Пользовательский серверный элемент Slider на ASP.NET AJAX: ScriptControl шаг за шагом

<ToolboxData("<{0}:Slider runat=server />")> _
Partial Public Class Slider
    Inherits ScriptControl
    Implements IPostBackDataHandler

    Public Event ValueChanged As EventHandler

    Public Property Minimum As Integer
        Get : Return CInt(If(ViewState("Min"), 0)) : End Get
        Set(v As Integer) : ViewState("Min") = v : End Set
    End Property
    Public Property Maximum As Integer
        Get : Return CInt(If(ViewState("Max"), 100)) : End Get
        Set(v As Integer) : ViewState("Max") = v : End Set
    End Property
    Public Property Value As Integer
        Get : Return CInt(If(ViewState("Val"), 0)) : End Get
        Set(v As Integer) : ViewState("Val") = Math.Max(Minimum, Math.Min(Maximum, v)) : End Set
    End Property

    Protected Overrides Sub RenderContents(w As HtmlTextWriter)
        w.Write("<div class=""sl-track""><div class=""sl-thumb"" role=""slider"" tabindex=""0""")
        w.Write(String.Format(" aria-valuemin=""{0}"" aria-valuemax=""{1}"" aria-valuenow=""{2}""", Minimum, Maximum, Value))
        w.Write("></div></div>")
        w.Write(String.Format("<input type=""hidden"" name=""{0}"" value=""{1}"" />", UniqueID, Value))
    End Sub
End Class

Protected Overrides Function GetScriptDescriptors() As IEnumerable(Of ScriptDescriptor)
    Dim d As New ScriptControlDescriptor("MyControls.Slider", ClientID)
    d.AddProperty("minimum", Minimum)
    d.AddProperty("maximum", Maximum)
    d.AddProperty("value", Value)
    Return New ScriptDescriptor() {d}
End Function

Protected Overrides Function GetScriptReferences() As IEnumerable(Of ScriptReference)
    Return New ScriptReference() {
        New ScriptReference("MyControls.Slider.js", Me.GetType().Assembly.FullName)}
End Function

Public Function LoadPostData(key As String, values As NameValueCollection) As Boolean _
        Implements IPostBackDataHandler.LoadPostData
    Dim posted As Integer
    If Not Integer.TryParse(values(key), posted) OrElse posted = Value Then Return False
    Value = posted ' сеттер сам ограничит диапазон
    Return True
End Function

Public Sub RaisePostDataChangedEvent() Implements IPostBackDataHandler.RaisePostDataChangedEvent
    RaiseEvent ValueChanged(Me, EventArgs.Empty)
End Sub

Type.registerNamespace('MyControls');
MyControls.Slider = function (el) {
  MyControls.Slider.initializeBase(this, [el]);
  this._minimum = 0; this._maximum = 100; this._value = 0;
};
MyControls.Slider.prototype = {
  initialize: function () {
    MyControls.Slider.callBaseMethod(this, 'initialize');
    this._thumb = this.get_element().querySelector('.sl-thumb');
    this._hidden = this.get_element().querySelector('input[type=hidden]');
    this._onKey = Function.createDelegate(this, this._keyDown);
    $addHandler(this._thumb, 'keydown', this._onKey);
    this._render();
  },
  dispose: function () {
    if (this._thumb) $removeHandler(this._thumb, 'keydown', this._onKey);
    MyControls.Slider.callBaseMethod(this, 'dispose');
  },
  _keyDown: function (e) {
    var k = e.keyCode, v = this._value;
    if (k === 37 || k === 40) v--; else if (k === 39 || k === 38) v++;
    else if (k === 36) v = this._minimum; else if (k === 35) v = this._maximum;
    else return;
    e.preventDefault(); this.set_value(v);
  },
  _render: function () {
    var pct = (this._value - this._minimum) / (this._maximum - this._minimum) * 100;
    this._thumb.style.left = pct + '%';
    this._thumb.setAttribute('aria-valuenow', this._value);
    this._hidden.value = this._value;
  },
  get_value: function () { return this._value; },
  set_value: function (v) {
    this._value = Math.max(this._minimum, Math.min(this._maximum, v));
    if (this._thumb) this._render();
  },
  set_minimum: function (v) { this._minimum = v; },
  set_maximum: function (v) { this._maximum = v; }
};
MyControls.Slider.registerClass('MyControls.Slider', Sys.UI.Control);