c# - WPF MVVM 分页

Question

我有一个分页 View 的用户控件，它有自己的 View 模型。我在具有数据网格的页面中添加了分页，该数据网格也具有单独的 View 模型。

我的问题是，每次在我的分页 View 模型中执行命令时，如何更新我在页面 View 模型中的 ObservableCollection？

这是我的 PagingControl.xaml

<StackPanel Width="Auto"
        Orientation="Horizontal">
        <Button                
            Margin="4,0"
            Content="&lt;&lt;"
            Command="{Binding FirstCommand}"/>
        <Button
            Margin="4,0"
            Content="&lt;"
            Command="{Binding PreviousCommand}"/>
        <StackPanel
            VerticalAlignment="Center"
            Orientation="Horizontal">
            <TextBlock
                Text="{Binding Start}"/>
            <TextBlock
                Text=" to "/>
            <TextBlock
                Text="{Binding End}"/>
            <TextBlock
                Text=" of "/>
            <TextBlock
                Text="{Binding TotalItems}"/>
        </StackPanel>
        <Button
            Margin="4,0"
            Content="&gt;"
            Command="{Binding NextCommand}"/>
        <Button
            Margin="4,0"
            Content="&gt;&gt;"
            Command="{Binding LastCommand}"/>
        <ComboBox Width="100" ItemsSource="{Binding ItemsPerPage}" SelectedValue="{Binding ItemCount}">
            <i:Interaction.Triggers>
                <i:EventTrigger EventName="SelectionChanged">
                <i:InvokeCommandAction Command="{Binding CountChangedCommand}"/>
                </i:EventTrigger>
            </i:Interaction.Triggers>
        </ComboBox>
    </StackPanel>

PagingViewModel.cs

public class PagingViewModel : ViewModelBase
{
    private ObservableCollection<DataModel> _data;

    private int start = 0;
    private int itemCount = 10;
    private int totalItems = 0;
    private readonly List<int> count;

    private ICommand _firstCommand;
    private ICommand _previousCommand;
    private ICommand _nextCommand;
    private ICommand _lastCommand;
    private ICommand _countchangedCommand;

    public ObservableCollection<DataModel> Data
    {
        get { return _data; }
        set
        {
            if (_data!= value)
            {
                _data= value;
                OnPropertyChanged("Data");
            }
        }
    }

    public PagingViewModel()
    {
        count = new List<int> { 10, 20, 30};
        RefreshData();
    }

    public int Start { get { return start + 1; } }

    public int End { get { return start + itemCount < totalItems ? start + itemCount : totalItems; } }

    public int TotalItems { get { return totalItems; } }

    public List<int> Count { get { return count; } }

    public int ItemCount { get { return itemCount; } set { itemCount = value; OnPropertyChanged("ItemCount"); } }

    public ICommand FirstCommand
    {
        get
        {
            if (_firstCommand == null)
            {
                _firstCommand = new RelayCommand
                (
                    param =>
                    {
                        start = 0;
                        RefreshData();
                    },
                    param =>
                    {
                        return start - itemCount >= 0 ? true : false;
                    }
                );
            }

            return _firstCommand;
        }
    }

    public ICommand PreviousCommand
    {
        get
        {
            if (_previousCommand == null)
            {
                _previousCommand = new RelayCommand
                (
                    param =>
                    {
                        start -= itemCount;
                        RefreshData();
                    },
                    param =>
                    {
                        return start - itemCount >= 0 ? true : false;
                    }
                );
            }

            return _previousCommand;
        }
    }

    public ICommand NextCommand
    {
        get
        {
            if (_nextCommand == null)
            {
                _nextCommand = new RelayCommand
                (
                    param =>
                    {
                        start += itemCount;
                        RefreshData();
                    },
                    param =>
                    {
                        return start + itemCount < totalItems ? true : false;
                    }
                );
            }

            return _nextCommand;
        }
    }

    public ICommand LastCommand
    {
        get
        {
            if (_lastCommand == null)
            {
                _lastCommand = new RelayCommand
                (
                    param =>
                    {
                        start = (totalItems / itemCount - 1) * itemCount;
                        start += totalItems % itemCount == 0 ? 0 : itemCount;
                        RefreshData();
                    },
                    param =>
                    {
                        return start + itemCount < totalItems ? true : false;
                    }
                );
            }

            return _lastCommand;
        }
    }

    public ICommand CountChangedCommand
    {
        get
        {
            if (_countchangedCommand == null)
            {
                _countchangedCommand = new RelayCommand
                (
                    param =>
                    {
                        start = 0;
                        RefreshData();
                    },
                    param =>
                    {
                        return ((totalItems - itemCount) > -10) ? true : false;
                    }
                );
            }

            return _countchangedCommand;
        }
    }

    public void RefreshData()
    {
        _data= GetData(start, itemCount, out totalItems);
DataViewModel vm = new DataViewModel(this);

        OnPropertyChanged("Start");
        OnPropertyChanged("End");
        OnPropertyChanged("TotalItems");
    }
}

这是我的页面的 View 模型:DataViewModel.cs

public class DataViewModel: ViewModelBase
{
    private ObservableCollection<DataModel> _data;

    public ObservableCollection<DataModel> Data
    {
        get { return _data; }
        set
        {
            if (_data!= value)
            {
                _data= value;
                OnPropertyChanged("Data");
            }
        }
    }

    public DataViewModel(PagingViewModel pagevm)
    {
        _data = new ObservableCollection<DataModel>();
        _data= pagevm.Data; 
    }

}

我的数据属性绑定(bind)到 DataView.xaml 中 DataGrid 的 ItemSource，DataContext 设置为 DataViewModel。

Answer 1

这是一个非常详细的问题!

对于您的刷新问题，我看到了几个选项:当在 RefreshData 中设置您的 Data 时，您应该使用属性的公共(public) setter 而不是 _data。如果不这样做，您将永远不会使用 OnPropertyChanged 来通知 View 您的整个集合已更改。

所以你需要替换:

_data= GetData(start, itemCount, out totalItems);

与:

Data= GetData(start, itemCount, out totalItems);

顺便说一下，您的 DataViewModel 对我来说毫无意义。您的 _audits 字段无处可见，ObservableCollection Data 从未在此 ViewModel 中设置。我相信你的问题一定来自于此。

附言:

另一方面，我有一些与您的问题没有直接关系的建议:

首先，当您想在设置之前检查某些 RelayCommand 或 else 是否为 null 时，您可能需要使用 ??接线员:https://msdn.microsoft.com/en-us/en-en/library/ms173224.aspx .

其次，我强烈建议您将 RelayCommands 行为放在方法中。当您最终得到许多命令时，在 lambda 表达式中维护 RelayCommand 真是一团糟。

这样你就可以替换这个:

public ICommand NextCommand
{
    get
    {
        if (_nextCommand == null)
        {
            _nextCommand = new RelayCommand
            (
                param =>
                {
                    start += itemCount;
                    RefreshData();
                },
                param =>
                {
                    return start + itemCount < totalItems ? true : false;
                }
            );
        }
           return _nextCommand;
    }
}

有了这个:

 public ICommand NextCommand
 {
     get
     {
         return _nextCommand = _nextCommand ?? new RelayCommand(Next, CanExecuteNext);
     }
 }

 private void Next()
 {
     start += itemCount;
     RefreshData();
 }

 private bool CanExecuteNext()
 {
     return start + itemCount < totalItems ? true : false;
 }