Xamarin: set labelText with an instance of ViewModel isnt updating UI - xamarin

Question

I defined a Label in Xaml
<Label Text="{Binding DeviceGuid}"/>
set the BindingContext inside my Page
BindingContext = new BluetoothViewModel();
and wrote the code for the getter and setter in the ViewModel
private string _deviceGuid;
public string DeviceGuid
{
get
{
return _deviceGuid;
}
set
{
if (_deviceGuid != value)
{
_deviceGuid = value;
OnPropertyChanged();
}
}
}
So thats the simple Thing :). The Binding works if I change the value inside the ViewModel.
Now here it comes:
There are some Backgroundtasks (or just other classes) that, in my opinion, should have Access to that property and if they will write it, the UI should update automatically.
I think its bad practice but I dont know how to realise it different.
I´ve already tried to create another instance of the viewmodel like
BluetoothViewModel a = new BluetoothViewModel();
a.DeviceGuid = "test";
Its calling the OnPropertyChanged() but isnt updating the UI ...
Thanks for your help in advance.

Answer 1

When you do this:
BluetoothViewModel a = new BluetoothViewModel();
a.DeviceGuid = "test";
You are creating another instance of the viewmodel that is not the one in your BindingContext.
Do this instead:
public BluetoothViewModel viewmodel;
BindingContext = viewmodel= new BluetoothViewModel();
And then:
viewmodel.DeviceGuid = "test";

Answer 2

The reason it must be happening is that you are not making these changes in the MainThread which is the thread responsible for making changes on the UI.
Do something like below where you change the property data:
Device.BeginInvokeOnMainThread(() => {
DeviceGuid="New string"; });
Update
What you should be doing is using the BindingContext and creating a new instance so your variable 'a' should look something like below
private BluetoothViewModel viewmodel;
BindingContext = viewmodel= new BluetoothViewModel ();
And then do this
viewmodel.DeviceGuid="New string";