对于源绑定到URL的图片，如何将显示的图片保存到磁盘上？

Question

假设我有一个

Image

，其

Source

来自绑定表达式，绑定到 URL（作为

string

）：

<Image Source="{Binding WebformatUrl}" />

WPF 下载图像并显示它。

有什么方法可以将 WPF 下载的图像保存到磁盘上吗？

Answer 1

一种方法是使用

IValueConverter

进行绑定：

<Window x:Class="wpf_image_source_to_disk.MainWindow"
        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"
        xmlns:local="clr-namespace:wpf_image_source_to_disk"
        mc:Ignorable="d"
        Title="MainWindow" Width="500" Height="300" >
    <Window.DataContext>
        <local:MainPageBindingContext/>
    </Window.DataContext>
    <Window.Resources>
        <local:StringToImageAsyncConverter x:Key="StringToImageAsyncConverter" />
    </Window.Resources>
    <Grid>
        <Image Source="{
            Binding WebFormatUrl,
            Converter={StaticResource StringToImageConverter}        
            }" Margin="20"/>
    </Grid>
</Window>

字符串到图像转换器

下载并返回图像后，保存方法被称为异步，并且（在本示例中）在默认编辑器（例如 MSPaint）中打开保存的文件。

class StringToImageConverter : IValueConverter
{
    public object? Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value is string url && !string.IsNullOrEmpty(url))
        {
            var bitmapImage = new BitmapImage();
            bitmapImage.BeginInit();
            bitmapImage.UriSource = new Uri(url);
            bitmapImage.CacheOption = BitmapCacheOption.OnLoad;
            bitmapImage.EndInit(); bitmapImage.DownloadCompleted += (s, e) =>
            {
                _ = SaveFileAsync(url, bitmapImage);
            };
            return bitmapImage;
        }
        else return new BitmapImage();
    }

    private async Task SaveFileAsync(string url, BitmapImage imageToSave)
    {
        JpegBitmapEncoder encoder = new JpegBitmapEncoder();
        encoder.Frames.Add(BitmapFrame.Create(imageToSave));

        var tmpFileNameForTest = Path.Combine(
            Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),
            Assembly.GetExecutingAssembly().GetName().Name,
            "Images",
            "tmp.jpg");
        Directory.CreateDirectory(Path.GetDirectoryName(tmpFileNameForTest));

        if(File.Exists(tmpFileNameForTest) )
        {
            File.Delete(tmpFileNameForTest);
        }
        using (FileStream fileStream = new FileStream(tmpFileNameForTest, FileMode.Create))
        {
            encoder.Save(fileStream);
        }
        await Task.Delay(TimeSpan.FromSeconds(1));
        {
            try
            {
                Process.Start(new ProcessStartInfo { UseShellExecute = true,  FileName = tmpFileNameForTest, });
            }
            catch
            {
                Debug.Fail("Couldn't start default editor");
            }
        }
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) =>
        throw new NotImplementedException();
}