MVVM模式的消息傳遞機制：Messenger.Default.Send 和 Register

public virtual void Send<TMessage>(TMessage message)
{
    SendToTargetOrType(message, null, null);
}

public virtual void Send<TMessage, TTarget>(TMessage message)//指定消息的目標類型TTarget。目標類型可以是任何類型，包括ViewModel、View、Control等。即接收方是屬於TTarget類的
{

     SendToTargetOrType(message, typeof(TTarget), null);
}

public virtual void Send<TMessage>(TMessage message, object token)//發送消息時，可以指定一個字元串類型的標識符，用於區分不同的消息。
{
     SendToTargetOrType(message, null, token);
}

public virtual void Register<TMessage>(object recipient, Action<TMessage> action)//第一個參數是要註冊的對象，第二個參數是一個 Action 委托，表示接收到消息後要執行的操作
{
     Register(recipient, null, receiveDerivedMessagesToo: false, action);
}

public virtual void Register<TMessage>(object recipient, bool receiveDerivedMessagesToo, Action<TMessage> action)//第二個參數是一個布爾值，指示是否接收派生類型的消息。false表示只接收TMessage類型的消息。

{

      Register(recipient, null, receiveDerivedMessagesToo, action);
}

public virtual void Register<TMessage>(object recipient, object token, Action<TMessage> action)
{
      Register(recipient, token, receiveDerivedMessagesToo: false, action);
}

1. 舉例：Send<TMessage, TTarget>(TMessage message)

public class MyMessage
{
    public string Text { get; set; }
}

// 發送消息
Messenger.Default.Send<MyMessage, MyViewModel>(new MyMessage { Text = "Hello World!" });

// 接收消息
Messenger.Default.Register<MyViewModel>(this, msg => {
    Console.WriteLine(msg.Text);
}); 

2. 舉例：Send<TMessage>(TMessage message, object token)

// Define a message class
public class MyMessage
{
    public string MessageText { get; set; }
}

// Register a recipient to receive MyMessage with a specific token
Messenger.Default.Register<MyMessage>(this, "myToken", message =>
{
    Console.WriteLine("Received message: " + message.MessageText);
});

// Send a MyMessage with the same token
var myMessage = new MyMessage { MessageText = "Hello world!" };
Messenger.Default.Send(myMessage, "myToken");