GH-373: Permissions Must Be Invoked on MainThread else Throw Exception (#487)

* Throw permission exception if not on main thread when we need to request.

* Add tests for not main thread permissions

DeviceTests/DeviceTests.Shared/Permissions_Tests.cs

@@ -37,5 +37,14 @@ namespace DeviceTests
 
             Assert.Equal(expectedStatus, status);
         }
+
+        [Fact]
+        public async Task Request_NotMainThread()
+        {
+            await Task.Run(async () =>
+            {
+                await Assert.ThrowsAsync<PermissionException>(() => Permissions.RequestAsync(PermissionType.LocationWhenInUse));
+            });
+        }
     }
 }

Xamarin.Essentials/Permissions/Permissions.shared.cs

@@ -12,8 +12,13 @@ namespace Xamarin.Essentials
         internal static Task<PermissionStatus> CheckStatusAsync(PermissionType permission) =>
             PlatformCheckStatusAsync(permission);
 
-        internal static Task<PermissionStatus> RequestAsync(PermissionType permission) =>
-            PlatformRequestAsync(permission);
+        internal static Task<PermissionStatus> RequestAsync(PermissionType permission)
+        {
+            if (!MainThread.IsMainThread)
+                throw new PermissionException("Permission request must be invoked on main thread.");
+
+            return PlatformRequestAsync(permission);
+        }
 
         internal static async Task RequireAsync(PermissionType permission)
         {