WPF 异步加载高清大图

不管什么东西，但凡太大了，总是让人又爱又恨啊！（很有道理的样子，大家鼓掌└(￣ ￣└)(┘￣ ￣)┘）

猿：老板，现在这社会啊，真是浮躁啊，之前还是什么1080P，然后就到了2K，现在又到了4K……他们是想把毛孔都看清楚？

老板：能不能做？

猿：已经做好了啊，但是……反正这次我们能说会道的X经理也没能忽……说服客户。

老板：是什么问题嘛，说出来我帮你想想办法。（老板真是好人啊，语重心长）

猿(ノへ￣、)：客户不是升级了4K摄像机吗，哎哟，那玩意儿，老长了，就跟火箭筒似的，扛起来那感觉(ง •̀_•́)ง，真是拉风……

老板：咳咳……

猿：嗯，4K，拍出来的照片分辨率4096x2160，一张图片就有30多M，然后……我们的程序挂了，内存崩了。╮(￣▽￣")╭

老板：嗯，这个问题嘛……

猿：昨晚我加班到凌晨三点，各种调试，各种查资料，终于搞定了，今天早上让小X过去更换了，然后……

老板：辛苦了，办公室里有吃的，晚上加班尽管吃！

猿：(￣▽￣")……

猿：然后内存是没爆了，但是大图出来的使用有2~3秒的空白……也不能操作……X经理正在给我们争取时间……

老板：这样啊，我给你一个我朋友的电话，他也是搞软件的，很牛Ｘ的……

嗯，各位童鞋，看到这里就应该明白，对话已经结束。这就开始我们的代码……

ＸＡＭＬ我们就简单点：用ListView做个列表，一个用于显示大图的Image，一个遮罩层Border 

 <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="420"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>
        <ListView  x:Name="lvImages" ScrollViewer.CanContentScroll="False" SelectionChanged="lvImages_SelectionChanged">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <Image Source="{Binding Converter={StaticResource ImageSourceConverter}, Mode=OneTime,ConverterParameter=400}" Width="400" />
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
        <ScrollViewer  Grid.Column="1" >
            <ScrollViewer.Background>
                <ImageBrush ImageSource="{Binding SelectedItem, ElementName=lvImages,Converter={StaticResource ImageSourceConverter}}" Stretch="Uniform"/>
            </ScrollViewer.Background>
            <Image Name="bigImage"   MouseWheel="bigImage_MouseWheel" RenderTransformOrigin="0.5,0.5">
                <Image.RenderTransform>
                    <ScaleTransform x:Name="scale"/>
                </Image.RenderTransform>
            </Image>
        </ScrollViewer>
        <Border Name="conver" Background="#60000000" Visibility="Hidden" Grid.ColumnSpan="2">
            <Path Data="M30.330999,48.228C31.299783,48.422099 32.299767,48.531501 33.330949,48.531501 34.360932,48.531501 35.363415,48.422099 36.335999,48.228L36.335999,66.659003 30.330999,66.659003z M43.268281,44.750998L52.394999,60.832034 47.175917,63.796996 38.038999,47.713359C39.994767,47.070048,41.753928,46.067427,43.268281,44.750998z M23.209264,44.598998C24.700174,45.940215,26.447487,46.971526,28.389,47.638133L19.002135,63.568 13.824999,60.518466z M47.700966,38.078002L63.757998,47.260428 60.782928,52.473998 44.726998,43.300773C46.044784,41.78905,47.055174,40.028529,47.700966,38.078002z M18.923298,37.936999C19.546999,39.89281,20.5418,41.665007,21.844,43.188594L5.7032509,52.206 2.7709999,46.960906z M18.4298,30.342997C18.240998,31.305102 18.131697,32.306304 18.131697,33.328299 18.131697,34.360797 18.237098,35.373698 18.435,36.3502L0.0077514648,36.370997 0,30.365096z M66.659098,30.285998L66.666998,36.294497 48.231798,36.313998C48.4272,35.347804 48.531301,34.352805 48.531301,33.328002 48.531301,32.294097 48.4272,31.284898 48.227999,30.308202z M60.963728,14.451999L63.895998,19.696799 47.743868,28.718999C47.120075,26.764499,46.123985,24.996299,44.817999,23.474099z M5.8831024,14.181999L21.936999,23.357735C20.616765,24.868125,19.607615,26.627213,18.960647,28.578999L2.9039993,19.403264z M47.663984,3.0889991L52.839998,6.1383522 43.456712,22.064999C41.96595,20.723838,40.213178,19.687466,38.276999,19.020785z M19.487081,2.8649989L28.617,18.94893C26.66778,19.585601,24.902254,20.590758,23.394619,21.910998L14.261999,5.8297189z M30.330999,0L36.335999,0 36.335999,18.430999C35.363416,18.2357 34.36093,18.127599 33.330949,18.127599 32.299765,18.127599 31.299782,18.2357 30.330999,18.430999z"
                          Stretch="Uniform" Margin="200" Fill="Red" RenderTransformOrigin="0.5,0.5">
                <Path.RenderTransform>
                    <RotateTransform x:Name="rotate" Angle="0"/>
                </Path.RenderTransform>
            </Path>
        </Border>
    </Grid>

后台代码：

private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            //高清大图，你懂的。
            var path = Combine(AppDomain.CurrentDomain.BaseDirectory, "images");//using static System.IO.Path;
            var files = Directory.GetFiles(path);
            lvImages.ItemsSource = files;
        }


        private async void lvImages_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
           //在选中项更改时进行处理，
            if (lvImages.SelectedIndex > -1)
            {
                //选中的项
                var current = e.AddedItems[0];
                
                bigImage.Source = null;

                //坐和放宽，很快就好。
                conver.Visibility = Visibility.Visible;
                rotate.BeginAnimation(RotateTransform.AngleProperty, new DoubleAnimation(360, TimeSpan.FromMilliseconds(1000))
                { RepeatBehavior = RepeatBehavior.Forever });

                //await一下
                try
                {
                    var source = await Task.Run<ImageSource>(() =>
                    {
                        var p = current.ToString();
                        var s = new BitmapImage();
                        s.BeginInit();
                        s.CacheOption = BitmapCacheOption.OnLoad;
                        //打开文件流
                        using (var stream = File.OpenRead(p))
                        {
                            s.StreamSource = stream;
                            s.EndInit();
                            //这一句很重要，少了UI线程就不认了。
                            s.Freeze();
                        }
                        return s;
                    });
                    //出炉
                    bigImage.Source = source;                  
                }
                catch { }
                //好了，不用转了
                rotate.BeginAnimation(RotateTransform.AngleProperty, null);
                conver.Visibility = Visibility.Hidden;
            }
            else
            {
                var s = bigImage.Source;
                s = null;
                bigImage.Source = null;
            }
        }

        private void bigImage_MouseWheel(object sender, MouseWheelEventArgs e)
        {
           //高清大图嘛，当然要放大了仔细看看
            var d = e.Delta / 1000.0;
            if (scale.ScaleX + d < 1)
            {
                scale.ScaleX = scale.ScaleY = 1;
                return;
            }
            scale.ScaleX += d;
            scale.ScaleY += d;
        }

一个简单的ImageSourceConverter：

 public class ImageSourceConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (value == null)
                return value;
            var source = new BitmapImage();
            source.BeginInit();
            source.UriSource = new Uri(value.ToString(), UriKind.Absolute);
            //设置解码的分辨率大小，也就是搞个缩略图
            source.DecodePixelWidth = parameter != null ? int.Parse(parameter.ToString()) : 100;
            source.EndInit();
            return source;
            
        }

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

看图：