Add fixed MapLayer once - windows-phone-8

Question

In my application I have a map for which the first thing I do (on launching) is to add a custom MapLayer (which I populate with many MapOverlays/pushpins).
When I browse to another page of the app and then return to the map page, everything (ie the MapLayer that was drawn on my map) is gone.
It takes time to add it all over again each time the user navigates to the map page so I would like it to be fixed, drawn/added just once.
Any suggestions?
edit, added the code [I removed some details, the structure remains the same]:
private async void drawStations()
{
SQLiteAsyncConnection conn = new SQLiteAsyncConnection("stasy.sqlite");
List<line1_stations> lines = await conn.QueryAsync<line1_stations>("select *...");
Microsoft.Phone.Maps.Controls.MapLayer layer = new Microsoft.Phone.Maps.Controls.MapLayer();
Pushpin p;
foreach (line1_stations a in lines)
{
double sLat = Convert.ToDouble(a.lat);
double sLon = Convert.ToDouble(a.lon);
p = new Pushpin();
p.Location = new GeoCoordinate(sLat, sLon);
p.Tap += img_Tap;
p.Content = "...";
p.Foreground = new SolidColorBrush(Colors.Transparent);
p.Width = 30;
p.Height = 30;
MapOverlay overlay1 = new MapOverlay();
overlay1.Content = p;
overlay1.GeoCoordinate = new GeoCoordinate(sLat, sLon);
overlay1.PositionOrigin = new Point(0.0, 1.0);
layer.Add(overlay1);
}
myMap.Layers.Add(layer);
}

Answer 1

You can try to create your MapLayer as a public property in App.xaml.cs and set it only when your app is running first time.
Then create an instance of app inside page with your map and add your MapLayer to your Map layers inside OnNavigatedTo event.

Answer 2

Define a flag in app level. here is the sample
// In App.xaml.cs
public static bool IsLaunched = false;
private void Application_Launching(object sender, LaunchingEventArgs e)
{
IsLaunched =true;
}
//In your map screen
protected override void OnNavigatedTo(NavigationEventArgs e)
{
if(App.IsLaunched)
{
drawStations();
//Set App flage to false
App.IsLaunched = false;
}
}
private async void drawStations()
{
SQLiteAsyncConnection conn = new SQLiteAsyncConnection("stasy.sqlite");
List<line1_stations> lines = await conn.QueryAsync<line1_stations>("select *...");
Microsoft.Phone.Maps.Controls.MapLayer layer = new Microsoft.Phone.Maps.Controls.MapLayer();
Pushpin p;
foreach (line1_stations a in lines)
{
double sLat = Convert.ToDouble(a.lat);
double sLon = Convert.ToDouble(a.lon);
p = new Pushpin();
p.Location = new GeoCoordinate(sLat, sLon);
p.Tap += img_Tap;
p.Content = "...";
p.Foreground = new SolidColorBrush(Colors.Transparent);
p.Width = 30;
p.Height = 30;
MapOverlay overlay1 = new MapOverlay();
overlay1.Content = p;
overlay1.GeoCoordinate = new GeoCoordinate(sLat, sLon);
overlay1.PositionOrigin = new Point(0.0, 1.0);
layer.Add(overlay1);
}
myMap.Layers.Add(layer);
}