Question

我正在制作一个程序，将图像从URL下载到bitmapimage并显示它。接下来我尝试使用jpegbitmapencoder将bitmapimage保存到硬盘驱动器。文件已成功创建，但实际的jpeg图像为空或1个黑色像素。

public Guid SavePhoto(string istrImagePath)
{
    ImagePath = istrImagePath;

    BitmapImage objImage = new BitmapImage(
        new Uri(istrImagePath, UriKind.RelativeOrAbsolute));
    PictureDisplayed.Source = objImage;
    savedCreationObject = objImage;

    Guid photoID = System.Guid.NewGuid();
    string photolocation = photoID.ToString() + ".jpg";  //file name
    FileStream filestream = new FileStream(photolocation, FileMode.Create);
    JpegBitmapEncoder encoder = new JpegBitmapEncoder();
    encoder.Frames.Add(BitmapFrame.Create(objImage));
    encoder.Save(filestream);

    return photoID;
}

这是保存和显示照片的功能。照片显示正确，但保存后再次显示我得到一个空的jpeg或1个黑色像素。

Answer 1

从Uri创建BitmapImage时，需要时间来下载图像。

如果您检查以下属性，则该值可能为TRUE

objImage.IsDownloading

因此，您将一个监听器附加到DownloadCompleted事件处理程序并将所有处理移动到该EventHandler。

objImage.DownloadCompleted += objImage_DownloadCompleted;

该处理程序的位置如下：

private void objImage_DownloadCompleted(object sender, EventArgs e)
{
  JpegBitmapEncoder encoder = new JpegBitmapEncoder();
  Guid photoID = System.Guid.NewGuid();
  String photolocation = photoID.ToString() + ".jpg";  //file name 

  encoder.Frames.Add(BitmapFrame.Create((BitmapImage)sender));

  using (var filestream = new FileStream(photolocation, FileMode.Create))
    encoder.Save(filestream);
}

您可能还想为DownloadFailed添加另一个EventHandler，以便正常处理任何错误情况。

修改

根据Ben的评论添加了完整的示例类：

public partial class MainWindow : Window { public MainWindow() { InitializeComponent(); SavePhoto("http://www.google.ca/intl/en_com/images/srpr/logo1w.png"); } public void SavePhoto(string istrImagePath) { BitmapImage objImage = new BitmapImage(new Uri(istrImagePath, UriKind.RelativeOrAbsolute)); objImage.DownloadCompleted += objImage_DownloadCompleted; } private void objImage_DownloadCompleted(object sender, EventArgs e) { JpegBitmapEncoder encoder = new JpegBitmapEncoder(); Guid photoID = System.Guid.NewGuid(); String photolocation = photoID.ToString() + ".jpg"; //file name encoder.Frames.Add(BitmapFrame.Create((BitmapImage)sender)); using (var filestream = new FileStream(photolocation, FileMode.Create)) encoder.Save(filestream); } }

Answer 2

扩展Chris Baxter的解决方案，此Converter使用本地版本（如果存在），否则下载并保存文件。

using System;
using System.Globalization;
using System.IO;
using System.Windows.Data;
using System.Windows.Media.Imaging;

namespace MyNamespace
{
    public sealed class UriToCachedImageConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            var url = value as string;
            if (url == null)
                return null;

            var webUri = new Uri(url, UriKind.Absolute);
            var filename = Path.GetFileName(webUri.AbsolutePath);

            var localFilePath = Path.Combine("C:\\MyImagesFolder\\", filename);

            if (File.Exists(localFilePath))
            {
                return BitmapFrame.Create(new Uri(localFilePath, UriKind.Absolute));
            }

            var image = new BitmapImage();
            image.BeginInit();
            image.CacheOption = BitmapCacheOption.OnLoad;
            image.UriSource = webUri;
            image.EndInit();

            SaveImage(image, localFilePath);

            return image;
        }

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

        public void SaveImage(BitmapImage image, string localFilePath)
        {
            image.DownloadCompleted += (sender, args) =>
            {
                var encoder = new PngBitmapEncoder();
                encoder.Frames.Add(BitmapFrame.Create((BitmapImage) sender));
                using (var filestream = new FileStream(localFilePath, FileMode.Create))
                {
                    encoder.Save(filestream);
                }
            };
        }
    }
}

并确保您可以访问xaml中的转换器

<UserControl xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:u="clr-namespace:MyNamespace"          
             d:DesignHeight="500" 
             d:DesignWidth="420">
    <UserControl.Resources>
        <ResourceDictionary>
            <u:UriToCachedImageConverter x:Key="UrlToCachedImageConverter" />
        </ResourceDictionary>
    </UserControl.Resources>            
</UserControl>

在图像上使用转换器

<Image Source="{Binding URL, Mode=OneWay, Converter={StaticResource UrlToCachedImageConverter}, IsAsync=true}"/>

将BitmapImage保存到文件

2 个答案: