1 . 如何在NPClass中存储自己的自定义数据?

只要实现NPClass的allocate和deallocate两个方法，并定义自己的扩展NPClass结构便可

static struct NPClass nativeScriptablePluginClass = {
    NP_CLASS_STRUCT_VERSION,
    JSCLASS_alloc,
    JSCLASS_free,
    NULL,
    JSCLASS_hasMethod,
    JSCLASS_invoke,
    NULL,
    JSCLASS_hasProperty,
    JSCLASS_getProperty,
    NULL,
    NULL,
};

//定义自己的扩展结构体
typedef struct MyScriptPluginClass MyScriptPluginClass;
struct MyScriptPluginClass {
    NPClass nativeClass;
    NPUTF8 *someStringData; //这是我们的自定义数据
};

//在自定义的alloc方法中返回为自定义NPClass申请的内存
NPObject* JSCLASS_alloc(NPP inst, NPClass *aClass) {
    return (NPObject *)malloc(sizeof(struct MyScriptPluginClass));
}

//相应的释放
void JSCLASS_free(NPObject *nobj) {
    //之后都可以通过种类型强转得到我们自定义的结构体
    MyScriptPluginClass *obj = (MyScriptPluginClass *)nobj;
    free(obj->someStringData);
    free(obj);
}

2 . 如何在插件中调用页面中的js方法?

//这里我们将实现一个可以调用window.eval的方法
void JSAPI_methodImpl_getLocation(NPObject *o) {
    NPString rt;
    MyScriptPluginClass *obj = (MyScriptPluginClass *)o;
    NPP _npp = obj->instance;
    NPObject *window = NULL;
    //static NPNetscapeFuncs* browser;
    //获取window对象
    browser->getvalue(_npp, NPNVWindowNPObject, &window);
    NPIdentifier eval = browser->getstringidentifier("eval");
    NPVariant evalResponse;
    NPVariant evalCode;

    //初始化我们需要eval的代码
    char *c_javascriptCode = calloc(2048, 1);
    sprintf(c_javascriptCode, "(function(){"
            "return window.location.href;\n"
            "})();");
    STRINGZ_TO_NPVARIANT(c_javascriptCode, evalCode);
    NPVariant args[] = {evalCode};

    //window.eval(mycode);
    browser->invoke(_npp, window, eval, args, 1, &evalResponse);

    free(c_javascriptCode);

    //判断一下js的返回值是否为我们期望的字符串类型
    if (NPVARIANT_IS_STRING(evalResponse)) {
        //得到我们想要的返回值
        rt = NPVARIANT_TO_STRING(evalResponse);
        fprintf(stdout, "window.location = %.*s", rt.UTF8Length, rt.UTF8Characters);
    }

    browser->releaseobject(window);
    browser->releasevariantvalue(&evalResponse);
}

3 . 如何管理内存
http://colonelpanic.net/2009/12/memory-management-in-npapi/