基于V8引擎的C++和JS的相互交互

1. 脚本引擎的基本功能

V8只是一个JS引擎。去除它的特点功能出处，它必须要实现JS引擎的几个基础功能：

• 脚本执行：
• 脚本可能是一个表达式；一段js代码；或者一个文件
• 执行表达式返回js表达式对应的值
  
• C++来取设JS的内容
• 获取JS内容的数据(包括基础数据类型、数组、日期等)、对象（类的一个实例）、类或函数
• 设置JS内容的数据
  
• JS来取设C++的内容
• C++为js动态添加类（例如：Date，可以通过new Date()来创建任意多个对象）
• C++为js动态添加全局对象（例如：Math，可以直接调用其全局方法如Math.min）

我们的目的是先学会怎么用，再去想为什么！

2. V8 脚本引擎基本功能实现

带着这几个目的去使用V8，找遍网上资料，发现很少有覆盖周全的。其实翻来覆去就是那几个资料，理论派居多；最可气的是按照那些例子去跑，怎么都会运行错误；而且在关键的地方全按照google伪代码例子照搬。不过理论派的阐述确实很一本正经；很精辟。也感谢先行者！

2. 1 V8之脚本运行

2.1.1 获取字符串：

void test_String()
{
    Handle<String> source = String::New("'Hello' + ', World!'");
    Handle<Script> script = Script::Compile(source);
    Handle<Value> result = script->Run();

    String::AsciiValue ascii(result);
    printf("%s\n", *ascii);
}
>>> Hello，World!

2.1.2 获取数组：

void test_Array()
{
    Handle<String> source = String::New("[1, 2, 'hello', 6+5]");
    Handle<Script> script = Script::Compile(source);
    Handle<Value> result = script->Run();

    String::AsciiValue ascii(result);
    printf("%s\n", *ascii);
}
>>> 1,2,hello,11

2. 2 V8之C++取设JS

2.2.1 获取成员之数据

void test_getjs_data(Handle<Context> pContext)
{
    Handle<String> source = String::New("var s1 = 8+5;");
    Handle<Script> script = Script::Compile(source);
    Handle<Value> result = script->Run();
    Handle<String> js_data = String::New("s1");
    Handle<Value> js_data_value = pContext->Global()->Get(js_data);

    String::AsciiValue ascii(js_data_value);
    printf("%s\n", *ascii);
}
>>> 13

2.2.2 获取成员之全局对象

void test_getjs_dataofObject(Handle<Context> pContext)
{
    Handle<String> source = String::New("function Point(x,y){this.x=x; this.y=y;} var pt=new Point(10,20);");
    Handle<Script> script = Script::Compile(source);
    Handle<Value> result = script->Run();

    Handle<String> js_data = String::New("pt");
    Handle<Value> js_data_value = pContext->Global()->Get(js_data);

    // Convert the result to an ASCII string and print it.
    {
        String::AsciiValue ascii(js_data_value);
        printf("pt = %s\n", *ascii);
    }
    Handle<Object> js_data_object = Handle<Object>::Cast(js_data_value);

    Handle<Value> key = String::New("x");
    Handle<Value> objX = js_data_object->Get(key);
    {
        String::AsciiValue ascii(objX);
        printf("pt.x = %s\n", *ascii);
    }
}
>>> pt = [object Object]
pt.x = 10

2.2.3 获取js 类

void test_getjs_dataofObjectClass(Handle<Context> pContext)
{
    Handle<String> source = String::New("function Point(x,y){this.x=x; this.y=y;} Point.prototype.show=function(){return '(x,y) = '+this.x+','+this.y;}");
    Handle<Script> script = Script::Compile(source);
    Handle<Value> result = script->Run();

    Handle<String> js_data = String::New("Point");
    Handle<Value> js_data_value = pContext->Global()->Get(js_data);

    // Convert the result to an ASCII string and print it.
    {
        String::AsciiValue ascii(js_data_value);
        printf("Point = %s\n", *ascii);
    }

    bool bIsFunction = js_data_value->IsFunction();
    if(bIsFunction)
    {
        printf("Point is function\n");
    }

    bool bIsObject = js_data_value->IsObject();
    if(bIsObject)
    {
        printf("Point is object\n");
    }

    Handle<Object> js_data_object = Handle<Object>::Cast(js_data_value);
    // var newObj = new Point(1,2);
    Handle<Value> argv[2] ;
    argv[0] = Int32::New(1);
    argv[1] = Int32::New(2);
    Handle<Value> newObj = js_data_object->CallAsConstructor(2, argv);
    {
        bool bIsFunction = newObj->IsFunction();
        if(bIsFunction) //-false-
        {
            printf("newObj is function\n");
        }

        bool bIsObject = newObj->IsObject();
        if(bIsObject) //-true-
        {
            printf("newObj is object\n");
        }
    }

    // newObj.show();
    {
        Handle<Object> obj = Handle<Object>::Cast(newObj);
        Handle<String> js_func_name = String::New("show");
        Handle<Value> js_func_ref = obj->Get(js_func_name);

        Handle<Function> js_func = Handle<Function>::Cast(js_func_ref);
        js_data_value = js_func->Call(obj, 0, NULL) ;

        String::AsciiValue ascii(js_data_value);
        printf("newObj.show() = %s\n", *ascii);
    }
}
>>> Point = function Point(x,y){this.x=x; this.y=y;}
Point is function
Point is object
newObj is object
newObj.show() = (x,y) = 1,2

2. 3 V8之JS取设C++

2.3.0 js 和 C++的关联

c++ 和 js对应。特别是为达到能在Js中使用var obj = new CObject()，参照CreateObjectToJs方法

void test_getc_loadObjectTemplate(Handle<ObjectTemplate> pObj)
{
    //-load c++'s data-
    pObj->SetAccessor(String::New("x"), XGetter, XSetter);

    //-load c++'s function-
    pObj->Set(String::New("setColor"),FunctionTemplate::New(set_color));

    //-load c++'s class-
    CreateObjectToJs(pObj);
}


Point* NewPointFunction(const Arguments & args)
{
    if(args.Length()==2)
    {
        Local<Value> v1 = args[0];
        Local<Value> v2 = args[1];

        return new Point( v1->Int32Value(), v2->Int32Value() );
    }
    else
        return new Point();
}

void PointWeakExternalReferenceCallback(Persistent<Value>, void* parameter)
{
    if (Point* cpp_object = static_cast<Point*>(parameter))
        delete cpp_object;
}

Persistent<External> NewWeakExternalPoint(void* parameter)
{
    Persistent<External> ret = Persistent<External>::New(External::New(parameter));
    ret.MakeWeak(parameter, PointWeakExternalReferenceCallback);
    return ret;
}

Handle<Value> PointFunctionInvocationCallback(const Arguments &args)
{
    if (!args.IsConstructCall())
        return Undefined();

    Point* cpp_object = NewPointFunction(args);
    if (!cpp_object)
        return ThrowException(String::New("Can not create Object in C++"));

    args.Holder()->SetInternalField(0, NewWeakExternalPoint(cpp_object));
    return Undefined();
}


void CreateObjectToJs(Handle<ObjectTemplate> pObj)
{
    Point* p = new Point(0, 0);
    Handle<FunctionTemplate> point_templ = FunctionTemplate::New(&PointFunctionInvocationCallback, External::New(p));
    point_templ->SetClassName(String::New("Point"));
    point_templ->InstanceTemplate()->SetInternalFieldCount(1);

    Handle<ObjectTemplate> point_proto = point_templ->PrototypeTemplate();
    point_proto->SetAccessor(String::New("x"), GetPointX, SetPointX);
    point_proto->SetAccessor(String::New("y"), GetPointY, SetPointY);
    point_proto->Set(String::New("show"), FunctionTemplate::New(ShowPoint));

    pObj->Set(String::New("Point"), point_templ);
}

2.3.1 全局函数

c++: 

Handle<Value> set_color(const Arguments & args)
{
    Handle<Value> rtn;
    if(args.Length() == 3)
    {
        int r = args[0]->Int32Value();
        int g = args[1]->Int32Value();
        int b = args[2]->Int32Value();

        char szTmp[80] = "";
        _stprintf(szTmp,"RGB%2X%02X%02X", r,g,b);
        rtn = String::New(szTmp);
    }
    else
        rtn = Undefined();

    return  rtn;
}

JS：

void test_getc_function(Handle<Context> pContext, Handle<ObjectTemplate> pObj)
{
    Handle<String> source = String::New("var data = setColor(255,128,0);");
    Handle<Script> script = Script::Compile(source);
    Handle<Value> result = script->Run();

    Handle<String> js_data = String::New("data");
    Handle<Value> js_data_value = pContext->Global()->Get(js_data);

    // Convert the result to an ASCII string and print it.
    String::AsciiValue ascii(js_data_value);
    printf("%s\n", *ascii);
}
>>> RGBFF8000

2.3.2 全局数据

c++:

int x = 0;
Handle<Value> XGetter(Local<String> key,const AccessorInfo& info)
{
    return Integer::New(x);
}

void XSetter(Local<String> key, Local<Value> value,const AccessorInfo& info)
{
    x = value->Int32Value();
}

JS:

void test_getc_data(Handle<Context> pContext, Handle<ObjectTemplate> pObj)
{
    Handle<String> source = String::New("var data1 = x; x=200; var data2=x;");
    Handle<Script> script = Script::Compile(source);
    Handle<Value> result = script->Run();

    {
        Handle<String> js_data = String::New("data1");
        Handle<Value> js_data_value = pContext->Global()->Get(js_data);
        String::AsciiValue ascii(js_data_value);
        printf("data1 = %s\n", *ascii);
    }

    {
        Handle<String> js_data = String::New("data2");
        Handle<Value> js_data_value = pContext->Global()->Get(js_data);
        String::AsciiValue ascii(js_data_value);
        printf("data2 = %s\n", *ascii);
    }
}
>>> data1 = 0
data2 = 200

2.3.3 类

c++:

struct Point
{
    Point()
    {
        x_ = 0;
        y_ = 0;
    }
    Point(int x, int y)
    {
        x_ = x;
        y_ = y;
    }

    int getX() const {
        return x_;
    }
    int getY() const { return y_; }
    void setX(int value) { x_ = value; }
    void setY(int value) { y_ = value; }
    bool isNull() const { return x_ == 0 && y_ == 0; }
    void show()
    {
        char szTmp[80] = "";
        _stprintf(szTmp, "x,y = %d,%d\n", x_, y_);
        printf(szTmp);
    }
    int x_, y_;
};


Handle<Value> GetPointX(Local<String> key,const AccessorInfo &info)
{
    Handle<Object> obj = info.This ();
    //Local<Object> self = info.Holder(); //
    Point& point = *static_cast<Point*> (Local<External>::Cast(obj->GetInternalField(0))->Value ());
    int value = point.x_;
    return Integer::New(value);
}

void SetPointX(Local<String> key, Local<Value> value,const AccessorInfo& info)
{
    Handle<Object> obj = info.This ();
    Point& point = *static_cast<Point*> (Local<External>::Cast(obj->GetInternalField(0))->Value ());
    point.x_ = value->Int32Value();
}

Handle<Value> GetPointY(Local<String> key,const AccessorInfo &info)
{
    Local<Object> self = info.Holder();
    Local<External> wrap = Local<External>::Cast(self->GetInternalField(0));
    void* ptr = wrap->Value();
    int value = static_cast<Point*>(ptr)->y_;
    return Integer::New(value);
}

void SetPointY(Local<String> key, Local<Value> value,const AccessorInfo& info)
{
    Local<Object> self = info.Holder();
    Local<External> wrap = Local<External>::Cast(self->GetInternalField(0));
    void* ptr = wrap->Value();
    static_cast<Point*>(ptr)->y_ = value->Int32Value();
}

Handle<Value> ShowPoint(const Arguments& args)
{
    Local<Object> self = args.Holder();
    //Local<Object> self = info.Holder();
    Local<External> wrap = Local<External>::Cast(self->GetInternalField(0));
    void* ptr = wrap->Value();
    static_cast<Point*>(ptr)->show();

    return Undefined();
}

JS:

void test_getc_object(Handle<Context> pContext, Handle<ObjectTemplate> pObj)
{
    test_getobject(pContext);return;

    // Create a string containing the JavaScript source code.
    Handle<String> source = String::New("var pt=new Point(10,20);");

    // Compile the source code.
    Handle<Script> script = Script::Compile(source);

    // Run the script to get the result.
    Handle<Value> result = script->Run();

    Handle<String> js_data = String::New("pt");
    Handle<Value> js_data_value = pContext->Global()->Get(js_data);

    // Convert the result to an ASCII string and print it.
    {
        String::AsciiValue ascii(js_data_value);
        printf("pt = %s\n", *ascii);
    }

    Handle<Object> js_data_object = Handle<Object>::Cast(js_data_value);

    Handle<Value> key = String::New("x");
    Handle<Value> objX = js_data_object->Get(key);
    {
        String::AsciiValue ascii(objX);
        printf("pt.x = %s\n", *ascii);
    }
}
>>> Point = function Point() { [native code] }
Point is function
Point is object
newObj is object
src obj.x = 3
last obj.x = 30
begin newObj.show() : x,y = 30,4
newObj.show() = undefined

3. 教训

• 在VS200X C++中，请都使用/MDD 编译选项
• 对于类，需要绑定一个构造函数，主要目的是用来生成C++对象。该对象千万不要使用局部变量，而要是指针
• 对于C++扩展的类，通过FunctionTemplate来实现；而不要使用ObjectTemplate
• 对于JS中的全局对象，可以通过ObjectTemplate来实现。实际上就是绑定全局API和变量