Dynamic loading of images in WPF

问题

I have a strange issue with WPF, I was loading images from the disk at runtime and adding them to a StackView container. However, the images were not displayed. After some debugging I found the trick, but it really doesn't make any sense. I've made a small demo app to identify the problem:

Create a new WPF project, and paste code as follows:

xaml:

<Window x:Class="wpfBug.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="300" Width="300" Loaded="Window_Loaded">
    <StackPanel Name="sp">
    </StackPanel>
</Window>

xaml.cs, paste below default usings:

namespace wpfBug
{
    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();
        }

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            Image i = new Image();
            BitmapImage src = new BitmapImage();
            src.BeginInit();
            src.UriSource = new Uri("picture.jpg", UriKind.Relative);
            src.EndInit();
            i.Source = src;
            i.Stretch = Stretch.Uniform;
            //int q = src.PixelHeight;        // Image loads here
            sp.Children.Add(i);
        }
    }
}

Copy a image to the bin/Debug folder and call it 'picture.jpg'

This program doesn't display anything, unless the commented line is uncommented.

Can anyone explain what I'm doing wrong, or why this happens? If you remove the image and run the program it generates an exception on the 'int q= ...' line. If that line is commented the program runs without exceptions even if no image is present. Loading an image only if nessesary makes sense, but then the image should be loaded when I add the Image control to the StackPanel.

Any ides ?

Edit: By the way, if you add the image as a resource, the 'int q = ..' line is not needed.

回答1:

It is because the Creation was delayed. If you want the picture to be loaded immediately, you can simply add this code into the init phase.

src.CacheOption = BitmapCacheOption.OnLoad;

like this:

src.BeginInit();
src.UriSource = new Uri("picture.jpg", UriKind.Relative);
src.CacheOption = BitmapCacheOption.OnLoad;
src.EndInit();

回答2:

In code to load resource in the executing assembly where my image 'Freq.png' was in the folder "Icons" and defined as "Resource".

        this.Icon = new BitmapImage(new Uri(@"pack://application:,,,/" 
             + Assembly.GetExecutingAssembly().GetName().Name 
             + ";component/" 
             + "Icons/Freq.png", UriKind.Absolute));

I also made a function if anybody would like it...

/// <summary>
/// Load a resource WPF-BitmapImage (png, bmp, ...) from embedded resource defined as 'Resource' not as 'Embedded resource'.
/// </summary>
/// <param name="pathInApplication">Path without starting slash</param>
/// <param name="assembly">Usually 'Assembly.GetExecutingAssembly()'. If not mentionned, I will use the calling assembly</param>
/// <returns></returns>
public static BitmapImage LoadBitmapFromResource(string pathInApplication, Assembly assembly = null)
{
    if (assembly == null)
    {
        assembly = Assembly.GetCallingAssembly();
    }

    if (pathInApplication[0] == '/')
    {
        pathInApplication = pathInApplication.Substring(1);
    }
    return new BitmapImage(new Uri(@"pack://application:,,,/" + assembly.GetName().Name + ";component/" + pathInApplication, UriKind.Absolute)); 
}

Usage:

        this.Icon = ResourceHelper.LoadBitmapFromResource("Icons/Freq.png");

回答3:

This is strange behavior and although I am unable to say why this is occurring, I can recommend some options.

First, an observation. If you include the image as Content in VS and copy it to the output directory, your code works. If the image is marked as None in VS and you copy it over, it doesn't work.

Solution 1: FileStream

The BitmapImage object accepts a UriSource or StreamSource as a parameter. Let's use StreamSource instead.

        FileStream stream = new FileStream("picture.png", FileMode.Open, FileAccess.Read);
        Image i = new Image();
        BitmapImage src = new BitmapImage();
        src.BeginInit();
        src.StreamSource = stream;
        src.EndInit();
        i.Source = src;
        i.Stretch = Stretch.Uniform;
        panel.Children.Add(i);

The problem: stream stays open. If you close it at the end of this method, the image will not show up. This means that the file stays write-locked on the system.

Solution 2: MemoryStream

This is basically solution 1 but you read the file into a memory stream and pass that memory stream as the argument.

        MemoryStream ms = new MemoryStream();
        FileStream stream = new FileStream("picture.png", FileMode.Open, FileAccess.Read);
        ms.SetLength(stream.Length);
        stream.Read(ms.GetBuffer(), 0, (int)stream.Length);

        ms.Flush();
        stream.Close();

        Image i = new Image();
        BitmapImage src = new BitmapImage();
        src.BeginInit();
        src.StreamSource = ms;
        src.EndInit();
        i.Source = src;
        i.Stretch = Stretch.Uniform;
        panel.Children.Add(i);

Now you are able to modify the file on the system, if that is something you require.

回答4:

You could try attaching handlers to various events of BitmapImage:

DecodeFailed
DownloadFailed
DownloadCompleted

They might tell you a little about what's going on, as far as the image is concerned.

回答5:

Here is the extension method to load an image from URI:

public static BitmapImage GetBitmapImage(
    this Uri imageAbsolutePath,
    BitmapCacheOption bitmapCacheOption = BitmapCacheOption.Default)
{
    BitmapImage image = new BitmapImage();
    image.BeginInit();
    image.CacheOption = bitmapCacheOption;
    image.UriSource = imageAbsolutePath;
    image.EndInit();

    return image;
}

Sample of use:

Uri _imageUri = new Uri(imageAbsolutePath);
ImageXamlElement.Source = _imageUri.GetBitmapImage(BitmapCacheOption.OnLoad);

Simple as that!

来源：https://stackoverflow.com/questions/569561/dynamic-loading-of-images-in-wpf

标签

wpf

image

dynamic