ToList in MainViewModel

Question

在ViewModel中，我从FirebaseDatabase加载数据，并在CollectionView中显示数据。

     public MainViewModel()
        {
            var collection = firebase
               .Child("Foto/")
               .AsObservable<Fotos>()
               .Subscribe((dbevent) =>
               {
                   if (dbevent.Object != null)
                   {
                       Foto.Add(dbevent.Object);
                   }
               });
        }

但是我想改变它，ToList，所以我使它降序基于BalId在数据库中。

想要使用GetAllFotosDesending()，但我不能使它在MainViewModel中工作。

  public async Task<List<Fotos>> GetAllFotosDesending()
    {
        return (await firebase
          .Child("Foto/")
          .OnceAsync<Fotos>()).Select(item => new Fotos
          {
              BalId = item.Object.BalId,
              RollNo = item.Object.RollNo,
              Foto = item.Object.Foto,
              Titel = item.Object.Titel,
              Fototekst = item.Object.Fototekst
          }).OrderByDescending(x => x.BalId).ToList();
    }

2选项，或使其在ToList中MainViewModel或添加GetAllFotosDesending()工作在MainViewModel中。最后一个选择也许更好？但是，当添加到MainViewModel时，我无法使它工作。

这是CollectionView和ItemsSource="{Binding Foto}"

   <CollectionView 
        x:Name="Dood"
        ItemsSource="{Binding Foto}">
        <CollectionView.ItemTemplate>
            <DataTemplate>
                <Grid ColumnDefinitions="Auto, *"
                      RowDefinitions="Auto, Auto, Auto, 1"
                      ColumnSpacing="10"
                      RowSpacing="5"
                      Padding="0,10">

                    <Image Source="{Binding Pic}"
                           Margin="20,0,0,10"
                            HeightRequest="70"
                            WidthRequest="70"
                            HorizontalOptions="Center"
                            VerticalOptions="Center"
                            Grid.RowSpan="3"
                            Grid.Row="0"
                            Grid.Column="0">
                        <Image.Clip>
                            <EllipseGeometry
                                        Center="35,35"
                                        RadiusX="35"
                                        RadiusY="35"/>
                        </Image.Clip>
                    </Image>

                    <Label Text="{Binding Titel}"
                           FontAttributes="Bold"
                           Grid.Column="1"
                           Grid.Row="0"/>

                    <Label Text="{Binding Email}"
                           Grid.Column="1"
                           Grid.Row="1"/>

                    <Label Text="{Binding RollNo}"
                           Grid.Column="1"
                           Grid.Row="2"/>

                    <BoxView Style="{StaticResource SeparatorLine}"
                             Grid.Column="0"
                             Grid.Row="3"
                             Grid.ColumnSpan="2"/>
                </Grid>
            </DataTemplate>
        </CollectionView.ItemTemplate>
    </CollectionView>

像这样装订

 public MainPage()
    {
        InitializeComponent();
        BindingContext = new MainViewModel();
    }

Answer 1

默认情况下，OrderBy没有字母数字比较，默认的Orderby只比较字母或数字，而不是两者的组合，您需要为您创建自己的字母数字比较器(特别是如果这些是字符串值)。

您可以使用这个基本的比较器来进行升序和降序：

public class AlphanumComparator : IComparer<object>
{
    private enum ChunkType { Alphanumeric, Numeric };

    private bool InChunk(char ch, char otherCh)
    {
        ChunkType type = ChunkType.Alphanumeric;

        if (char.IsDigit(otherCh))
        {
            type = ChunkType.Numeric;
        }

        return (type != ChunkType.Alphanumeric || !char.IsDigit(ch))
            && (type != ChunkType.Numeric || char.IsDigit(ch));
    }

    public int Compare(object x, object y)
    {
        string firstString = x as string;
        string secondString = y as string;
        if (string.IsNullOrWhiteSpace(firstString) || string.IsNullOrWhiteSpace(secondString))
        {
            return 0;
        }

        int firstMarker = 0, secondMarker = 0;

        while ((firstMarker < firstString.Length) || (secondMarker < secondString.Length))
        {
            if (firstMarker >= firstString.Length)
            {
                return -1;
            }
            else if (secondMarker >= secondString.Length)
            {
                return 1;
            }
            char firstCh = firstString[firstMarker];
            char secondCh = secondString[secondMarker];

            StringBuilder thisChunk = new StringBuilder();
            StringBuilder thatChunk = new StringBuilder();

            while ((firstMarker < firstString.Length) && (thisChunk.Length == 0 || InChunk(firstCh, thisChunk[0])))
            {
                thisChunk.Append(firstCh);
                firstMarker++;

                if (firstMarker < firstString.Length)
                {
                    firstCh = firstString[firstMarker];
                }
            }

            while ((secondMarker < secondString.Length) && (thatChunk.Length == 0 || InChunk(secondCh, thatChunk[0])))
            {
                thatChunk.Append(secondCh);
                secondMarker++;

                if (secondMarker < secondString.Length)
                {
                    secondCh = secondString[secondMarker];
                }
            }

            int result = 0;
            // If both chunks contain numeric characters, sort them numerically
            if (char.IsDigit(thisChunk[0]) && char.IsDigit(thatChunk[0]))
            {
                int thisNumericChunk = Convert.ToInt32(thisChunk.ToString());
                int thatNumericChunk = Convert.ToInt32(thatChunk.ToString());
                if (thisNumericChunk < thatNumericChunk)
                {
                    result = -1;
                }

                if (thisNumericChunk > thatNumericChunk)
                {
                    result = 1;
                }
            }
            else
            {
                result = thisChunk.ToString().CompareTo(thatChunk.ToString());
            }

            if (result != 0)
            {
                return result;
            }
        }

        return 0;
    }
}

一旦完成了，您就可以如下所示使用它：

OrderByDescending(x => x.BalId, new AlphanumComparator());
OrderBy(x => x.BalId, new AlphanumComparator());

Answer 2

就这样解决了。

 public async Task<List<Fotos>> GetAllFotosDesending()
    {
        return (await firebase
          .Child("Foto/")
          .OnceAsync<Fotos>()).Select(item => new Fotos
          {
              BalId = item.Object.BalId,
              RollNo = item.Object.RollNo,
              Foto = item.Object.Foto,
              Titel = item.Object.Titel,
              Fototekst = item.Object.Fototekst
          }).OrderByDescending(x => x.BalId).ToList();
    }
    public async void InitializeAsync()
    {          
        Fotos = await GetAllFotosDesending();           
    }
  

        public MainViewModel()
    {
        InitializeAsync();
    }