Ignore the Binding initialization
<p>The inital problem is coming from a <a href="https://github.com/Emixam23/XamarinByEmixam23/tree/master/Detailed%20Part/Controls/Map/MapPolylineProject" rel="nofollow">personal project about the polyline of the <strong>Xamarin.Forms.Map</strong></a> where the initialization is realized by a binding from the XAML part..</p>

<p>Let me be clear by an example :</p>

<p>I have an object <strong>CustomMap.cs</strong>  which inherit from <code>Xamarin.Forms.Map</code> <em>(This file is in the PCL part -> CustomControl/CustomMap.cs)</em></p>

<pre class="lang-cs prettyprint-override"><code>public class CustomMap : Map, INotifyPropertyChanged
{
    public static readonly BindableProperty PolylineAddressPointsProperty =
        BindableProperty.Create(nameof(PolylineAddressPoints), typeof(List&lt;string&gt;), typeof(CustomMap), null);
    public List&lt;string&gt; PolylineAddressPoints
    {
        get { return (List&lt;string&gt;)GetValue(PolylineAddressPointsProperty); }
        set
        {
            SetValue(PolylineAddressPointsProperty, value);
            this.GeneratePolylineCoordinatesInner();
        }
    }   
    // ...
}
</code></pre>

<p><em>As you can see, I have a bindable property with an assessor and the XAML doesn't seem to use this assessor..</em></p>

<p>So the <strong>MainPge.xaml</strong> part of the page, where the control is called, looks like that:</p>

<pre class="lang-xml prettyprint-override"><code>&lt;?xml version="1.0" encoding="utf-8" ?&gt;
&lt;ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:control="clr-namespace:MapPolylineProject.CustomControl;assembly=MapPolylineProject"
         x:Class="MapPolylineProject.Page.MainPage"&gt;

  &lt;ContentPage.Content&gt;
    &lt;control:CustomMap x:Name="MapTest" PolylineAddressPoints="{Binding AddressPointList}"
                                       VerticalOptions="Fill" HorizontalOptions="Fill"/&gt;

  &lt;/ContentPage.Content&gt;

&lt;/ContentPage&gt;
</code></pre>

<p>The <strong>MainPge.xaml.cs</strong> part:</p>

<pre class="lang-cs prettyprint-override"><code>public partial class MainPage : ContentPage
{
    public List&lt;string&gt; AddressPointList { get; set; }

    public MainPage()
    {
        base.BindingContext = this;

        AddressPointList = new List&lt;string&gt;()
        {
            "72230 Ruaudin, France",
            "72100 Le Mans, France",
            "77500 Chelles, France"
        };

        InitializeComponent();

        //MapTest.PolylineAddressPoints = AddressPointList;
    }
}
</code></pre>

<p>So, everything is fine if I edit the <code>PolylineAddressPoints</code> from the object instance <em>(if the commented part isnt' commented..)</em>, but if I init the value from the XAML (from the <code>InitializeComponent();</code>), it doesn't work, the <code>SetValue</code>, in the <code>Set {}</code>, of the <code>CustomMap.PolylineAddressPoints</code>, isn't called..</p>

<p>I then searched on the web about it and get something about the <strong>Dependency Properties?</strong> or something like that. So I tried some solutions but, from WPF, so some methods, such as <code>DependencyProperty.Register();</code>. So yeah, I can't find the way to solve my problem..</p>

<p>I also though about something, if <code>DependencyProperty.Register();</code> would exists in <strong>Xamarin.Forms</strong>, then it means I would have to do it for each values?  Because, if every value has to be set by a <em>XAML binding logic</em>, it would not work, I would have to register every value, doesn't it?</p>

<p>I'm sorry if I'm not clear, but I'm so lost about this problem.. Please, do not hesitate to ask for more details, thank in advance !</p>

<p><strong>Finaly, the initial problem is that I'm trying to set a value of an object/control, from the XAML. Doing this by a Binding doesn't work, it seems like it ignored.. However, it does work if I do the following:</strong></p>

<pre><code>MapTest.PolylineAddressPoints = AddressPointList;  
</code></pre>