高级C#信使

作者：Ilya Suzdalnitski

译自：http://wiki.unity3d.com/index.php/Advanced_CSharp_Messenger

描述
前言
1. MissingReferenceException的原因和解决方案
信使

描述

这是C#的一个高级版本的消息系统。当加载了一个新的场景（level）之后，它会自动清空事件表。这将防止程序员意外的调用已销毁的方法，从而有助于防止出现很多MissingReferenceExceptions。这个消息系统是基于Rod Hyde的CSharpMessenger和Magnus Wolffelt的CSharpMessenger Extended而研发的。

前言

我们的项目一旦引入了消息系统（CSharpMessenger Extended），我们就开始面对非常奇怪的BUGs。每次广播一个消息，U3D就会抛出MissingReferenceExceptions这个异常。提示会说消息处理器声明所在类已经被销毁。这个问题无处不在，并且没有一个合理的解释。不过，把消息处理器代码放到try-cache块中可以解决这个问题。我们知道，在代码中有大量的try-cache块不是一个好的解决方案。我们花费了一些时间，终于找到了问题的所在。

MissingReferenceException的原因和解决方案

是这样的，当加载一个新场景（level）（或者重新加载当前场景）时，MissingReferenceException的BUG就会出现。例如：我们有一个消息"start game"，声明如下：

public class MainMenu : MonoBehaviour {    
    void Start ()
    {
        Messenger.AddListener("start game", StartGame);
    }
 
    void StartGame()
    {
        Debug.Log("StartGame called in" + gameObject);  //This is the line that would throw an exception
    }
 
    void StartGameButtonPressed()
    {
        Messenger.Broadcast("start game");
    }
}

乍一看，代码完全没有问题。但是在重新加载了这个场景之后，U3D将会抛出一个异常，提示说MainMenu对象已经被销毁。但是没有代码会销毁MainMenu脚本对象。

实际发生的是：

我们在信使中添加了一个"start game"消息监听器。
StartGameButtonPressed方法被调用，广播了"start game"消息。
我们使用Application.LoadLevel重新加载了这个场景（level）。
重复第1步。
重复第2步。

在相应的步骤，信使的事件表里是这个样子的：

在第1步：{ "start game", mainMenu1- > StartGame(); }
在第4步：{ "start game", mainMenu1- > StartGame(); } { "start game", mainMenu2- > StartGame(); }

所以在第4步我们有两个"start game"消息处理器——第1个是已经销毁的MainMenu对象（在重新加载场景时被销毁），第2个是当前有效的MainMenu对象。结果是这样的，当我们在重新加载场景后广播"start game"消息时，信使把两个消息处理器（已经销毁的和当前有效的）都调用了。这里就是MissingReferenceException出现的源头。那么解决方法显而易见——在卸载场景之后清空eventTable。在程序员方面不用做任何事来清空这个表，这一过程将自动完成。

信使

我们很高兴向你展示一个高级版本的C#消息系统。

用法

事件监听器

void OnPropCollected( PropType propType ) {
    if (propType == PropType.Life)
        livesAmount++;
}

注册事件监听器

void Start() {
    Messenger.AddListener< Prop >( "prop collected", OnPropCollected );
}

注销事件监听器

    Messenger.RemoveListener< Prop > ( "prop collected", OnPropCollected );

广播事件

public void OnTriggerEnter(Collider _collider) 
{
    Messenger.Broadcast< PropType > ( "prop collected", _collider.gameObject.GetComponent<Prop>().propType );
}

清空信使

当加载一个新的场景（level）时，信使会自动清空它的eventTable。这将确保信使的eventTable被清空，并且将使我们免于意外的MissingReferenceExceptions。如果你想手动清空管理器的eventTable，你可以调用Messenger.Cleanup()。

永久信使

如果你想要某个消息幸免于Cleanup，使用Messenger.MarkAsPermanent(string)来标记它既可。它可能用于这样的场合：某个类响应不同场景（level）的消息广播。

杂项

打印所有消息

为了调试的目的，你可以把信使的shouldLogAllMessages设置成true。这样，在调用信使的任何方法时都会打印消息。

从其他信使过渡

为了快速把旧的消息系统CSharpMessenger转变成高级的消息系统，请执行下面的步骤：

在MonoDevelop中打开“在文件中查找/替换”对话框。
在查找域输入：Messenger<([^<>]+)>.([A-Za-z0-9_]+)
在替换域输入：Messenger.$2<$1>
选择域：所有解决方案
勾选“正则搜索”复选框
按下“替换”按钮

代码

想要信使顺利运作需要两个文件：Callback.cs和Messenger.cs。

Callback.cs

public delegate void Callback();
public delegate void Callback<T>(T arg1);
public delegate void Callback<T, U>(T arg1, U arg2);
public delegate void Callback<T, U, V>(T arg1, U arg2, V arg3);

Messenger.cs

/*
 * Advanced C# messenger by Ilya Suzdalnitski. V1.0
 * 
 * Based on Rod Hyde's "CSharpMessenger" and Magnus Wolffelt's "CSharpMessenger Extended".
 * 
 * Features:
     * Prevents a MissingReferenceException because of a reference to a destroyed message handler.
     * Option to log all messages
     * Extensive error detection, preventing silent bugs
 * 
 * Usage examples:
     1. Messenger.AddListener<GameObject>("prop collected", PropCollected);
        Messenger.Broadcast<GameObject>("prop collected", prop);
     2. Messenger.AddListener<float>("speed changed", SpeedChanged);
        Messenger.Broadcast<float>("speed changed", 0.5f);
 * 
 * Messenger cleans up its evenTable automatically upon loading of a new level.
 * 
 * Don't forget that the messages that should survive the cleanup, should be marked with Messenger.MarkAsPermanent(string)
 * 
 */
 
//#define LOG_ALL_MESSAGES
//#define LOG_ADD_LISTENER
//#define LOG_BROADCAST_MESSAGE
#define REQUIRE_LISTENER
 
using System;
using System.Collections.Generic;
using UnityEngine;
 
static internal class Messenger {
    #region Internal variables
 
    //Disable the unused variable warning
#pragma warning disable 0414
    //Ensures that the MessengerHelper will be created automatically upon start of the game.
    static private MessengerHelper messengerHelper = ( new GameObject("MessengerHelper") ).AddComponent< MessengerHelper >();
#pragma warning restore 0414
 
    static public Dictionary<string, Delegate> eventTable = new Dictionary<string, Delegate>();
 
    //Message handlers that should never be removed, regardless of calling Cleanup
    static public List< string > permanentMessages = new List< string > ();
    #endregion
    #region Helper methods
    //Marks a certain message as permanent.
    static public void MarkAsPermanent(string eventType) {
#if LOG_ALL_MESSAGES
        Debug.Log("Messenger MarkAsPermanent \t\"" + eventType + "\"");
#endif
 
        permanentMessages.Add( eventType );
    }
 
 
    static public void Cleanup()
    {
#if LOG_ALL_MESSAGES
        Debug.Log("MESSENGER Cleanup. Make sure that none of necessary listeners are removed.");
#endif
 
        List< string > messagesToRemove = new List<string>();
 
        foreach (KeyValuePair<string, Delegate> pair in eventTable) {
            bool wasFound = false;
 
            foreach (string message in permanentMessages) {
                if (pair.Key == message) {
                    wasFound = true;
                    break;
                }
            }
 
            if (!wasFound)
                messagesToRemove.Add( pair.Key );
        }
 
        foreach (string message in messagesToRemove) {
            eventTable.Remove( message );
        }
    }
 
    static public void PrintEventTable()
    {
        Debug.Log("\t\t\t=== MESSENGER PrintEventTable ===");
 
        foreach (KeyValuePair<string, Delegate> pair in eventTable) {
            Debug.Log("\t\t\t" + pair.Key + "\t\t" + pair.Value);
        }
 
        Debug.Log("\n");
    }
    #endregion
 
    #region Message logging and exception throwing
    static public void OnListenerAdding(string eventType, Delegate listenerBeingAdded) {
#if LOG_ALL_MESSAGES || LOG_ADD_LISTENER
        Debug.Log("MESSENGER OnListenerAdding \t\"" + eventType + "\"\t{" + listenerBeingAdded.Target + " -> " + listenerBeingAdded.Method + "}");
#endif
 
        if (!eventTable.ContainsKey(eventType)) {
            eventTable.Add(eventType, null );
        }
 
        Delegate d = eventTable[eventType];
        if (d != null && d.GetType() != listenerBeingAdded.GetType()) {
            throw new ListenerException(string.Format("Attempting to add listener with inconsistent signature for event type {0}. Current listeners have type {1} and listener being added has type {2}", eventType, d.GetType().Name, listenerBeingAdded.GetType().Name));
        }
    }
 
    static public void OnListenerRemoving(string eventType, Delegate listenerBeingRemoved) {
#if LOG_ALL_MESSAGES
        Debug.Log("MESSENGER OnListenerRemoving \t\"" + eventType + "\"\t{" + listenerBeingRemoved.Target + " -> " + listenerBeingRemoved.Method + "}");
#endif
 
        if (eventTable.ContainsKey(eventType)) {
            Delegate d = eventTable[eventType];
 
            if (d == null) {
                throw new ListenerException(string.Format("Attempting to remove listener with for event type \"{0}\" but current listener is null.", eventType));
            } else if (d.GetType() != listenerBeingRemoved.GetType()) {
                throw new ListenerException(string.Format("Attempting to remove listener with inconsistent signature for event type {0}. Current listeners have type {1} and listener being removed has type {2}", eventType, d.GetType().Name, listenerBeingRemoved.GetType().Name));
            }
        } else {
            throw new ListenerException(string.Format("Attempting to remove listener for type \"{0}\" but Messenger doesn't know about this event type.", eventType));
        }
    }
 
    static public void OnListenerRemoved(string eventType) {
        if (eventTable[eventType] == null) {
            eventTable.Remove(eventType);
        }
    }
 
    static public void OnBroadcasting(string eventType) {
#if REQUIRE_LISTENER
        if (!eventTable.ContainsKey(eventType)) {
            throw new BroadcastException(string.Format("Broadcasting message \"{0}\" but no listener found. Try marking the message with Messenger.MarkAsPermanent.", eventType));
        }
#endif
    }
 
    static public BroadcastException CreateBroadcastSignatureException(string eventType) {
        return new BroadcastException(string.Format("Broadcasting message \"{0}\" but listeners have a different signature than the broadcaster.", eventType));
    }
 
    public class BroadcastException : Exception {
        public BroadcastException(string msg)
            : base(msg) {
        }
    }
 
    public class ListenerException : Exception {
        public ListenerException(string msg)
            : base(msg) {
        }
    }
    #endregion
 
    #region AddListener
    //No parameters
    static public void AddListener(string eventType, Callback handler) {
        OnListenerAdding(eventType, handler);
        eventTable[eventType] = (Callback)eventTable[eventType] + handler;
    }
 
    //Single parameter
    static public void AddListener<T>(string eventType, Callback<T> handler) {
        OnListenerAdding(eventType, handler);
        eventTable[eventType] = (Callback<T>)eventTable[eventType] + handler;
    }
 
    //Two parameters
    static public void AddListener<T, U>(string eventType, Callback<T, U> handler) {
        OnListenerAdding(eventType, handler);
        eventTable[eventType] = (Callback<T, U>)eventTable[eventType] + handler;
    }
 
    //Three parameters
    static public void AddListener<T, U, V>(string eventType, Callback<T, U, V> handler) {
        OnListenerAdding(eventType, handler);
        eventTable[eventType] = (Callback<T, U, V>)eventTable[eventType] + handler;
    }
    #endregion
 
    #region RemoveListener
    //No parameters
    static public void RemoveListener(string eventType, Callback handler) {
        OnListenerRemoving(eventType, handler);   
        eventTable[eventType] = (Callback)eventTable[eventType] - handler;
        OnListenerRemoved(eventType);
    }
 
    //Single parameter
    static public void RemoveListener<T>(string eventType, Callback<T> handler) {
        OnListenerRemoving(eventType, handler);
        eventTable[eventType] = (Callback<T>)eventTable[eventType] - handler;
        OnListenerRemoved(eventType);
    }
 
    //Two parameters
    static public void RemoveListener<T, U>(string eventType, Callback<T, U> handler) {
        OnListenerRemoving(eventType, handler);
        eventTable[eventType] = (Callback<T, U>)eventTable[eventType] - handler;
        OnListenerRemoved(eventType);
    }
 
    //Three parameters
    static public void RemoveListener<T, U, V>(string eventType, Callback<T, U, V> handler) {
        OnListenerRemoving(eventType, handler);
        eventTable[eventType] = (Callback<T, U, V>)eventTable[eventType] - handler;
        OnListenerRemoved(eventType);
    }
    #endregion
 
    #region Broadcast
    //No parameters
    static public void Broadcast(string eventType) {
#if LOG_ALL_MESSAGES || LOG_BROADCAST_MESSAGE
        Debug.Log("MESSENGER\t" + System.DateTime.Now.ToString("hh:mm:ss.fff") + "\t\t\tInvoking \t\"" + eventType + "\"");
#endif
        OnBroadcasting(eventType);
 
        Delegate d;
        if (eventTable.TryGetValue(eventType, out d)) {
            Callback callback = d as Callback;
 
            if (callback != null) {
                callback();
            } else {
                throw CreateBroadcastSignatureException(eventType);
            }
        }
    }
 
    //Single parameter
    static public void Broadcast<T>(string eventType, T arg1) {
#if LOG_ALL_MESSAGES || LOG_BROADCAST_MESSAGE
        Debug.Log("MESSENGER\t" + System.DateTime.Now.ToString("hh:mm:ss.fff") + "\t\t\tInvoking \t\"" + eventType + "\"");
#endif
        OnBroadcasting(eventType);
 
        Delegate d;
        if (eventTable.TryGetValue(eventType, out d)) {
            Callback<T> callback = d as Callback<T>;
 
            if (callback != null) {
                callback(arg1);
            } else {
                throw CreateBroadcastSignatureException(eventType);
            }
        }
    }
 
    //Two parameters
    static public void Broadcast<T, U>(string eventType, T arg1, U arg2) {
#if LOG_ALL_MESSAGES || LOG_BROADCAST_MESSAGE
        Debug.Log("MESSENGER\t" + System.DateTime.Now.ToString("hh:mm:ss.fff") + "\t\t\tInvoking \t\"" + eventType + "\"");
#endif
        OnBroadcasting(eventType);
 
        Delegate d;
        if (eventTable.TryGetValue(eventType, out d)) {
            Callback<T, U> callback = d as Callback<T, U>;
 
            if (callback != null) {
                callback(arg1, arg2);
            } else {
                throw CreateBroadcastSignatureException(eventType);
            }
        }
    }
 
    //Three parameters
    static public void Broadcast<T, U, V>(string eventType, T arg1, U arg2, V arg3) {
#if LOG_ALL_MESSAGES || LOG_BROADCAST_MESSAGE
        Debug.Log("MESSENGER\t" + System.DateTime.Now.ToString("hh:mm:ss.fff") + "\t\t\tInvoking \t\"" + eventType + "\"");
#endif
        OnBroadcasting(eventType);
 
        Delegate d;
        if (eventTable.TryGetValue(eventType, out d)) {
            Callback<T, U, V> callback = d as Callback<T, U, V>;
 
            if (callback != null) {
                callback(arg1, arg2, arg3);
            } else {
                throw CreateBroadcastSignatureException(eventType);
            }
        }
    }
    #endregion
}
 
//This manager will ensure that the messenger's eventTable will be cleaned up upon loading of a new level.
public sealed class MessengerHelper : MonoBehaviour {
    void Awake ()
    {
        DontDestroyOnLoad(gameObject);    
    }
 
    //Clean up eventTable every time a new level loads.
    public void OnLevelWasLoaded(int unused) {
        Messenger.Cleanup();
    }
}

高级C#信使（译） - Unity维基百科