Xamarin MVVM Databinding not refreshing - xamarin

Question

i am struggling with the mvvm data binding. I am not using any framework for the mvvm, I got a very basic base class for my view models. I uploaded my example-app with my problem to GitHub, find the link below.
My problem:
I got a simple app with an tab menu. there are 2 tabs called "TabA" and "TabB". Both views have a simple view model. The view models are referencing a manager class which holds the data. The Manager class has to objects (objects of my datamodel-class which just contains a string and implements INotifyPropertyChanged) in an observablecollection. There is also a Property in the Manager which references the current choosen object (its just one of the 2 objects from the list).
There are 2 actions which can be done by "TabB". The first one works as expected. If you enter some new string into the entry an hit the first button, it updates the string of the current choosen object and updates the label in TabA.
What is not working? With the second Button in my "TabB" class you switch the value of the current choosen object in the Manager. In the debugger I can see that the value is changed, but the Label in "TabA" does not recognize that it has to update the value.
Can you help me?
https://github.com/dercdev/MVVM-Xamarin
With the help of Jason I came to something like this:
In my TabAViewModel I subscribed the event of the Manager:
public TabAViewModel()
{
_mgr = Manager.Instance;
_mgr.PropertyChanged += new PropertyChangedEventHandler(obj_PropertyChanged);
}
Then I raise the event:
private void obj_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
OnPropertyChanged("CurrentData");
}
Which updates the label of the view.
Is that okay or is there a "better" way to do it?

Answer 1

As far as I know, the better way is to use INotifyPropertyChanged. If you want to implement Notify, I think you need to implement INotifyPropertyChanged interface, you can create one class name ViewModelBase that inheriting INotifyPropertyChanged, like this:
public class ViewModelBase:INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public void RaisePropertyChanged(string propertyName)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
}
Then you can call RaisePropertyChanged method to inotify when property changed,
private string _text;
public string Text
{
get
{
return _text;
}
set
{
_text = value;
RaisePropertyChanged("Text");
}
}
ObservableCollection implements INotifyPropertyChanged, allowing the collection to notify the user when the contents of the collection have changed - and specifically, what changed within the collection. For example, if you add an item to the collection, the CollectionChanged event will be raised with properties that tell you the index of the new item as well as including the item in a list.
So ObservableCollection _list don't need to call RaisePropertyChanged method.
https://learn.microsoft.com/en-us/dotnet/api/system.collections.objectmodel.observablecollection-1.system-componentmodel-inotifypropertychanged-propertychanged?view=netframework-4.7.2