silverlight - INotifyPropertyChanged 背后的概念是什么？

Question

所有使用 MVVM 的 Silverlight 示例都使用名为 IPropertyChanged 的​​接口(interface)。它背后的概念是什么，为什么我们需要在设置一些值时引发事件？

例如:-

public class UserNPC:INotifyPropertyChanged
{
    private string name;
    public string Name { 
        get { return name; } 
        set { name = value; onPropertyChanged(this, "Name"); } 
    }
    public int grade;
    public int Grade { 
        get { return grade; } 
        set { grade = value; onPropertyChanged(this, "Grade"); } 
    }

    // Declare the PropertyChanged event
    public event PropertyChangedEventHandler PropertyChanged;

    // OnPropertyChanged will raise the PropertyChanged event passing the
    // source property that is being updated.
    private void onPropertyChanged(object sender, string propertyName)
    {
        if (this.PropertyChanged != null)
        {
            PropertyChanged(sender, new PropertyChangedEventArgs(propertyName));
        }
    }
}

INotifyPropertyChanged 的​​确切目的是什么？

Answer 1

您有以下依赖项:

查看 → 绑定(bind) → 模型

现在，概念如下:

如果您的模型对象中的某些数据发生变化，您需要提出 PropertyChanged事件。为什么？因为 Binding 对象已经用数据对象的 PropertyChanged 注册了一个方法。事件。

因此，当您的 Model 对象发生变化时，您所要做的就是引发事件，然后您就完成了。

当您这样做时，Binding 对象会通过您的事件收到有关更改的通知。 Binding 对象反过来让 View 对象知道发生了什么事。然后，View 对象可以在必要时更新 UI。

代码示例

这里有一个可编译的例子。设置几个断点，按 F11 单步执行代码，看看幕后发生了什么。请注意，此示例具有以下依赖关系: View → 模型。我遗漏了 Binding 对象。

using System;
using System.ComponentModel;

namespace INotifyPropertyChangedDemo
{
    class Program
    {
        static void Main(string[] args)
        {
            // Create 2 listeners.
            View1 view1 = new View1();
            View2 view2 = new View2();

            // Create 1 data object.
            Model model = new Model();

            // Connect listener with data object.
            model.PropertyChanged += new PropertyChangedEventHandler(view1.MyPropertyChangedEventHandler);
            model.PropertyChanged += new PropertyChangedEventHandler(view2.MyPropertyChangedEventHandler);

            // Let data object publish change notification.
            model.FirstName = "new name";

            // Check whether all listeners got notified.
            // ... via console.
        }

        public class Model : INotifyPropertyChanged
        {
            public event PropertyChangedEventHandler PropertyChanged;

            private string firstName;
            public string FirstName
            {
                get { return firstName; }
                set
                {
                    if (firstName != value)
                    {
                        firstName = value;
                        if (PropertyChanged != null)
                        {
                            PropertyChanged(this, new PropertyChangedEventArgs("FirstName"));
                        }
                    }
                }
            }
        }

        public class View1
        {
            public void MyPropertyChangedEventHandler(object source, PropertyChangedEventArgs arg)
            {
                Console.WriteLine("Listener 1: Changed Property: {0}", arg.PropertyName);
                string newValue = ((Model) source).FirstName;
                Console.WriteLine("Listener 1: Changed Property Value: {0}", newValue);
            }
        }

        public class View2
        {
            public void MyPropertyChangedEventHandler(object source, PropertyChangedEventArgs arg)
            {
                Console.WriteLine("Listener 2: Changed Property: {0}", arg.PropertyName);
                string newValue = ((Model)source).FirstName;
                Console.WriteLine("Listener 2: Changed Property Value: {0}", newValue);
            }
        }
    }
}