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使用Node.js为其他程序编写扩展的基本方法_node.js

程序员文章站 2022-04-13 08:16:01
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准备开始

首先我们用下面的目录结构来创建一个节点通知(node-notify)文件夹.

复制代码 代码如下:

.
|-- build/ # This is where our extension is built.
|-- demo/
| `-- demo.js # This is a demo Node.js script to test our extension.
|-- src/
| `-- node_gtknotify.cpp # This is the where we do the mapping from C++ to Javascript.
`-- wscript # This is our build configuration used by node-waf

这个看起来很漂亮的tree 用通用的 tree 生成.

现在让我来创建测试脚本demo.js 和决定我们扩展的API前期看起来应该像:

// This loads our extension on the notify variable.
// It will only load a constructor function, notify.notification().
var notify = require("../build/default/gtknotify.node"); // path to our extension
 
var notification = new notify.notification();
notification.title = "Notification title";
notification.icon = "emblem-default"; // see /usr/share/icons/gnome/16x16
notification.send("Notification message");

编写我们的Node.js扩展
Init方法

为了创建一个Node.js扩展，我们需要编写一个继承node::ObjectWrap的C++类。 ObjectWrap 实现了让我们更容易与Javascript交互的公共方法

我们先来编写类的基本框架：

#include  // v8 is the Javascript engine used by QNode
#include 
// We will need the following libraries for our GTK+ notification
#include 
#include 
#include 
 
using namespace v8;
 
class Gtknotify : node::ObjectWrap {
 private:
 public:
  Gtknotify() {}
  ~Gtknotify() {}
  static void Init(Handle target) {
   // This is what Node will call when we load the extension through require(), see boilerplate code below.
  }
};
 
/*
 * WARNING: Boilerplate code ahead.
 *
 * See https://www.cloudkick.com/blog/2010/aug/23/writing-nodejs-native-extensions/ & http://www.freebsd.org/cgi/man.cgi?query=dlsym
 * 
 * Thats it for actual interfacing with v8, finally we need to let Node.js know how to dynamically load our code.
 * Because a Node.js extension can be loaded at runtime from a shared object, we need a symbol that the dlsym function can find,
 * so we do the following: 
 */
 
v8::Persistent Gtknotify::persistent_function_template;
extern "C" { // Cause of name mangling in C++, we use extern C here
 static void init(Handle target) {
  Gtknotify::Init(target);
 }
 // @see http://github.com/ry/node/blob/v0.2.0/src/node.h#L101
 NODE_MODULE(gtknotify, init);
}

现在，我们必须把下面的代码编写到我们的Init()方法中：

声明构造函数，并将其绑定到我们的目标变量。var n = require("notification");将绑定notification() 到 n:n.notification().

// Wrap our C++ New() method so that it's accessible from Javascript
  // This will be called by the new operator in Javascript, for example: new notification();
  v8::Local local_function_template = v8::FunctionTemplate::New(New);
   
  // Make it persistent and assign it to persistent_function_template which is a static attribute of our class.
  Gtknotify::persistent_function_template = v8::Persistent::New(local_function_template);
   
  // Each JavaScript object keeps a reference to the C++ object for which it is a wrapper with an internal field.
  Gtknotify::persistent_function_template->InstanceTemplate()->SetInternalFieldCount(1); // 1 since a constructor function only references 1 object
  // Set a "class" name for objects created with our constructor
  Gtknotify::persistent_function_template->SetClassName(v8::String::NewSymbol("Notification"));
   
  // Set the "notification" property of our target variable and assign it to our constructor function
  target->Set(String::NewSymbol("notification"), Gtknotify::persistent_function_template->GetFunction());

声明属性:n.title 和n.icon.
 

  // Set property accessors
  // SetAccessor arguments: Javascript property name, C++ method that will act as the getter, C++ method that will act as the setter
  Gtknotify::persistent_function_template->InstanceTemplate()->SetAccessor(String::New("title"), GetTitle, SetTitle);
  Gtknotify::persistent_function_template->InstanceTemplate()->SetAccessor(String::New("icon"), GetIcon, SetIcon);
  // For instance, n.title = "foo" will now call SetTitle("foo"), n.title will now call GetTitle()
声明原型方法：n.send()
 
  // This is a Node macro to help bind C++ methods to Javascript methods (see https://github.com/joyent/node/blob/v0.2.0/src/node.h#L34)
  // Arguments: our constructor function, Javascript method name, C++ method name
  NODE_SET_PROTOTYPE_METHOD(Gtknotify::persistent_function_template, "send", Send);
现在我们的Init()方法看起来应该是这样的：

// Our constructor
static v8::Persistent persistent_function_template;
 
static void Init(Handle target) {
 v8::HandleScope scope; // used by v8 for garbage collection
 
 // Our constructor
 v8::Local local_function_template = v8::FunctionTemplate::New(New);
 Gtknotify::persistent_function_template = v8::Persistent::New(local_function_template);
 Gtknotify::persistent_function_template->InstanceTemplate()->SetInternalFieldCount(1); // 1 since this is a constructor function
 Gtknotify::persistent_function_template->SetClassName(v8::String::NewSymbol("Notification"));
 
 // Our getters and setters
 Gtknotify::persistent_function_template->InstanceTemplate()->SetAccessor(String::New("title"), GetTitle, SetTitle);
 Gtknotify::persistent_function_template->InstanceTemplate()->SetAccessor(String::New("icon"), GetIcon, SetIcon);
 
 // Our methods
 NODE_SET_PROTOTYPE_METHOD(Gtknotify::persistent_function_template, "send", Send);
 
 // Binding our constructor function to the target variable
 target->Set(String::NewSymbol("notification"), Gtknotify::persistent_function_template->GetFunction());
}
剩下要做的就是编写我们在Init方法中用的C++方法：New,GetTitle,SetTitle,GetIcon,SetIcon,Send

构造器方法: New()

New() 方法创建了我们自定义类的新实例(一个 Gtknotify 对象)，并设置一些初始值，然后返回该对象的 JavaScript 处理。这是 JavaScript 使用 new 操作符调用构造函数的期望行为。

 
std::string title;
std::string icon;
 
// new notification()
static Handle New(const Arguments& args) {
 HandleScope scope;
 Gtknotify* gtknotify_instance = new Gtknotify();
 // Set some default values
 gtknotify_instance->title = "Node.js";
 gtknotify_instance->icon = "terminal";
 
 // Wrap our C++ object as a Javascript object
 gtknotify_instance->Wrap(args.This());
 
 return args.This();
}
getters 和 setters: GetTitle(), SetTitle(), GetIcon(), SetIcon()

下面主要是一些样板代码，可以归结为 C++ 和 JavaScript (v8) 之间的值转换。

// this.title
static v8::Handle GetTitle(v8::Local<:string> property, const v8::AccessorInfo& info) {
 // Extract the C++ request object from the JavaScript wrapper.
 Gtknotify* gtknotify_instance = node::ObjectWrap::Unwrap(info.Holder());
 return v8::String::New(gtknotify_instance->title.c_str());
}
// this.title=
static void SetTitle(Local property, Local value, const AccessorInfo& info) {
 Gtknotify* gtknotify_instance = node::ObjectWrap::Unwrap(info.Holder());
 v8::String::Utf8Value v8str(value);
 gtknotify_instance->title = *v8str;
}
// this.icon
static v8::Handle GetIcon(v8::Local<:string> property, const v8::AccessorInfo& info) {
 // Extract the C++ request object from the JavaScript wrapper.
 Gtknotify* gtknotify_instance = node::ObjectWrap::Unwrap(info.Holder());
 return v8::String::New(gtknotify_instance->icon.c_str());
}
// this.icon=
static void SetIcon(Local property, Local value, const AccessorInfo& info) {
 Gtknotify* gtknotify_instance = node::ObjectWrap::Unwrap(info.Holder());
 v8::String::Utf8Value v8str(value);
 gtknotify_instance->icon = *v8str;
}

原型方法: Send()

首先我们抽取 C++ 对象的 this 引用，然后使用对象的属性来构建通知并显示。

// this.send()
static v8::Handle Send(const Arguments& args) {
 v8::HandleScope scope;
 // Extract C++ object reference from "this"
 Gtknotify* gtknotify_instance = node::ObjectWrap::Unwrap(args.This());
 
 // Convert first argument to V8 String
 v8::String::Utf8Value v8str(args[0]);
 
 // For more info on the Notify library: http://library.gnome.org/devel/libnotify/0.7/NotifyNotification.html
 Notify::init("Basic");
 // Arguments: title, content, icon
 Notify::Notification n(gtknotify_instance->title.c_str(), *v8str, gtknotify_instance->icon.c_str()); // *v8str points to the C string it wraps
 // Display the notification
 n.show();
 // Return value
 return v8::Boolean::New(true);
}

编译扩展

node-waf 是一个构建工具，用来编译 Node 的扩展，这是 waf 的基本封装。构建过程可通过名为 wscript 的文件进行配置。

def set_options(opt):
 opt.tool_options("compiler_cxx")
 
def configure(conf):
 conf.check_tool("compiler_cxx")
 conf.check_tool("node_addon")
 # This will tell the compiler to link our extension with the gtkmm and libnotifymm libraries.
 conf.check_cfg(package='gtkmm-2.4', args='--cflags --libs', uselib_store='LIBGTKMM')
 conf.check_cfg(package='libnotifymm-1.0', args='--cflags --libs', uselib_store='LIBNOTIFYMM')
 
def build(bld):
 obj = bld.new_task_gen("cxx", "shlib", "node_addon")
 obj.cxxflags = ["-g", "-D_FILE_OFFSET_BITS=64", "-D_LARGEFILE_SOURCE", "-Wall"]
 # This is the name of our extension.
 obj.target = "gtknotify"
 obj.source = "src/node_gtknotify.cpp"
 obj.uselib = ['LIBGTKMM', 'LIBNOTIFYMM']

现在我们已经准备好要开始构建了，在*目录下运行如下命令：


node-waf configure && node-waf build

如果一切正常，我们将得到编译过的扩展，位于：./build/default/gtknotify.node ，来试试：

$ node
> var notif = require('./build/default/gtknotify.node');
> n = new notif.notification();
{ icon: 'terminal', title: 'Node.js' }
> n.send("Hello World!");
true

上述的代码将在你的屏幕右上方显示一个通知信息。

打成npm包

这是非常酷的, 但是怎样与Node社区分享你的努力的成果呢? 这才是npm主要的用途: 使它更加容易扩展和分发.

打npm的扩展包是非常简单的. 你所要做的就是在你的*目录中创建一个包含你的扩展信息的文件package.json :

{
 // 扩展的名称 (不要在名称中包含node 或者 js, 这是隐式关键字).
 // 这是通过require() 导入扩展的名称.
 
 "name" : "notify",
 
 // Version should be http://semver.org/ compliant
 
 "version" : "v0.1.0"
 
 // 这些脚本将在调用npm安装和npm卸载的时候运行.
 
 , "scripts" : {
   "preinstall" : "node-waf configure && node-waf build"
   , "preuninstall" : "rm -rf build/*"
  }
 
 // 这是构建我们扩展的相对路径.
 
 , "main" : "build/default/gtknotify.node"
 
 // 以下是可选字段:
 
 , "description" : "Description of the extension...."
 , "homepage" : "https://github.com/olalonde/node-notify"
 , "author" : {
   "name" : "Olivier Lalonde"
   , "email" : "olalonde@gmail.com"
   , "url" : "http://www.syskall.com/"
  }
 , "repository" : {
   "type" : "git"
   , "url" : "https://github.com/olalonde/node-notify.git"
  }
}

关于package.json 格式的更多细节, 可以通过 npm help json 获取文档. 注意 大多数字段都是可选的.


你现在可以在你的*目录中通过运行npm install 来安装你的新的npm包了. 如果一切顺利的话, 应该可以简单的加载你的扩展 var notify = require('你的包名');. 另外一个比较有用的命令式 npm link 通过这个命令你可以创建一个到你开发目录的链接,当你的代码发生变化时不必每次都去安装/卸载.

假设你写了一个很酷的扩展, 你可能想要在*npm库发布到网上. 首先你要先创建一个账户:


$ npm adduser

下一步, 回到你的根目录编码并且运行:

 
$ npm publish

就是这样, 你的包现在已经可以被任何人通过npm install 你的包名命令来安装了.

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