Question

我正在从WPF视图设置对ViewModel对象的绑定，但绑定似乎没有触发。我在MVVM中没有做太多工作，所以我想我要问看是否有一个原因导致在加载页面时没有触发对图像源属性的绑定。

这是页面的XAML：

<Page x:Class="DallasPrintManager.PrintPage"
  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:viewmodel="clr-namespace:DallasPrintManager.ViewModel"
  mc:Ignorable="d" 
  d:DesignHeight="300" d:DesignWidth="900"
  DataContext="{Binding Main, Source={StaticResource PrintPage}}">
<Grid >
    <Grid.ColumnDefinitions>
        <ColumnDefinition  />
        <ColumnDefinition Width="200" />
    </Grid.ColumnDefinitions>
    <ScrollViewer HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto">
        <Image Source="{Binding ImageDisplay}" />
    </ScrollViewer>
</Grid>
</Page>

这是ViewModel：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ComponentModel;
using System.Windows.Media.Imaging;
using System.Drawing;
using System.Windows.Input;
using DallasPrintManager.Commands;
using System.Net;
using System.IO;
using System.Windows;

public class PrintPageViewModel : INotifyPropertyChanged
{
    private BitmapImage _imageDisplay;

    public PrintPageViewModel()
    {
        ImageDisplay = getImage();
    }

    private BitmapImage getImage()
    {
        try
        {
            WebClient wc = new WebClient();
            byte[] imageData = wc.DownloadData("http://localhost/TestImage.tif");
            MemoryStream ms = new MemoryStream();
            ms.Write(imageData, 0, imageData.Length);
            System.Windows.Media.Imaging.BitmapImage wpfImg = new System.Windows.Media.Imaging.BitmapImage();
            wpfImg.BeginInit();
            wpfImg.StreamSource = ms;
            wpfImg.EndInit();
            return wpfImg;
            //return (Bitmap)Bitmap.FromStream(ms);
        }
        catch (WebException e)
        {
            MessageBox.Show("Error fetching document:\n\n" + e.Message);
            return null;
        }
        catch (Exception e)
        {
            if (e.Source == "System.Drawing")
                MessageBox.Show("Error reading document.");
            Console.Out.Write(e);
            return null;
        }
    }

    public BitmapImage ImageDisplay
    {
        get
        {
            return _imageDisplay;
        }
        set
        {
            if (value != _imageDisplay)
            {
                _imageDisplay = value;
                OnPropertyChanged("ImageDisplay");
            }
        }

    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this,
                new PropertyChangedEventArgs(propertyName));
        }
    }

}

viewmodel在app.xaml中实例化，并由打印页面绑定。

Answer 1

查看“调试输出”窗口。你看到任何绑定错误吗？他们几乎告诉你你需要什么。

Answer 2

如果未显示调试输出窗口，则可以使用 CTRL + Alt + O 启用它。

有关详细信息，请在XAML中添加xmlns:diag="clr-namespace:System.Diagnostics;assembly=WindowsBase"，并在绑定中添加diag:PresentationTraceSources.TraceLevel=High：

<Page x:Class="DallasPrintManager.PrintPage"
  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:viewmodel="clr-namespace:DallasPrintManager.ViewModel"
  xmlns:diag="clr-namespace:System.Diagnostics;assembly=WindowsBase"
  [...]
  DataContext="{Binding Main, Source={StaticResource PrintPage}}">

和

<Image Source="{Binding ImageDisplay, diag:PresentationTraceSources.TraceLevel=High}" />

如果绑定使用INotifyPropertyChanged，您还应确保绑定的属性实现UpdateSourceTrigger=PropertyChanged。否则他们不会更新。

您甚至可以创建一个dummyConverter：

public class DebugDummyConverter : IValueConverter
    {
            public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
            {
                    Debugger.Break();
                    return value;
            }

            public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
            {
                    Debugger.Break();
                    return value;
            }
    }

并将其添加到绑定Converter={StaticResource DebugDummyConverter}中以获取有关绑定值的信息，但这仅在绑定正确设置时才有效，否则您将无法获得任何信息。

有关详细信息，请参阅http://www.wpf-tutorial.com/data-binding/debugging/。

为什么这个绑定在MVVM中不起作用？

2 个答案: