一、概述

Messenger中文解釋為信使的意思，顧名思義，在MvvmLight中，它的主要作用是用於View和ViewModel、ViewModel和ViewModel之間的通訊。

考慮以下場景：

現在有一個主窗體，主窗體上有一個按鈕，點選按鈕的時候，彈出一個子窗體，用於實時的往主窗體上新增新的項。

這時候Messenger就發揮其強大的威力了。

Messenger類的主要互動模式就是資訊的傳送和接收（類似訊息的釋出訂閱）。

MVVM Light Messenger 旨在通過簡單的設計模式來精簡此場景：任何物件都可以是接收端；任何物件都可以是傳送端；任何物件都可以是訊息。

Messenger的工作流程如下：

二、View和ViewModel之間通訊

在View中註冊訊息 Messenger.Default.Register<string>(this, "ShowSubWindowToken", ShowSubWindow)，相當於訂閱服務。

訊息標誌token：ShowSubWindowToken，用於標識只閱讀某個或者某些Sender傳送的訊息，並執行相應的處理，所以Sender那邊的token要保持一致

訊息處理Action：ShowSubWindow，引數型別為string，用來執行接收到訊息後的後續工作。

在ViewModel中傳送訊息 Messenger.Default.Send("Show subwindow","ShowSubWindowToken")，相當於釋出事件。

傳遞的訊息引數為Show subwindow，訊息token為ShowSubWindowToken，需要與接收者註冊的訊息的Token一致。

程式碼片段如下：

using GalaSoft.MvvmLight;
using GalaSoft.MvvmLight.Command;
using GalaSoft.MvvmLight.Messaging;
using MvvmLightClosableTabControl.Models;
using System.Collections.ObjectModel;
using System.Windows;

namespace MvvmLightClosableTabControl.ViewModel
{
    
 
public class Page3ViewModel:ViewModelBase
    {
        public  Page3ViewModel()
        {
            Messenger.Default.Register<string>(this,  "AddItemToken", AddItem);
        }
           
        private ObservableCollection<ListBoxItemModel> listBoxData = new ObservableCollection<ListBoxItemModel>()
        {
              new ListBoxItemModel(){ Img="/MvvmLightClosableTabControl;component/Img/1.png",Info="Honey Peach " },
              new ListBoxItemModel(){ Img="/MvvmLightClosableTabControl;component/Img/2.png",Info="Tomato" },
              new ListBoxItemModel(){ Img="/MvvmLightClosableTabControl;component/Img/3.png",Info="Banana" },
              new ListBoxItemModel(){ Img="/MvvmLightClosableTabControl;component/Img/4.png",Info="Chilli " },
              new ListBoxItemModel(){ Img="/MvvmLightClosableTabControl;component/Img/5.png",Info="Apple" },
        };
        /// <summary>
        /// LisBox資料模板
        /// </summary>
        public ObservableCollection<ListBoxItemModel> ListBoxData
        {
            get { return listBoxData; }
            set { listBoxData = value; RaisePropertyChanged(() => ListBoxData); }
        }


        private int selectedIndex = -1;
        public int SelectedIndex
        {
            get { return selectedIndex; }
            set
            {
                selectedIndex = value;
                RaisePropertyChanged();
                string selValue = $"ImgPath: {listBoxData[selectedIndex].Img}\r\nInfo: {listBoxData[selectedIndex].Info}";
                ViewModelLocator.DialogService.ShowInfoDialog($"您當前選擇的是:\r\n{selValue}");
            }
        }
        #region Command
        private RelayCommand addItemCommand;

        public RelayCommand AddItemCommand
        {
            get
            {
                if (addItemCommand == null)
                {
                    addItemCommand = new RelayCommand(AddOneItem);
                }
                return addItemCommand;
            }
            set { addItemCommand = value; }
        }
        private void AddOneItem()
        {
            int a = 3;
            Messenger.Default.Send("Show subwindow","ShowSubWindowToken");
        }
        #endregion
        private void AddItem(string msg)
        {
            ListBoxData.Add(new ListBoxItemModel() { Img = "/MvvmLightClosableTabControl;component/Img/1.png", Info = msg });
            MessageBox.Show(msg + " added completed!");
        }
    }
}

using GalaSoft.MvvmLight.Messaging;
using System.Windows;
using System.Windows.Controls;

namespace MvvmLightClosableTabControl.Pages
{
    /// <summary>
    /// Interaction logic for Page3.xaml
    /// </summary>
    public partial class Page3 : Page
    {
        public Page3()
        {
            InitializeComponent();
            Messenger.Default.Register<string>(this, "ShowSubWindowToken", ShowSubWindow);
        }
        private void ShowSubWindow(string msg)
        {
            Page3SubWindow myWindow = new Page3SubWindow()
            {
                WindowStartupLocation = WindowStartupLocation.CenterOwner, Owner = Application.Current.MainWindow
            };
            myWindow.ShowDialog();
        }
         

    }
}

<Page x:Class="MvvmLightClosableTabControl.Pages.Page3"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
      xmlns:local="clr-namespace:MvvmLightClosableTabControl.Pages"
      mc:Ignorable="d" 
      d:DesignHeight="300" d:DesignWidth="300"
      Title="Page3">

    <Page.DataContext>
        <Binding Path="Page3" Source="{StaticResource Locator}"/>
    </Page.DataContext>
    <Grid Background="#FFBBB415">
        <Grid.RowDefinitions>
            <RowDefinition Height="70"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <Label Content="This is page3." VerticalAlignment="Center" HorizontalAlignment="Center" FontSize="40"></Label>
        <StackPanel Grid.Row="1" >
            <GroupBox Header="Messenger顯示子視窗新增使用者" FontSize="16" Foreground="DarkGreen">
                <StackPanel Orientation="Horizontal">
                    <StackPanel>

                        <Button Content="新增條目" Command="{Binding AddItemCommand}"></Button>
                        <ListBox ItemsSource="{Binding ListBoxData}" DisplayMemberPath="Info" SelectedIndex="{Binding SelectedIndex}">

                        </ListBox>
                    </StackPanel>
                </StackPanel>
             
          
            </GroupBox>
                
                
        </StackPanel>
    </Grid>
</Page>

三、ViewModel和ViewModel之間通訊