在 Xamarin 和 MVVM 中绑定 ObservableCollection Listview 时不刷新特定标签

Question

public class Zicker : INotifyPropertyChanged
{
    public class MyClass
    {
        public string HeyName { get; set; }
        public string HeySurname { get; set; }
        public int HeyAge { get; set; }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected void RaisePropertyChanged([CallerMemberName] string name = null)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged.Invoke(this, new PropertyChangedEventArgs(name));
        }
    }

    private ObservableCollection<MyClass> _yourList = new ObservableCollection<MyClass>();
    public ObservableCollection<MyClass> YourList
    {
        get
        {
            return _yourList;
        }
        set
        {
            _yourList = value;
            RaisePropertyChanged("YourList");
            RaisePropertyChanged("BindMeLabel");
        }
    }

    public int BindMeLabel
    {
        get { return _yourList.Sum(a => a.HeyAge); }
    }

    public void WonCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        RaisePropertyChanged("BindMeLabel");
    }

    public List<string> heresamplenames = new List<string> { "Mohamed", "Zaran", "Ivan" };
    public List<string> heresamplesurnames = new List<string> { "Pakou", "Simmone", "Zagoev" };
    public List<int> heresampleages = new List<int> { 17,33,50 };

    public Zicker()
    {
        ObservableCollection<MyClass> vs = new ObservableCollection<MyClass>();
        for (int i = 0; i < 3; i++)
        { vs.Add(new MyClass { HeyName = heresamplenames[i], HeySurname = heresamplesurnames[i], HeyAge = heresampleages[i] }); }
        YourList = vs; YourList.CollectionChanged += WonCollectionChanged;
    }
}

<ContentPage.Content>
    <StackLayout Orientation="Vertical" HorizontalOptions="Center" VerticalOptions="Center">
        <ContentView HorizontalOptions="Fill" VerticalOptions="Fill">
            <ListView HorizontalOptions="Center" VerticalOptions="Center" HasUnevenRows="True" ItemsSource="{Binding YourList}">
                <ListView.ItemTemplate>
                    <DataTemplate>
                        <ViewCell>
                            <ViewCell.View>
                                <Grid>
                                    <Grid.ColumnDefinitions>
                                        <ColumnDefinition Width="1*"></ColumnDefinition>
                                        <ColumnDefinition Width="1*"></ColumnDefinition>
                                        <ColumnDefinition Width="1*"></ColumnDefinition>
                                    </Grid.ColumnDefinitions>
                                    <Label HorizontalTextAlignment="Center" VerticalTextAlignment="Center" Text="{Binding Path=HeyName}" Grid.Column="0" FontSize="12" TextColor="Black"></Label>
                                    <Label HorizontalTextAlignment="Center" VerticalTextAlignment="Center" Text="{Binding Path=HeySurname}" FontSize="12" TextColor="Black" Grid.Column="1"/>
                                    <Entry HorizontalTextAlignment="Center" VerticalOptions="Center" Text="{Binding Path=HeyAge}" FontSize="12" Keyboard="Numeric" TextColor="Black" Grid.Column="2"/>
                                </Grid>
                            </ViewCell.View>
                        </ViewCell>
                    </DataTemplate>
                </ListView.ItemTemplate>
            </ListView>
        </ContentView>
        <Label Text="{Binding BindMeLabel}" HorizontalOptions="CenterAndExpand" VerticalOptions="CenterAndExpand" FontSize="40" TextColor="Black"></Label>
    </StackLayout>
</ContentPage.Content>

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

我的问题：在这个列表中，有三个名字、姓氏和年龄。在底部，还有一个标签，应该显示为 Ages 集合的总和。

当 UI 启动时，Label 运行良好。但是，如果我尝试更改任何年龄条目，绑定标签就会出现大问题。

我想使用 MVVM 结构，但由于这个问题，标签绑定刚刚启动。

Answer 1

如果您要更新HeyName 属性，绑定不会更新，因为MyClass 类没有实现INotifyPropertyChanged。

尝试用以下代码替换 MyClass 类：

public class MyClass : INotifyPropertyChanged
{
    private string name;
    private string surname;
    private int age;

    public string HeyName
    {
        get => name;
        set
        {
            name = value;
            RaisePropertyChanged("HeyName");
        }
    }

    public string HeySurname
    {
        get => surname;
        set
        {
            surname = value;
            RaisePropertyChanged("HeySurname");
        }
    }

    public int HeyAge
    {
        get => age;
        set
        {
            age = value;
            RaisePropertyChanged("HeyAge");
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected void RaisePropertyChanged([CallerMemberName] string name = null)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged.Invoke(this, new PropertyChangedEventArgs(name));
        }
    }
}

编辑：

抱歉，更新HeyAge 属性时不会调用CollectionChanged，因为它仅在集合更改时调用，但不会在集合中某个项目的属性时调用变化。

尝试将OnAgeChanged 事件添加到MyClass 类中，并在HeyAge 属性更改时调用它：

public class MyClass : INotifyPropertyChanged
{
    public event EventHandler OnAgeChanged;

    public int HeyAge
    {
        get => age;
        set
        {
            age = value;
            RaisePropertyChanged("HeyAge");
            OnAgeChanged?.Invoke(this, EventArgs.Empty);
        }
    }
    ...
    ...

然后，当您将新的MyClass 对象添加到集合中时，将事件注册到ViewModel 中，如下所示：

public Zicker()
{
    ObservableCollection<MyClass> vs = new ObservableCollection<MyClass>();
    for (int i = 0; i < 3; i++)
    {
        var test = new MyClass()
        {
            HeyName = heresamplenames[i],
            HeySurname = heresamplesurnames[i],
            HeyAge = heresampleages[i],
        };
        test.OnAgeChanged += Test_OnAgeChanged;
        vs.Add(test);
    }

    YourList = vs;
    YourList.CollectionChanged += WonCollectionChanged;
}

private void Test_OnAgeChanged(object sender, EventArgs e)
{
    RaisePropertyChanged("BindMeLabel");
}

请注意，WonCollectionChanged 不再需要了。 另请注意，变量vs 不是必需的，您可以直接在YourList 对象中工作。