the `LoadOnDemandCommand` will not be triggered. Since the appointments for that visible date range have been loaded already.

* The `LoadOnDemandCommand` is triggered when the Schedule [ResourceCollection](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.SfScheduler.html#Syncfusion_UI_Xaml_Scheduler_SfScheduler_ResourceCollection) is updated to load the appointments based on the changed resource collection.

* The `LoadOnDemandCommand` will be triggered when the [ResourceGroupType](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.SfScheduler.html#Syncfusion_UI_Xaml_Scheduler_SfScheduler_ResourceGroupType) is changed.

## Load On Demand for recurring appointment
The scheduler will add the occurrences of recurrence series based on the visible date range, use the [RecurrenceHelper.GetRecurrenceDateTimeCollection](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.RecurrenceHelper.html#Syncfusion_UI_Xaml_Scheduler_RecurrenceHelper_GetRecurrenceDateTimeCollection_System_String_System_DateTime_System_Nullable_System_DateTime__System_Nullable_System_DateTime__) to compare and load the recurrence appointment on demand in the ItemsSource.

* The recurrence appointment should be added to the Scheduler [ItemsSource](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.SfScheduler.html#Syncfusion_UI_Xaml_Scheduler_SfScheduler_ItemsSource) until the date of recurrence ends.

* If [RecurrenceRule](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.ScheduleAppointment.html#Syncfusion_UI_Xaml_Scheduler_ScheduleAppointment_RecurrenceRule) added with count or end date, use the [RecurrenceHelper.GetRecurrenceDateTimeCollection](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.RecurrenceHelper.html#Syncfusion_UI_Xaml_Scheduler_RecurrenceHelper_GetRecurrenceDateTimeCollection_System_String_System_DateTime_System_Nullable_System_DateTime__System_Nullable_System_DateTime__) method to get the recurrence date collection and compare recursive dates in the current visible date range. Then add the recurrence appointment in the scheduler [ItemsSource](https://help.syncfusion.com/cr/wpf/Syncfusion.UI.Xaml.Scheduler.SfScheduler.html#Syncfusion_UI_Xaml_Scheduler_SfScheduler_ItemsSource) If the recursive dates are in the current visible date range.

* If the `RecurrenceRule` is added without an end date, then the recurrence appointment should be added in the scheduler `ItemsSource` when all the visible dates changed from the recurrence start date.

N> [View sample in GitHub](https://github.com/SyncfusionExamples/load-on-demand-appointments-wpf-scheduler)