6

使用 MVVM 模式,我有一个 Model、ViewModel 和 View,其中包含一个 ListView。ListView 绑定到 ViewModel 的成员,该成员是 Model 类的 ObservableCollection。我可以使初始显示的绑定起作用,并且可以在对视图执行操作时更新模型类上的属性以获取适当的行,但是我无法刷新视图,从 ObservableCollection 中的模型类中提取数据。ListView 类不包含使无效或强制刷新的方法,这将解决我的问题。更新模型上的数据后如何让 ListView 刷新?

这是我正在尝试做的一个简单示例:每行包含一个按钮和标签。单击按钮后,我可以更新标签,这将反映在屏幕上。我需要做的是更新模型,这反过来又会强制更新视图。但是,我无法让它发挥作用。在实际应用中,Model的更新是在业务逻辑层,而不是在视图中,之后需要强制刷新ListView。

示例代码:

using System;
using System.Collections.ObjectModel;
using Xamarin.Forms;

namespace ListViewTest
{   
    public class Model
    {
        public static int ids = 0;
        public Model(string count) 
        {
            Id = +1;
            Count = count;
        }
        public int Id { get; set; }
        public string Count { get; set; }
    }

    public class ModelList : ObservableCollection<Model>
    {
    }

    public class ViewModel
    {
        ModelList list = new ModelList();
        public ModelList ViewModelList
        {
            get { return list; }
            set 
            { 
                list = value; 
            }
        }
    }

    public partial class MainPage : ContentPage
    {   
        public ViewModel viewModel;

        public class DataCell : ViewCell
        {
            public DataCell()
            {
                var Btn = new Button();
                Btn.Text = "Click";
                var Data = new Label();
                Data.SetBinding(Label.TextProperty,"Count");
                Btn.Clicked += (object sender, EventArgs e) => 
                {
                    Model model = (Model)(((Button)sender).Parent.BindingContext);
                    int count = Convert.ToInt32(model.Count);
                    count++;
                    model.Count = count.ToString();

                    // Need to refresh ListView from data source here... How???
                };

                StackLayout s = new StackLayout();
                s.Orientation = StackOrientation.Horizontal;
                s.Children.Add(Btn);
                s.Children.Add(Data);
                this.View = s;
            }
        }

        public MainPage ()
        {
            viewModel = new ViewModel();
            viewModel.ViewModelList.Add(new Model("0"));
            viewModel.ViewModelList.Add(new Model("0"));
            InitializeComponent();
        }

        public void InitializeComponent()
        {
            ListView listView = new ListView
            {
                ItemsSource = viewModel.ViewModelList,
                ItemTemplate = new DataTemplate(() =>
                    {
                        return new DataCell();
                    })
            };
            Content = listView;
        }
    }
}
4

2 回答 2

9

我认为您的模型需要实现 INotifyPropertyChanged。所以 UI 知道模型中的值已经改变

像这样的东西:

public class Model : INotifyPropertyChanged
{
    // boiler-plate
    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
    }
    protected bool SetField<T>(ref T field, T value, string propertyName)
    {
        if (EqualityComparer<T>.Default.Equals(field, value)) return false;
        field = value;
        OnPropertyChanged(propertyName);
        return true;
    }

    // props
    private string _count;
    public string Count 
    {
        get { return _count; }
        set { SetField(ref _count, value, "Count"); }
    }
}
于 2014-07-29T15:16:56.753 回答
2

从 ViewModelList 属性中删除“set”方法。每当您使用 ObservableCollection 时,只需更改该集合中的项目即可。永远不要用另一个 ObservableCollection 替换它。

于 2014-10-07T11:45:20.833 回答