WPF计时器和UI更新问题

我的应用程序出现问题，它在更改自定义属性后没有正确更新UI，该属性使用DisplayMemberBinding="{Binding property}"绑定到GridView列。

XAML:

<ListView x:Name="downloadList" HorizontalAlignment="Left" Height="293" Margin="0,126,0,0" VerticalAlignment="Top" Width="810" Grid.IsSharedSizeScope="True" MouseDoubleClick="DownloadList_MouseDoubleClick">
    <ListView.View>
        <GridView x:Name="DownloadGridView">
            <GridViewColumn x:Name="c_filename" Header="File name" Width="{Binding Source={x:Static p:Settings.Default}, Path=downloadList_fileName_Width, Mode=TwoWay}" DisplayMemberBinding="{Binding fileName}" />
            <GridViewColumn x:Name="c_size" Header="Size" Width="{Binding Source={x:Static p:Settings.Default}, Path=downloadList_size_Width, Mode=TwoWay}" DisplayMemberBinding="{Binding formattedFileSize}" />
            <GridViewColumn x:Name="c_downloaded" Header="Downloaded" Width="{Binding Source={x:Static p:Settings.Default}, Path=downloadList_downloaded_Width, Mode=TwoWay}" DisplayMemberBinding="{Binding sizeProgress}" />
            <GridViewColumn x:Name="c_status" Header="Status" Width="{Binding Source={x:Static p:Settings.Default}, Path=downloadList_status_Width, Mode=TwoWay}" DisplayMemberBinding="{Binding Status}"/>
        </GridView>
    </ListView.View>
</ListView>

这是我的自定义类属性：

using System;
using System.Runtime.CompilerServices;
using System.Text;
using System.ComponentModel;
namespace DownloadManager
{
public class DownloadItem : INotifyPropertyChanged
{
    private string _filepath;
    public string filePath
    {
        get { return _filepath; }
        set
        {
            _filepath = value;
            RaisePropertyChanged();
        }
    }
    private int _sizeprogress;
    public int sizeProgress
    {
        get { return _sizeprogress; }
        set
        {
            _sizeprogress = value;
            RaisePropertyChanged();
        }
    }
// and so on...
    public event PropertyChangedEventHandler PropertyChanged;
    public void RaisePropertyChanged(
        [CallerMemberName] string caller = "")
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(caller));
        }
    }
}
}

计时器：编辑以显示我尝试做什么的真实示例

System.Windows.Threading.DispatcherTimer updateTimer = new System.Windows.Threading.DispatcherTimer();
updateTimer.Tick += new EventHandler(updateTimer_Tick);
updateTimer.Interval = new TimeSpan(0, 0, 1);

private void updateTimer_Tick(object sender, EventArgs e)
{
    foreach (DownloadItem item in downloadList.Items)
    {
        long BytesReceived = item.filePath.Length;
        item.sizeProgress = BytesReceived;
    }
}

item.filePath包含正在下载的文件的路径，使用FileStream进行写入

我的目标是每秒读取文件大小并显示它

问题：UI，在本例中是绑定到sizeProgress的列，只更新了一次，只在第一次勾选时更新，然后什么都不更新。应用程序仍然运行，没有任何异常。。

我真的不知道问题出在哪里。

如果您需要更多信息/代码，请告诉我。谢谢。

long BytesReceived = item.filePath.Length;