Question

我正在尝试学习ObservableCollection<T>和ListBox的VirtualizingStackPanel。

MyTestData类中有一个图像，绑定到Image内的ListBox.ItemTemplate（缩略图属性）。问题是，当我滚动时，Thumbnail属性被调用两次。这是一个问题还是WPF如何运作？

窗口：

<Window x:ClassModifier="internal" x:Class="Virtual.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"
    mc:Ignorable="d"
    Title="MainWindow" SizeToContent="Height" Width="525" WindowStartupLocation="CenterScreen">
<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="*"></ColumnDefinition>
        <ColumnDefinition Width="*"></ColumnDefinition>
    </Grid.ColumnDefinitions>

    <StackPanel Orientation="Vertical">
        <Button x:Name="btnAdd1" Content="Add 1" Click="btnAdd_Click"></Button>
        <Button x:Name="btnAdd10" Content="Add 10" Click="btnAdd_Click"></Button>
        <Button x:Name="btnAdd100" Content="Add 100" Click="btnAdd_Click"></Button>
        <Button x:Name="btnAdd1000" Content="Add 1000" Click="btnAdd_Click"></Button>
        <Button x:Name="btnAdd1000000" Content="Add 1.000.000" Click="btnAdd_Click"></Button>
        <TextBox x:Name="tbDebug" Margin="0,10,0,25" Height="250" AcceptsReturn="True">

        </TextBox>

    </StackPanel>
    <Button x:Name="btnAdd" Content="Add" VerticalAlignment="Bottom" Click="btnAdd_Click"></Button>

    <ListBox x:Name="lbList2" Margin="10,0,0,25" Height="350"
       Grid.Column="1"
       ScrollViewer.CanContentScroll="True"
       ScrollViewer.IsDeferredScrollingEnabled="False"
       VirtualizingStackPanel.IsVirtualizing="True"
       VirtualizingStackPanel.VirtualizationMode="Recycling">
        <ListBox.ItemsPanel>
            <ItemsPanelTemplate>
                <VirtualizingStackPanel></VirtualizingStackPanel>
            </ItemsPanelTemplate>
        </ListBox.ItemsPanel>
        <ListBox.ItemTemplate>
            <DataTemplate>
                <StackPanel Orientation="Horizontal">
                    <Border Margin="3,0,0,0" BorderBrush="Black" BorderThickness="1">
                        <Image VerticalAlignment="Top" HorizontalAlignment="Left" MaxWidth="30" MaxHeight="30" SnapsToDevicePixels="True" Source="{Binding Thumbnail, UpdateSourceTrigger=PropertyChanged}" />
                    </Border>
                    <Label Content="{Binding Name}"></Label>
                </StackPanel>
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>

    <Button x:Name="btnAdd2" Margin="10,0,0,0" Grid.Column="1" Content="Add" VerticalAlignment="Bottom" Click="btnAdd_Click"></Button>
</Grid>
</Window>

代码：

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace Virtual
{
    internal class MyTestData
    {
        public string Name { get; set; }

        private BitmapSource _thumbnail = null;
        public BitmapSource Thumbnail
        {
            get
            {
                if (_thumbnail == null)
                    _thumbnail = new BitmapImage(new Uri("c:\\300.jpg"));

                Console.Text += "\n" + Name + " --> THUMBNAIL";
                Console.ScrollToEnd();

                return _thumbnail;
            }
        }

        public MyTestData(string oName)
        {
            Name = oName;
        }

        internal TextBox Console;
    }

    internal partial class MainWindow : Window
    {
        public List<string> OItems1 { get; set; }
        public ObservableCollection<MyTestData> OItems2 { get; set; }

        public MainWindow()
        {
            InitializeComponent();

            OItems1 = new List<string>();
            OItems2 = new ObservableCollection<MyTestData>();

            lbList2.ItemsSource = OItems2;
        }

        private void OAdd(int c)
        {
            for (int i = 0; i < c; i++)
            {
                OItems2.Add(new MyTestData("oDATA = " + (OItems2.Count + 1).ToString())
                {
                    Console = tbDebug
                });
            }
        }

        private void btnAdd_Click(object sender, RoutedEventArgs e)
        {
            if (sender == btnAdd1)
                OAdd(1);
            else if (sender == btnAdd10)
                OAdd(10);
            else if (sender == btnAdd100)
                OAdd(100);
            else if (sender == btnAdd1000)
                OAdd(1000);
            else if (sender == btnAdd1000000)
                OAdd(1000000);
        }
    }
}

Answer 1

这是一个问题还是WPF如何运作？

后者。请参阅Microsoft在Connect上的以下答案：

框架（或任何其他人）有权以任何理由随意调用属性获取者。不能保证它只会被调用一次。

意外行为：绑定引擎获取ImageSource两次： https://connect.microsoft.com/VisualStudio/feedback/details/770169/unexpected-behaviour-binding-engine-getting-imagesource-twice

所以这种行为是预期的。

WPF ListBox，两次调用绑定属性

1 个答案: