Trouble with XAML and data binding

Hello, I'm making a very simple app with Xamarin forms where I've products with a quantity for each one. Everything is saved in a database:

[PrimaryKey, AutoIncrement]
public int ID { get; set; }
public string Name { get; set; }
public int Quantity { get; set; }

From a ListView I select my product, and I want to get its informations, Name and Quantity. No problem so far. Then I have a Stepper to increment this quantity, but pressing on it, nothing change from my Quantity.
My XAML:

<?xml version="1.0" encoding="UTF-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="ProductXAML" Title="Product">

    <StackLayout VerticalOptions ="FillAndExpand">

        <StackLayout Orientation="Horizontal">
            <Label Text="Name :" />
            <Label Text="{Binding Name}" />
        </StackLayout>

        <StackLayout Orientation="Horizontal">
            <Label Text="Quantity :" />
            <Label Text="{Binding Quantity}" />
            <Stepper Minimum="0" Increment="1" ValueChanged="OnQuantityChanged" Value="{Binding Quantity}" />
        </StackLayout>

    </StackLayout>

</ContentPage>

And now the C#

public partial class ProductXAML : ContentPage, INotifyPropertyChanged {

    public event PropertyChangedEventHandler PropertyChanged;

    StockItem _stockItem;

    public ProductXAML (StockItem stockItem)
    {
        _stockItem = stockItem;
        BindingContext = _stockItem;

        InitializeComponent ();
    }

    void OnQuantityChanged(object sender, ValueChangedEventArgs e) {

        _stockItem.Quantity = (int)e.NewValue;

        Debug.WriteLine (e.NewValue);

        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs("Quantity"));
    }
}

Also I've this warning, not sure how to resolve it: `ProductXAML.PropertyChanged hides inherited member Xamarin.Forms.BindableObject.PropertyChanged. Use the new keyword if hiding was intended (CS0108)

Thanks for your help!