Windows Phone 在读取网络图片之前先显示默认图片

1.新建一个控件WindowsPhoneControl1

WindowsPhoneControl1.xaml

<UserControl x:Class="DefaultImage.WindowsPhoneControl1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    FontFamily="{StaticResource PhoneFontFamilyNormal}"
    FontSize="{StaticResource PhoneFontSizeNormal}"
    Foreground="{StaticResource PhoneForegroundBrush}"
    d:DesignHeight="" d:DesignWidth="">

    <Grid x:Name="LayoutRoot">
        <Image Height="" HorizontalAlignment="Left" Margin="10,10,0,0" Name="image1" Stretch="Fill" VerticalAlignment="Top" Width="" Source="/DefaultImage;component/hotelphoto_alterimg.png" />
    </Grid>
</UserControl>

WindowsPhoneControl1.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using System.Windows.Threading;
using System.Windows.Media.Imaging;

namespace DefaultImage
{
    public partial class WindowsPhoneControl1 : UserControl
    {
        public WindowsPhoneControl1()
        {
            InitializeComponent();
            DispatcherTimer timer = new DispatcherTimer();
            timer.Tick += new EventHandler(timer_Tick);
            timer.Interval = new TimeSpan(, , , , );
            timer.Start();
        }

        private string myImageSource;
        public string MyImageSource
        {
            get
            {
                return myImageSource;
            }
            set
            {
                myImageSource = value;
                WebClient wc = new WebClient();
                wc.OpenReadCompleted += new OpenReadCompletedEventHandler(wc_OpenReadCompleted);
                wc.OpenReadAsync(new Uri(myImageSource));
            }
        }

        void wc_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
        {
            BitmapImage bitmapImage = new BitmapImage();
            bitmapImage.SetSource(e.Result);
            image1.Source = bitmapImage;
        }

        void timer_Tick(object sender, EventArgs e)
        {
            MyImageSource = this.DataContext.ToString();
        }
    }
}

MainPage.xaml

<phone:PhoneApplicationPage
    x:Class="DefaultImage.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:phone="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone"
    xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d" d:DesignWidth="" d:DesignHeight=""
    FontFamily="{StaticResource PhoneFontFamilyNormal}"
    FontSize="{StaticResource PhoneFontSizeNormal}"
    Foreground="{StaticResource PhoneForegroundBrush}"
    SupportedOrientations="Portrait" Orientation="Portrait"
    shell:SystemTray.IsVisible="True" xmlns:my="clr-namespace:DefaultImage">

    <!--LayoutRoot is the root grid where all page content is placed-->
    <Grid x:Name="LayoutRoot" Background="Transparent">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>

        <!--TitlePanel contains the name of the application and page title-->
        <StackPanel x:Name="TitlePanel" Grid.Row="" Margin="12,17,0,28">
            <TextBlock x:Name="ApplicationTitle" Text="MY APPLICATION" Style="{StaticResource PhoneTextNormalStyle}"/>
            <TextBlock x:Name="PageTitle" Text="page name" Margin="9,-7,0,0" Style="{StaticResource PhoneTextTitle1Style}"/>
        </StackPanel>

        <!--ContentPanel - place additional content here-->
        <Grid x:Name="ContentPanel" Grid.Row="" Margin="12,0,12,0">
            <ListBox Height="" HorizontalAlignment="Left" ItemsSource="{Binding}" Margin="10,10,0,0" Name="listBox1" VerticalAlignment="Top" Width="">
                <ListBox.ItemTemplate>
                    <DataTemplate>
                        <my:WindowsPhoneControl1 DataContext="{Binding MySource}"/>
                    </DataTemplate>
                </ListBox.ItemTemplate>
            </ListBox>
        </Grid>
    </Grid>
</phone:PhoneApplicationPage> 

MainPage.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using Microsoft.Phone.Controls;

namespace DefaultImage
{
    public partial class MainPage : PhoneApplicationPage
    {
        // Constructor
        public MainPage()
        {
            InitializeComponent();
            var list = new List<MyImage> {
                new MyImage(){ MySource= "http://i.microsoft.com/global/ImageStore/PublishingImages/Asset/thumbnails/icon_monthsecurityupdates_40x40.png"},
                 //new MyImage(){ MySource= "http://i.microsoft.com/global/ImageStore/PublishingImages/Asset/thumbnails/icon_mssecurityessentials_40x40.png"}
            };
            listBox1.DataContext = list;
        }
    }
    public class MyImage
    {
        public string MySource { get; set; }
    }

}