Xamarin形式的绑定数据问题

Question

我是Xamarin Forms（XF）的新手。当我尝试绑定数据时，遇到了一个我不理解的问题。如果我通过接口（如选择器）更新任何属性，则PropertyChanged存在。但是，如果我通过代码更新任何属性，则PropertyChanged不存在。我尝试通过分配PropertyChanged = delegate {}来解决它，PropertyChanged存在，但是handler(this, new PropertyChangedEventArgs(propertyName));无法更新我界面上的属性值。这是我的示例：

• 在MainPage.xaml中：

<Picker Grid.Row="1" Title="Select a component" ItemsSource="{Binding ComponentTree.Children}" ItemDisplayBinding="{Binding Name}" SelectedItem="{Binding SelectedGrandFatherComponent}" />
<Picker Grid.Row="2" Title="Select a component" ItemsSource="{Binding SelectedGrandFatherComponent.Children}" ItemDisplayBinding="{Binding Name}" SelectedItem="{Binding SelectedFatherComponent}" />
<Picker Grid.Row="3" x:Name="SelectedComponentPicker" Title="Select a component" ItemsSource="{Binding SelectedFatherComponent.Children}" ItemDisplayBinding="{Binding Name}" SelectedItem="{Binding SelectedComponent}" />
<ScrollView Grid.Row="4">
  <interfaces:SimpleTreeView BindingContext="{Binding SelectedComponent}" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand" />
</ScrollView>
<Label Text="{Binding SelectedComponentStructure}"></Label>

• 在TreeCardView.xaml中：

<Button Grid.Column="1" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand" Command="{Binding Appear}"  BackgroundColor="Transparent" CommandParameter="{Binding Name}" />

• 在TreeNode.cs中（类似于树中每个节点的控制器（TreeCardView.xaml）：]：>

public TreeNode()
{
  Appear = new Command<string>((x) => ShowUp(x));
}
public static void ShowUp(string name)
{
  StructureDesignViewModel instance = new StructureDesignViewModel();
  StructureDesignViewModel.HandleSelectedComponent delegateIns = new StructureDesignViewModel.HandleSelectedComponent(instance.SetSelectedComponent);
  delegateIns(name);
}

• 在StructureDesignViewModel.cs中（例如MainPage.xaml的控件）：

public class StructureDesignViewModel : ObservableObject, INotifyPropertyChanged
    {
        private static readonly StructureDesignViewModel instance = new StructureDesignViewModel();

        public static StructureDesignViewModel Instance
        {
            get
            {
                return instance;
            }
        }
        public static IList<Component> Components
        {
            get
            {
                return ComponentData.Components;
            }
        }
        TreeNode selectedGrandFatherComponent;
        TreeNode selectedFatherComponent;
        TreeNode selectedComponent;
        string selectedComponentStructure;

        public TreeNode BaseTree
        {
            get; set;
        }

        public TreeNode ComponentTree
        {
            get; set;
        }


        void GrowUp(TreeNode node, List<Component> components)
        {
            int len = components.Count;
            for (int i = 0; i < len; i++)
            {
                var currentNode = node.Children.Add(new TreeNode { Name = components[i].Name, Status = components[i].Status, IsExpanded = false, Modals = components[i].Modals });
                GrowUp((TreeNode)currentNode, components[i].Childs);
            }
        }
        public StructureDesignViewModel()
        {
            ComponentTree = new SimpleTreeView().ViewModel.MyTree;
            ComponentTree = new TreeNode { Name = "Component Root", Status = 0, IsExpanded = true };
            GrowUp(ComponentTree, (List<Component>)Components);
        }
        public TreeNode SelectedGrandFatherComponent
        {
            get { return selectedGrandFatherComponent; }
            set
            {
                if (selectedGrandFatherComponent != value)
                {
                    selectedGrandFatherComponent = value;
                    OnPropertyChanged();
                }
            }
        }

        public TreeNode SelectedFatherComponent
        {
            get
            {
                if (selectedGrandFatherComponent?.Children.Count > 0)
                {
                    selectedFatherComponent = (HMIStudio.Shared.Interfaces.TreeNode)selectedGrandFatherComponent.Children[0];
                }
                return selectedFatherComponent;
            }
            set
            {
                if (selectedFatherComponent != value)
                {
                    selectedFatherComponent = value;
                    OnPropertyChanged();
                }
            }
        }

        public TreeNode SelectedComponent
        {
            get
            {
                if (selectedFatherComponent?.Children.Count > 0)
                {
                    selectedComponent = (HMIStudio.Shared.Interfaces.TreeNode)selectedFatherComponent.Children[0];
                }
                return selectedComponent;
            }
            set
            {
                if (selectedComponent != value)
                {
                    selectedComponent = value;
                    OnPropertyChanged();
                }
            }
        }

        public string SelectedComponentStructure
        {
            get
            {
                return selectedComponentStructure;
            }
            set
            {
                if (selectedComponentStructure != value)
                {
                    Set("SelectedComponentStructure", ref selectedComponentStructure, value);
                }
            }
        }
        public event PropertyChangedEventHandler PropertyChanged = delegate { };

        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
                if (propertyName == "SelectedGrandFatherComponent")
                {
                    OnPropertyChanged(nameof(SelectedFatherComponent));
                }
                else if (propertyName == "SelectedFatherComponent")
                {
                    OnPropertyChanged(nameof(SelectedComponent));
                }
            }
        }
        public delegate void HandleSelectedComponent(string Name);
        public void SetSelectedComponent(string name)
        {
            Console.WriteLine("Showing selected component {0}", name);
            SelectedComponentStructure = name;
        }
    }

• 在ObservableObject.cs中：

public class ObservableObject : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        protected virtual void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }

        protected virtual void Set<T>(string propertyName, ref T backingField, T newValue, Action beforeChange = null, Action afterChange = null)
        {
            if (string.IsNullOrEmpty(propertyName))
                throw new ArgumentException("propertyName");

            if (backingField == null && newValue == null)
                return;

            if (backingField != null && backingField.Equals(newValue))
                return;

            if (beforeChange != null)
                beforeChange();

            backingField = newValue;

            OnPropertyChanged(propertyName);

            if (afterChange != null)
                afterChange();
        }
    }
我的程序的流程：

• [StructureDesignViewModel从ComponentData.Components获取数据（树结构数据）以传递到视图中的ItemsSource。
• [当我选择GrandFatherComponent时，GrandFatherComponent.Children是下一个选取器的ItemSource。
• [当我单击SimpleTreeView/SelectedComponent的节点时，我调用函数Appear将节点的名称分配给SelectedComponentStructure。

我的问题是SelectedComponentStructure已更改，但界面未更新。

感谢您的阅读！最好的问候！

我是Xamarin Forms（XF）的新手。当我尝试绑定数据时，遇到了一个我不理解的问题。如果我通过接口（如选择器）更新任何属性，则存在PropertyChanged。但是，如果我更新任何...

Answer 1

StructureDesignViewModel