ListView with Activity Indicator

Hello, Attached screen shot will help you understand the scenario, I have a welcome page having "Place Order" button and Order Page, on clicking Place Order button order page will be loaded by using given code:

Navigation.PushAsync(new Order());

I am populating ListView on Order form load by using given code:

public Order()
    {
        this.InitializeComponent();
        GetSupplierItem();
        orderDetails = new List<OrderDetail>();

        NavigationPage.SetTitleView(this, new Image()
        {
            Source = "MatlobaMiqdar.png"
        });
        NavigationPage.SetHasBackButton(this, false);
    }


public void GetSupplierItem()
    {
        Mainlistview.ItemsSource = Baqala.ServiceHandler.GetSupplierItemForOrderBySupplierID(1);
}

I want to show the loading animation till the listview loaded, to achieve this I have used ActivityIndicator but it does not worked, I tried a lot, here is my .xaml code:

<?xml version="1.0" encoding="utf-8" ?>

<ContentPage.Content>

    <StackLayout Padding="4" BackgroundColor="#f0f0f0" HorizontalOptions="CenterAndExpand" VerticalOptions="CenterAndExpand">

        <ActivityIndicator x:Name="activity" Color="YellowGreen" IsRunning="{Binding IsBusy}" IsVisible="{Binding IsBusy}"   />

        <ListView  x:Name="Mainlistview"  RowHeight="74" HasUnevenRows="True" SeparatorColor="YellowGreen" SelectionMode="None" ItemTapped="Mainlistview_ItemTapped" >


            <ListView.ItemTemplate>
                <DataTemplate>

                    <ViewCell>



                        <StackLayout  Orientation="Horizontal" Margin="2,2,2,4" Spacing="1" >


                            <StackLayout Padding="2">
                                <Image x:Name="ItemPicture" Source="{Binding Item.ItemPicture, StringFormat='http://baqala-pk.com/Images/Vegetableimages/{0}'}" WidthRequest="70" HeightRequest="70"  />
                            </StackLayout>

                            <StackLayout Orientation="Vertical" VerticalOptions="StartAndExpand" HorizontalOptions="StartAndExpand" Padding="0" Margin="4,0,0,0">

                                <Label x:Name="ItemName" Text="{Binding Item.ItemName}" TextColor="Orange" LineBreakMode="NoWrap"  />
                                <Label x:Name="Price" Text="{Binding Price, StringFormat='RS.{0}/KG'}" HorizontalOptions="StartAndExpand" TextColor="#503026" FontFamily="calbiri"  LineBreakMode="NoWrap" />


                            </StackLayout>






                            <StackLayout WidthRequest="130" Padding="0" HorizontalOptions="EndAndExpand"  Margin="0,0,6,0">

                                <Grid  HorizontalOptions="EndAndExpand" WidthRequest="120" HeightRequest="70" Padding="0,0,0,0" Margin="0,0,0,0" RowSpacing="0">
                                    <Grid.ColumnDefinitions>
                                        <ColumnDefinition Width="*" />
                                    </Grid.ColumnDefinitions>
                                    <Grid.RowDefinitions>
                                        <RowDefinition Height="22" />
                                        <RowDefinition Height="*" />
                                    </Grid.RowDefinitions>


                                    <Frame Grid.Row="0" Grid.Column="0" CornerRadius="5" Padding="0" BackgroundColor="Gray" HorizontalOptions="End" WidthRequest="64" Margin="1">
                                        <Label  x:Name="ItemName2" Grid.Row="0" Grid.Column="0" Text="Rs.0" TextColor="White" FontSize="11" HorizontalTextAlignment="Center" VerticalTextAlignment="Center" />
                                    </Frame>



                                    <Frame Grid.Row="1" Grid.Column="0" CornerRadius="6" Padding="0" BackgroundColor="LightGray" HorizontalOptions="End" WidthRequest="160">
                                        <StackLayout WidthRequest="160" HeightRequest="22" Orientation="Horizontal" Padding="0" Margin="0" HorizontalOptions="End" >

                                            <Button x:Name="DeductQty" Text="-" Clicked="OnClicked_DeductQty" CommandParameter="{Binding .}" Padding="0" Margin="0" BorderColor="YellowGreen" HorizontalOptions="End" HeightRequest="20" WidthRequest="24" IsEnabled="False" TextColor="White" Font="Bold, 20" />
                                            <Entry x:Name="Qty2" WidthRequest="60" HeightRequest="14" Text="0" FontSize="Small" MaxLength="2" Margin="0" HorizontalOptions="Center" InputTransparent="True" TextColor="Black" ></Entry>
                                            <Button x:Name="AddQty" Text="+"  Clicked="OnClicked_AddQty" CommandParameter="{Binding .}" Padding="0" Margin="0" BorderColor="YellowGreen" HorizontalOptions="Start" HeightRequest="20" WidthRequest="24" BackgroundColor="YellowGreen" TextColor="White" Font="Bold, 20" />


                                        </StackLayout>
                                    </Frame>
                                </Grid>
                            </StackLayout>
                        </StackLayout>
                    </ViewCell>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>

        <Frame FlowDirection="MatchParent" CornerRadius="5" Margin="2" Padding="4" BackgroundColor="#d3d3d3" HorizontalOptions="CenterAndExpand" VerticalOptions="CenterAndExpand" WidthRequest="400" >
            <StackLayout Orientation="Vertical" HorizontalOptions="Center" VerticalOptions="Center">
                <StackLayout Orientation="Horizontal" HorizontalOptions="Center" VerticalOptions="Center" >
                    <Label x:Name="lblOrderTotal1" Font="Bold, 14" TextColor="Red" Text="Total: Rs." />
                    <Label x:Name="lblOrderTotal" Font="Bold, 14" TextColor="Red" Text="0.00" />
                </StackLayout>
                <Button x:Name="butPlaceOrder" Margin="0,2,0,0" Clicked="OnClicked_butPlaceOrder" HeightRequest="30" WidthRequest="160" FontSize="14" TextColor="White" BackgroundColor="Orange" Padding="0" CornerRadius="15"  Text="Continue" IsVisible="False" />
            </StackLayout>
        </Frame>
    </StackLayout>

</ContentPage.Content>