How to refresh a StatusBar based on several ViewModels? - windows-phone-8.1

Question

I develop an Universal App, that uses MVVM Light and Navegar. In this app, the main page contains a Pivot, where each PivotItem is bounded to a ViewModel.
This page looks like this:
The XAML looks like this:
<Page
...>
<Page.DataContext>
<Binding Path="MainViewModelInstance" Source="{StaticResource Locator}"/>
</Page.DataContext>
<Page.Resources>
...
</Page.Resources>
<Grid>
<!-- HOME -->
<Pivot x:Uid="homePivot"
x:Name="homePivot"
CommonNavigationTransitionInfo.IsStaggerElement="True"
TitleTemplate="{StaticResource AppPivotTitle}"
Style="{StaticResource CustomizedPivot}">
<Pivot.Background>
<ImageBrush Stretch="Fill"/>
</Pivot.Background>
<PivotItem x:Uid="homePivotitemHome"
Margin="19,14.5,19,0"
CommonNavigationTransitionInfo.IsStaggerElement="True"
Header="Home">
<PivotItem.Background>
<ImageBrush ImageSource="{Binding Infos.background_image, Converter={StaticResource InfosUIImageFondToStringConverter}}"
Stretch="UniformToFill"
Opacity="0.75"/>
</PivotItem.Background>
</PivotItem.ContentTemplate>
</PivotItem>
<!-- NEWS -->
<PivotItem x:Uid="homePivotitemNews"
Margin="19,14.5,0,0"
CommonNavigationTransitionInfo.IsStaggerElement="True" >
<PivotItem.DataContext>
<Binding Path="NewsViewModel" Source="{StaticResource Locator}"/>
</PivotItem.DataContext>
<PivotItem.ContentTemplate>
<DataTemplate>
<Grid>
<ListView ItemsSource="{Binding News}"
IsItemClickEnabled="True"
SelectionMode="Single"
ContinuumNavigationTransitionInfo.ExitElementContainer="True">
...
</ListView>
</Grid>
</DataTemplate>
</PivotItem.ContentTemplate>
</PivotItem>
<!-- CONTACTS -->
<PivotItem x:Uid="homePivotitemContacts"
Margin="19,14.5,0,0"
CommonNavigationTransitionInfo.IsStaggerElement="True"
Header="Contacts">
<PivotItem.DataContext>
<Binding Path="ContactsViewModel" Source="{StaticResource Locator}"/>
</PivotItem.DataContext>
<PivotItem.ContentTemplate>
<DataTemplate>
<Grid>
<ListView ItemsSource="{Binding Contacts}"
IsItemClickEnabled="True"
SelectionMode="Single"
ContinuumNavigationTransitionInfo.ExitElementContainer="True">
...
</ListView>
</Grid>
</DataTemplate>
</PivotItem.ContentTemplate>
</PivotItem>
...
</Pivot>
</Grid>
</Page>
So the page is bounded to a DataContext "MainViewModelInstance", that is used for the first PivotItem. The others PivotItems have their own DataContext: "NewsViemodel", "ContactsViewModel", ...
On each ViewModel, a constrctor calls a Load() method that updates the datas of this HomePage.
I would like to display a StatusBarProgressIndicator when one of the ViewModels is loading datas, and stopping it when all datas has been loaded.
Currently I show the StatusBarProgressIndicatorby calling a "ShowStatusBarProgress()" method in the "MainViewModel":
private async void LoadData()
{
ShowStatusbarProgress();
// Load data
...
HideStatusbarProgress();
}
private async void ShowStatusbarProgress()
{
progressbar.Text = "Please Wait...";
await progressbar.ShowAsync();
}
private async void HideStatusbarProgress()
{
await progressbar.HideAsync();
}
But I don't see how to do for taking into account all the "Load()" methods of the ViewModels. I heard about Messenger services, but I don't see how it can work in my case.