移动端开发：如何用CommunityToolkit.MVVM替代.NET MessagingCenter？

我将移动端应用从Xamarin迁移到MAUI后，收到提示称MessagingCenter已被弃用，需改用CommunityToolkit.MVVM。已安装该工具包，但代码转换时遇到问题。

原MessagingCenter代码 #

public Command MgmtCommand
{
    get
    {
        return new Command((Vacation vacation) =>
        {
            //Unsubscribe from events - precautionary step to ensure that there are no existing subscriptions for the specified events
            MessagingCenter.Unsubscribe(this, "AddVacation");
            MessagingCenter.Unsubscribe(this, "UpdateVacation");

            //Navigate to the VacationAddView, passing a vacation if available
            Application.Current.MainPage.Navigation.PushAsync(new VacationMgmtView(vacation));

            //Subscribe to a MessagingCenter event for refreshing data when a new vacation is added
            MessagingCenter.Subscribe<Vacation>(this, "AddVacation", async (data) =>
            {
                //Refresh the vacation list data asynchronously
                await RefreshVacationListData();
                //Unsubscribe from the MessagingCenter event after refreshing data
                MessagingCenter.Unsubscribe<Vacation>(this, "AddVacation");
            });

            //Subscribe to a MessagingCenter event for refreshing data when a new vacation is updated
            MessagingCenter.Subscribe<Vacation>(this, "UpdateVacation", async (data) =>
            {
                // Refresh the vacation list data asynchronously
                await RefreshVacationListData();
                // Unsubscribe from the MessagingCenter event after refreshing data
                MessagingCenter.Unsubscribe<Vacation>(this, "UpdateVacation");
            });
        });
    }
}

当前尝试的错误代码 #

private async Task NavigateAndHandleAddUpdateAsync()
{
    WeakReferenceMessenger.Default.Register<OpenWindowMessage>(this, HandleOpenWindowMessage);
}

private async Task HandleOpenWindowMessage(object recipient, OpenWindowMessage message)
{
    switch (message.Value)
    {
        case true:
            Application.Current.MainPage.Navigation.PushAsync(new VacationMgmtView(vacation));
            break;
        default:
            //await DisplayAlert("Alert", "Secondary window was closed", "OK");
            break;
    }
}

迁移后的正确实现 #

1. 定义强类型消息类 #

CommunityToolkit.MVVM的WeakReferenceMessenger依赖强类型消息，需先创建对应消息类：

using CommunityToolkit.Mvvm.Messaging.Messages;

// 对应添加假期的消息
public class AddVacationMessage : ValueChangedMessage<Vacation>
{
    public AddVacationMessage(Vacation value) : base(value)
    {
    }
}

// 对应更新假期的消息
public class UpdateVacationMessage : ValueChangedMessage<Vacation>
{
    public UpdateVacationMessage(Vacation value) : base(value)
    {
    }
}

2. 重构MgmtCommand代码 #

替换原MessagingCenter逻辑为WeakReferenceMessenger的实现：

using CommunityToolkit.Mvvm.Messaging;

public Command MgmtCommand
{
    get
    {
        return new Command(async (Vacation vacation) =>
        {
            // 预先取消现有订阅，避免重复监听
            WeakReferenceMessenger.Default.Unregister<AddVacationMessage>(this);
            WeakReferenceMessenger.Default.Unregister<UpdateVacationMessage>(this);

            // 导航到假期管理页面
            await Application.Current.MainPage.Navigation.PushAsync(new VacationMgmtView(vacation));

            // 订阅添加假期完成的消息
            WeakReferenceMessenger.Default.Register<AddVacationMessage>(this, async (recipient, message) =>
            {
                await RefreshVacationListData();
                // 处理完成后取消订阅
                WeakReferenceMessenger.Default.Unregister<AddVacationMessage>(recipient);
            });

            // 订阅更新假期完成的消息
            WeakReferenceMessenger.Default.Register<UpdateVacationMessage>(this, async (recipient, message) =>
            {
                await RefreshVacationListData();
                // 处理完成后取消订阅
                WeakReferenceMessenger.Default.Unregister<UpdateVacationMessage>(recipient);
            });
        });
    }
}

3. 在VacationMgmtView中发送消息 #

当假期添加或更新操作完成后，在对应的ViewModel中发送消息：

// 添加假期完成时发送消息
WeakReferenceMessenger.Default.Send(new AddVacationMessage(addedVacation));

// 更新假期完成时发送消息
WeakReferenceMessenger.Default.Send(new UpdateVacationMessage(updatedVacation));

错误代码问题说明 #

你之前的尝试存在以下问题：

• 使用了不匹配业务逻辑的OpenWindowMessage，未对应原代码中的Add/Update事件
• 未遵循原逻辑中的订阅时机（导航后订阅）和一次性订阅（处理后取消）的设计
• 缺少强类型消息的定义，导致消息传递逻辑无法和原功能对齐

内容的提问来源于stack exchange，提问作者CipherIS