在通过XAML设置依赖属性之前初始化它

Question

我有一个只读的依赖属性名为＆＃34; Periods＆＃34;。我已使用ContentPropertyAttribute将其设置为content属性。但是，由于Periods是集合类型依赖项属性，因此必须在构造函数中初始化它以确保不形成单例。我试过这个，但是在运行时，尽管在XAML中设置，但属性似乎没有值。我推断我必须在XAML设置之后初始化属性，然后重写它。我在我的假设中是对的，如果是这样，我可以在XAML文件中设置之前初始化该属性。

编辑1： 当初始化值不为空或在控件加载后手动设置值时，控件按预期工作，这使我认为我的假设是正确的。任何帮助将不胜感激。

以下是我的代码的相关部分：

<ContentProperty("Periods")>
Public Class Day
    Inherits System.Windows.Controls.Control



#Region "Constructers"
    Shared Sub New()
        'This OverrideMetadata call tells the system that this element wants to provide a style that is different than its base class.
        'This style is defined in themes\generic.xaml
        DefaultStyleKeyProperty.OverrideMetadata(GetType(Day), New FrameworkPropertyMetadata(GetType(Day)))
    End Sub
    Public Sub New()
        MyBase.New()
        Me.SetValue(PeriodsPropertyKey, New ObservableCollection(Of Period))

    End Sub
    Protected Overrides Sub OnInitialized(e As EventArgs)
        MyBase.OnInitialized(e)

    End Sub
#End Region

#Region "Properties"
    Public ReadOnly Property Periods() As ObservableCollection(Of Period)
        Get
            Return Me.GetValue(PeriodsProperty)
        End Get
    End Property

    Private Shared ReadOnly PeriodsPropertyKey As DependencyPropertyKey = DependencyProperty.RegisterReadOnly("Periods", GetType(ObservableCollection(Of Period)), GetType(Day), New FrameworkPropertyMetadata(Nothing))
    Public Shared ReadOnly PeriodsProperty As DependencyProperty = PeriodsPropertyKey.DependencyProperty
#End Region

这是XAML：

    <local:Timetable Margin="50,25,21,68" UseLayoutRounding="True" PixelToMinuteRatio="2" StartTime="9:00"
                             x:Name="Timetable1">
                <local:Day DayName="Sunday">
                    <local:Period Background="#72000000" VerticalAlignment="Top" Day="1" StartTime="9:00"
                              EndTime="10:20" Margin="0,0,1,0" TimetableStartTime="{Binding ElementName=Timetable1, Path=StartTime}"
                              TimetableEndTime="{Binding ElementName=Timetable1, Path=EndTime}" />



                </local:Day>
                <local:Day DayName="Monday">
                    <local:Period Background="#72000000" VerticalAlignment="Top" Day="1" StartTime="9:00"
                              EndTime="10:20" Margin="0,0,1,0" TimetableStartTime="{Binding ElementName=Timetable1, Path=StartTime}"
                              TimetableEndTime="{Binding ElementName=Timetable1, Path=EndTime}" />

                    <local:Period Background="#72000000" VerticalAlignment="Top" Day="1" StartTime="10:20"
                              EndTime="11:00" Margin="0,0,1,0" TimetableStartTime="{Binding ElementName=Timetable1, Path=StartTime}"
                              TimetableEndTime="{Binding ElementName=Timetable1, Path=EndTime}" />


                </local:Day>
                <local:Day DayName="Tuesday">
                    <local:Period Background="#72000000" VerticalAlignment="Top" Day="1" StartTime="9:00"
                              EndTime="10:20" Margin="0,0,1,0" TimetableStartTime="{Binding ElementName=Timetable1, Path=StartTime}"
                              TimetableEndTime="{Binding ElementName=Timetable1, Path=EndTime}" />

                    <local:Period Background="#72000000" VerticalAlignment="Top" Day="1" StartTime="10:20"
                              EndTime="11:00" Margin="0,0,1,0" TimetableStartTime="{Binding ElementName=Timetable1, Path=StartTime}"
                              TimetableEndTime="{Binding ElementName=Timetable1, Path=EndTime}" />

                    <local:Period Background="#72000000" VerticalAlignment="Top" Day="1" StartTime="11:15"
                              EndTime="11:55" Margin="0,0,1,0" TimetableStartTime="{Binding ElementName=Timetable1, Path=StartTime}"
                              TimetableEndTime="{Binding ElementName=Timetable1, Path=EndTime}" />


                </local:Day>

</local:Timetable>