用户控件不将更改传播到绑定源

我正在为Metro UI开发ImagePicker用户控件。它的原理很简单:它显示图像，当点击图像时，打开一个文件对话框，允许更改当前图像。要实现这一点，用户控件只需公开一个ImageSource属性，包装后的图像将绑定到该属性。

<local:ImagePicker Source="{Binding PictureUri, Mode=TwoWay}"/>

在启动时，绑定工作正常，并显示来自视图模型提供的PictureUri属性的图片。问题是，当我点击图像并选择一个新的图像时，显示新的图像，但是绑定值没有在我的视图模型中更新，尽管有双向绑定模式。我相信这个问题来自我的用户控制代码，但我不明白为什么值不传播到视图模型时，它实际上是传播到包装图像…

这里是XAML部分

<UserControl x:Name="ImagePickerUserControl"
    x:Class="ImageUserControl.ImagePicker"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Grid x:Name="ImagePickerRootGrid" Background="Gray">
        <Image Source="{Binding Source, ElementName=ImagePickerUserControl}"/>
    </Grid>
</UserControl>

代码部分，很抱歉这么长，但我相信这里的一切都很重要。

public sealed partial class ImagePicker : UserControl
{
    public ImagePicker()
    {
        this.InitializeComponent();
        // Hookup event to handle when the control is tapped
        this.Tapped += ImagePicker_Tapped;
    }
    public static readonly DependencyProperty SourceProperty =
        DependencyProperty.Register("Source", typeof(ImageSource), typeof(ImagePicker),
        new PropertyMetadata(null, new PropertyChangedCallback(ImagePicker.OnSourceChanged)));
    public ImageSource Source
    {
        get
        {
            return (ImageSource)this.GetValue(ImagePicker.SourceProperty);
        }
        set
        {
            this.SetValue(ImagePicker.SourceProperty, value);
        }
    }
    private static void OnSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        // Update Visual State
    }
    private async void ImagePicker_Tapped(object sender, TappedRoutedEventArgs e)
    {
        // Pick up a new picture
        FileOpenPicker filePicker = new FileOpenPicker();
        filePicker.FileTypeFilter.Add(".jpg");
        filePicker.FileTypeFilter.Add(".jpeg");
        filePicker.FileTypeFilter.Add(".png");
        var pngFile = await filePicker.PickSingleFileAsync();
        // If the user picked up a file
        if (pngFile != null)
        {
            BitmapImage bitmap = new BitmapImage();
            await bitmap.SetSourceAsync(await pngFile.OpenReadAsync());
            // Update the source image
            this.Source = bitmap;
        }
    }
}

我相信这个问题只是我的一个错误，但我不能理解这里发生了什么。如果你想试着运行项目并更好地查看代码，我上传并分享到SkyDrive上:ImageUserControl

感谢您的耐心阅读这么长的帖子。

using System.ComponentModel;
using Windows.UI.Xaml.Media;
namespace ImageUserControl
{
    public class PictureViewModel : INotifyPropertyChanged
    {
        private ImageSource pictureUri;
        public ImageSource PictureUri 
        { 
            get
            {
                return this.pictureUri;
            }
            set 
            {
                if (this.pictureUri != value)
                {
                    this.pictureUri = value;
                    this.RaisePropertyChanged("PictureUri");
                }
            }
        }
        public event PropertyChangedEventHandler PropertyChanged;
        public void RaisePropertyChanged(string propertyName)
        {
            var handler = this.PropertyChanged;
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }
}

using System;
using Windows.UI.Xaml.Media.Imaging;
namespace ImageUserControl
{
    public class ViewModelLocator
    {
        static ViewModelLocator()
        {}
        public PictureViewModel Main
        {
            get
            {
                return new PictureViewModel
                {                    
                    PictureUri = new BitmapImage(new Uri("ms-resource:/Files/Assets/eels.jpg"))
                };
            }
        }
    }
}