如何在视图模型重新加载绑定数据时显示 'Loading...' 覆盖

Question

我想做一件听起来很简单，做起来却很难的事情。

让我们假设我有一些内容绑定到缓慢加载操作。例如，从本地 SQL 检索并需要几秒钟的可观察列表。当发生这种情况时，我想用 "Loading ..." 文本或任何其他 'please wait' 类型的内容覆盖内容呈现器（例如 Groupbox）。

我很快得出结论，简单地在操作前后切换绑定到 UI 的布尔标志是行不通的。在整个操作完成之前，UI 不会刷新。可能是因为手术 CPU 密集，我不知道。

我现在正在研究 Adorners，但是我在 'busy indicator' 覆盖的上下文中搜索它时得到的信息很少。大约 5 年前，Internet 上只有一些解决方案，但我无法使用它们中的任何一个。

问题：

听起来很简单 - 如何在 View Model 正在更新绑定数据的同时临时在屏幕上显示一些内容？

Answer 1

这里是一个示例，说明如何在 ViewModel\Model 处理一些较长的任务时设置具有 "Loading" 显示的视图。

Window

<Window x:Class="Loading.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"
    xmlns:local="clr-namespace:Loading"
    mc:Ignorable="d"
    Title="MainWindow" Height="350" Width="525">
<Window.Resources>
    <local:VisibilityConverter x:Key="visibilityConverter" />
</Window.Resources>
<Window.DataContext>
    <local:ViewModel x:Name="viewModel" />
</Window.DataContext>
<Grid>
    <Button Content="Perform" VerticalAlignment="Bottom" HorizontalAlignment="Center" Width="100" Height="30" Command="{Binding HandleRequestCommand}" />
    <Border Visibility="{Binding Path=IsLoading,Converter={StaticResource visibilityConverter}}" Background="#AAAAAAAA" Margin="5">
        <TextBlock Text="Loading..." VerticalAlignment="Center" HorizontalAlignment="Center" />
    </Border>
</Grid>

VisibilityConverter.cs（简单的辅助转换器）

class VisibilityConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return (bool)value ? Visibility.Visible : Visibility.Collapsed;
    }

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

ViewModel.cs

public class ViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    private bool isLoading;

    public ViewModel()
    {
        HandleRequestCommand = new Command(HandleRequest);
    }

    public bool IsLoading
    {
        get
        {
            return isLoading;
        }
        set
        {
            if (value != isLoading)
            {
                isLoading = value;
                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(IsLoading)));
            }
        }
    }

    public ICommand HandleRequestCommand
    {
        get;
    }

    public void HandleRequest()
    {
        IsLoading = true;

        Task.Factory.StartNew(LongRunningOperation);
    }

    private void LongRunningOperation()
    {
        // *** INSERT LONG RUNNING OPERATION ***

        Dispatcher.CurrentDispatcher.Invoke(() => IsLoading = false);
    }
}

Answer 2

I quickly came to the conclusion that simply switching a boolean flag bound to the UI, before and after the operation doesn't work. The UI does not get refreshed until the entire operation completes. Maybe because the operation is CPU-intensive, I don't know.

是的，如果您实际上是在后台线程上执行 long-running 操作，它应该可以工作。

请参考下面的简单例子

查看：

<Window x:Class="WpfApplication2.Window1"
        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"
        xmlns:local="clr-namespace:WpfApplication2"
        mc:Ignorable="d"
        xmlns:s="clr-namespace:System;assembly=mscorlib"
        Title="Window1" Height="300" Width="300">
    <Window.DataContext>
        <local:Window1ViewModel />
    </Window.DataContext>
    <Window.Resources>
        <BooleanToVisibilityConverter x:Key="BooleanToVisibilityConverter" />
    </Window.Resources>
    <Grid>
        <TextBlock>Content...</TextBlock>
        <Grid Background="Yellow" Visibility="{Binding IsLoading, Converter={StaticResource BooleanToVisibilityConverter}}">
            <TextBlock HorizontalAlignment="Center" VerticalAlignment="Center">Loading...</TextBlock>
        </Grid>
    </Grid>
</Window>

查看模型：

public class Window1ViewModel : INotifyPropertyChanged
{
    public Window1ViewModel()
    {
        IsLoading = true;
        //call the long running method on a background thread...
        Task.Run(() => LongRunningMethod())
            .ContinueWith(task =>
            {
                //and set the IsLoading property back to false back on the UI thread once the task has finished
                IsLoading = false;
            }, System.Threading.CancellationToken.None, TaskContinuationOptions.None, TaskScheduler.FromCurrentSynchronizationContext());
    }

    public void LongRunningMethod()
    {
        System.Threading.Thread.Sleep(5000);
    }

    private bool _isLoading;
    public bool IsLoading
    {
        get { return _isLoading; }
        set { _isLoading = value; NotifyPropertyChanged(); }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
}

如何在视图模型重新加载绑定数据时显示 'Loading...' 覆盖

How to show 'Loading...' overlay while the View Model reloads the bound data

wpf

xaml

adorner