Unity使用C++作为游戏逻辑脚本的研究（二）

文章申明：本文来自JacksonDunstan的博客系列文章内容摘取和翻译，版权归其所有，附上原文的链接，大家可以有空阅读原文：C++ Scripting( in Unity)

上一篇文章写完，有同学觉得有点晦涩，其实可以多认真看两遍源码，仔细琢磨一下，就会有一种茅塞顿开的感觉：D。今天继续上文，深入讨论一下C++作为游戏脚本的研究，本文会较长，需要写一些示例代码做讲解。

一、对C#指针(引用)的封装

在上文，我们提到，C++对C#的调用，是基于C#的函数指针（引用）而来的，比如在C++中：

//return transform handle  || function pointer name  || take a handle to the go
int32_t                    (*GameObjectGetTransform)   (int32_t thiz);

为了拓展性，我们都会倾向于对于这种int32_t类型的数据做一个封装，自然容易想到用一个结构体（结构体默认为public）

namespace System
{
     struct Object
     {
         int32_t Handle;
     }
}

利用继承的特点，我们可以延伸出其他类型的结构体定义：

namespace UnityEngine
{
     struct Vector3 {float x; float y; float z;};

     struct Transform:System::Object
     {
         void SetPosition(Vector3 val)
         {
               TransformSetPosition(Handle, val);
         }
     }

     struct GameObject:System::Object
     {
          GameObject()
          {
               Handle = GameObjectNew();
          }

          Transform GetPosition()
          {
                Transform transform;
                transform.Handle = GameObjectGetTransform(Handle);
                return transform;
          }
     }
}

二、对内存管理的控制

在C#部分，对于托管部分，是基于垃圾自动回收机制的，对于C++部分，相对较为简单的回收，可以基于计数的回收机制，当对象的引用计数为零的时候执行垃圾回收，那么对于我们可以定义两个全局变量来做相关的计数统计：

//global
int32_t managedObjectsRefCountLen;
int32_t *managedObjectsRefCounts;

//.....

//init
managedObjectsRefCountLen = maxManagedObjects;//c#会传入该数据
managedObjectsRefCounts = (int32_t*)calloc(maxManagedObjects, sizeof(int32_t));

这样在GameObject的初始化和解析的时候可以执行相关的内存管理操作：

GameObject()
{
     Handle = GameObjectNew();
     managedObjectsRefCounts[Handle]++;
}

~GameObject()
{
     if(--managedObjectsRefCounts[Handle] == )
     {
         ReleaseObject(Handle);
     }
}

对于其他的结构体，可以利用宏定义来实现类似的结构体定义中的操作。综上，可以实现在传递的时候对int32_t类型数据的封装，其次可以内嵌内存操作。整体代码对于c#的修改不多，对于C++的修改较多。

三、代码部分

对于c#部分的代码，基本不修改，只是修改一下Init函数，添加内存管理相关的数据和函数，具体代码如下：

....
//初始化函数及相关委托的修改
public delegate void InitDelegate(int maxManagedObjects, IntPtr releaseObject,
IntPtr gameObjectNew, IntPtr gameObjectGetTransform, IntPtr transformSetPosition);
....
...
#if UNITY_EDITOR_OSX || UNITY_EDITOR_LINUX
...
#elif UNITY_EDITOR_WIN
...
#else
    [DllImport("NativeScript")]
    static extern void Init(int maxManagedObjects, IntPtr releaseObject,
    IntPtr gameObjectNew, IntPtr gameObjectGetTransform,
    IntPtr transformSetPosition);
...
#endif

//新增释放
delegate void ReleaseObjectDelegate(int handle);
...
//修改Awake函数中对于初始化的操作
    void Awake()
    {
      ...

      //init c++ libraray
      const int maxManagedObjects = ;
      ObjectStore.Init(maxManagedObjects);
      Init(maxManagedObjects,
      Marshal.GetFunctionPointerForDelegate(new ReleaseObjectDelegate(ReleaseObject)),
      Marshal.GetFunctionPointerForDelegate(new GameObjectNewDelegate(GameObjectNew)),
      Marshal.GetFunctionPointerForDelegate(new GameObjectGetTransformDelegate(GameObjectGetTransform)),
      Marshal.GetFunctionPointerForDelegate(new TransformSetPositionDelegate(TransformSetPosition)));
    }

...

//c# function for c++ to call
static void ReleaseObject(int handle)
{
    ObjectStore.Remove(handle);
}
...

C++部分的代码修改较多，我就copy一下作者的工程源码吧 ：D

// For assert()
#include <assert.h>

// For int32_t, etc.
#include <stdint.h>

// For malloc(), etc.
#include <stdlib.h>

// For std::forward
#include <utility>

// Macro to put before functions that need to be exposed to C#
#ifdef _WIN32
    #define DLLEXPORT extern "C" __declspec(dllexport)
#else
    #define DLLEXPORT extern "C"
#endif

////////////////////////////////////////////////////////////////
// C# struct types
////////////////////////////////////////////////////////////////

namespace UnityEngine
{
    struct Vector3
    {
        float x;
        float y;
        float z;

        Vector3()
            : x(0.0f)
            , y(0.0f)
            , z(0.0f)
        {
        }

        Vector3(
            float x,
            float y,
            float z)
            : x(x)
            , y(y)
            , z(z)
        {
        }
    };
}

////////////////////////////////////////////////////////////////
// C# functions for C++ to call
////////////////////////////////////////////////////////////////

namespace Plugin
{
    using namespace UnityEngine;

    void (*ReleaseObject)(
        int32_t handle);

    int32_t (*GameObjectNew)();

    int32_t (*GameObjectGetTransform)(
        int32_t thiz);

    void (*TransformSetPosition)(
        int32_t thiz,
        Vector3 val);
}

////////////////////////////////////////////////////////////////
// Reference counting of managed objects
////////////////////////////////////////////////////////////////

namespace Plugin
{
    int32_t managedObjectsRefCountLen;
    int32_t* managedObjectRefCounts;

    void ReferenceManagedObject(int32_t handle)
    {
        assert(handle >=  && handle < managedObjectsRefCountLen);
        if (handle != )
        {
            managedObjectRefCounts[handle]++;
        }
    }

    void DereferenceManagedObject(int32_t handle)
    {
        assert(handle >=  && handle < managedObjectsRefCountLen);
        if (handle != )
        {
            int32_t numRemain = --managedObjectRefCounts[handle];
            if (numRemain == )
            {
                ReleaseObject(handle);
            }
        }
    }
}

////////////////////////////////////////////////////////////////
// Mirrors of C# types. These wrap the C# functions to present
// a similiar API as in C#.
////////////////////////////////////////////////////////////////

namespace System
{
    struct Object
    {
        int32_t Handle;

        Object(int32_t handle)
        {
            Handle = handle;
            Plugin::ReferenceManagedObject(handle);
        }

        Object(const Object& other)
        {
            Handle = other.Handle;
            Plugin::ReferenceManagedObject(Handle);
        }

        Object(Object&& other)
        {
            Handle = other.Handle;
            other.Handle = ;
        }
    };
 //宏定义操作
#define SYSTEM_OBJECT_LIFECYCLE(ClassName, BaseClassName) \
    ClassName(int32_t handle) \
        : BaseClassName(handle) \
    { \
    } \
    \
    ClassName(const ClassName& other) \
        : BaseClassName(other) \
    { \
    } \
    \
    ClassName(ClassName&& other) \
        : BaseClassName(std::forward<ClassName>(other)) \
    { \
    } \
    \
    ~ClassName() \
    { \
        DereferenceManagedObject(Handle); \
    } \
    \
    ClassName& operator=(const ClassName& other) \
    { \
        DereferenceManagedObject(Handle); \
        Handle = other.Handle; \
        ReferenceManagedObject(Handle); \
        return *this; \
    } \
    \
    ClassName& operator=(ClassName&& other) \
    { \
        DereferenceManagedObject(Handle); \
        Handle = other.Handle; \
        other.Handle = ; \
        return *this; \
    }
}

namespace UnityEngine
{
    using namespace System;
    using namespace Plugin;

    struct GameObject;
    struct Component;
    struct Transform;

    struct GameObject : Object
    {
        SYSTEM_OBJECT_LIFECYCLE(GameObject, Object)
        GameObject();
        Transform GetTransform();
    };

    struct Component : Object
    {
        SYSTEM_OBJECT_LIFECYCLE(Component, Object)
    };

    struct Transform : Component
    {
        SYSTEM_OBJECT_LIFECYCLE(Transform, Component)
        void SetPosition(Vector3 val);
    };

    GameObject::GameObject()
        : GameObject(GameObjectNew())
    {
    }

    Transform GameObject::GetTransform()
    {
        return Transform(GameObjectGetTransform(Handle));
    }

    void Transform::SetPosition(Vector3 val)
    {
        TransformSetPosition(Handle, val);
    }
}

////////////////////////////////////////////////////////////////
// C++ functions for C# to call
////////////////////////////////////////////////////////////////

// Init the plugin
DLLEXPORT void Init(
    int32_t maxManagedObjects,
    void (*releaseObject)(int32_t),
    int32_t (*gameObjectNew)(),
    int32_t (*gameObjectGetTransform)(int32_t),
    void (*transformSetPosition)(int32_t, UnityEngine::Vector3))
{
    using namespace Plugin;

    // Init managed object ref counting
    managedObjectsRefCountLen = maxManagedObjects;
    managedObjectRefCounts = (int32_t*)calloc(
        maxManagedObjects,
        sizeof(int32_t));

    // Init pointers to C# functions
    ReleaseObject = releaseObject;
    GameObjectNew = gameObjectNew;
    GameObjectGetTransform = gameObjectGetTransform;
    TransformSetPosition = transformSetPosition;
}

// Called by MonoBehaviour.Update
DLLEXPORT void MonoBehaviourUpdate()
{
    using namespace UnityEngine;

    static int32_t numCreated = ;
    if (numCreated < )
    {
        GameObject go;
        Transform transform = go.GetTransform();
        float comp = (float)numCreated;
        Vector3 position(comp, comp, comp);
        transform.SetPosition(position);
        numCreated++;
    }
}

四、c#和Unity API 的导出

写到上面部分，基本对于c#和c++之间的操作有一个整体的较为完整的讲解，还有一个没有提起，那就是，怎么将 unity 的API导出给C++使用呢？作者给出了一个导出方式：JSON导出这让熟悉c#导出到lua的同学可以发现异曲同工之妙，其基本的导出设计为：

{
    "Assemblies": [
        {
            "Path": "/Applications/Unity/Unity.app/Contents/Managed/UnityEngine.dll",
            "Types": [
                {
                    "Name": "UnityEngine.Object",
                    "Constructors": [],
                    "Methods": [],
                    "Properties": [],
                    "Fields": []
                },
                {
                    "Name": "UnityEngine.GameObject",
                    "Constructors": [
                        {
                            "Types": []
                        }
                    ],
                    "Properties": [ "transform" ],
                    "Fields": []
                },
                {
                    "Name": "UnityEngine.Component",
                    "Constructors": [],
                    "Methods": [],
                    "Properties": [],
                    "Fields": []
                },
                {
                    "Name": "UnityEngine.Transform",
                    "Constructors": [],
                    "Methods": [],
                    "Properties": [ "position" ],
                    "Fields": []
                }
            ]
        }
    ]
}

整体设计简介易懂，当然，并不是所有的c#特性都可以被导出，json的导出不支持：Array/out and ref/ delegate/ generic functions and types/ struct types，不知后期作者是否考虑扩展对这些不兼容的特效的导出。

使用json导出，整体的修改和使用非常简单，比如对Component，需要添加对其transform特性的导出，那么只需要修改为：

"Properties":["transform"]

那么，保存后重新导出，就可以得到transform特性。

此外，对于.Net的一些API， 也可以使用JSON导出的方式：

{
    "Path": "/Applications/Unity/Unity.app/Contents/Mono/lib/mono/unity/System.dll",
    "Types": [
        {
            "Name": "System.Diagnostics.Stopwatch",
            "Constructors": [
                {
                    "Types": []
                }
            ],
            "Methods": [
                {
                    "Name": "Start",
                    "Types": []
                },
                {
                    "Name": "Reset",
                    "Types": []
                }
            ],
            "Properties": [ "ElapsedMilliseconds" ],
            "Fields": []
        }
    ]
}

基于上面的各个部分，整体的游戏工程，可以分为2个部分：逻辑代码部分和binding相关的部分，作者给出的工程规划：

Assets
|- Game.cpp                  // Game-specific code. Can rename this file, add headers, etc.
|- NativeScriptTypes.json    // JSON describing which .NET types the game wants to expose to C++
|- NativeScriptConstants.cs  // Game-specific constants such as plugin names and paths
|- NativeScript/             // C++ scripting system. Drop this into your project.
   |- Editor/
      |- GenerateBindings.cs // Code generator
   |- Bindings.cs            // C# code to expose functionality to C++
   |- ObjectStore.cs         // Object handles system
   |- Bindings.h             // C++ wrapper types for C# (declaration)
   |- Bindings.cpp           // C++ wrapper types for C# (definition)
   |- BootScript.cs          // MonoBehaviour to boot up the C++ plugin
   |- BootScene.unity        // Scene with just BootScript on an empty GameObject

对于NativeScript来说，相当于基本的binding相关的东西，对于任何工程都适用，对于其他部分，则根据具体的工程来设计。基于这样的设计，需要做到三个基本规范：

1、需要定义一个全局的类：

public static class NativeScriptConstants
{
    /// <summary>
    /// Name of the plugin used by [DllImport] when running outside the editor
    /// </summary>
    public const string PluginName = "NativeScript";

    /// <summary>
    /// Path to load the plugin from when running inside the editor
    /// </summary>
#if UNITY_EDITOR_OSX
    public const string PluginPath = "/NativeScript.bundle/Contents/MacOS/NativeScript";
#elif UNITY_EDITOR_LINUX
    public const string PluginPath = "/NativeScript.so";
#elif UNITY_EDITOR_WIN
    public const string PluginPath = "/NativeScript.dll";
#endif

    /// <summary>
    /// Maximum number of simultaneous managed objects that the C++ plugin uses
    /// </summary>
    public const int MaxManagedObjects = ;

    /// <summary>
    /// Path within the Unity project to the exposed types JSON file
    /// </summary>
    public const string ExposedTypesJsonPath = "NativeScriptTypes.json";
}

2、NativeScriptConstants.ExposedTypesJsonPath需要指向前面所提到的json导出文件；

3、在C++代码部分，需要定义2个函数用来执行相关的更新

// Called when the plugin is initialized
void PluginMain()
{
}

// Called for MonoBehaviour.Update
void PluginUpdate()
{
}

最后，整体的工程可以在github上找到，给出工程的链接：

jacksondunstan/UnityNativeScripting

Over!