BitmapImage DownloadCompleted DownloadFailed 为什么经常不会进

[HFtime]

BitmapImage bmi = new BitmapImage(url);
            if (!bmi.IsDownloading)
            {

            }
            else
            {
                Messenger.Default.Send("加载");
                bmi.DownloadFailed += new EventHandler<ExceptionEventArgs>(delegate(object sender, ExceptionEventArgs ex)
                {
                });
                bmi.DownloadCompleted += new EventHandler(delegate(object sender, EventArgs ex)
                {

                });
            }