system_assignation/node_modules/node-addon-api/doc/object_wrap.md

Object Wrap

The Napi::ObjectWrap class is used to bind the lifetime of C++ code to a JavaScript object. Once bound, each time an instance of the JavaScript object is created, an instance of the C++ class will also be created. When a method is called on the JavaScript object which is defined as an InstanceMethod, the corresponding C++ method on the wrapped C++ class will be invoked.

In order to create a wrapper it's necessary to extend the Napi::ObjectWrapclass which contains all the plumbing to connect JavaScript code with a C++ object. Classes extending Napi::ObjectWrap can be instantiated from JavaScript using the new operator, and their methods can be directly invoked from JavaScript. The wrap word refers to a way of grouping methods and state of the class because it will be necessary write custom code to bridge each of your C++ class methods.

Example

#include <napi.h>

class Example : public Napi::ObjectWrap<Example> {
  public:
    static Napi::Object Init(Napi::Env env, Napi::Object exports);
    Example(const Napi::CallbackInfo &info);

  private:
    static Napi::FunctionReference constructor;
    double _value;
    Napi::Value GetValue(const Napi::CallbackInfo &info);
    Napi::Value SetValue(const Napi::CallbackInfo &info);
};

Napi::Object Example::Init(Napi::Env env, Napi::Object exports) {
    // This method is used to hook the accessor and method callbacks
    Napi::Function func = DefineClass(env, "Example", {
        InstanceMethod("GetValue", &Example::GetValue),
        InstanceMethod("SetValue", &Example::SetValue)
    });

    // Create a peristent reference to the class constructor. This will allow
    // a function called on a class prototype and a function
    // called on instance of a class to be distinguished from each other.
    constructor = Napi::Persistent(func);
    // Call the SuppressDestruct() method on the static data prevent the calling
    // to this destructor to reset the reference when the environment is no longer
    // available.
    constructor.SuppressDestruct();
    exports.Set("Example", func);
    return exports;
}

Example::Example(const Napi::CallbackInfo &info) : Napi::ObjectWrap<Example>(info) {
    Napi::Env env = info.Env();
    // ...
    Napi::Number value = info[0].As<Napi::Number>();
    this->_value = value.DoubleValue();
}

Napi::FunctionReference Example::constructor;

Napi::Value Example::GetValue(const Napi::CallbackInfo &info){
    Napi::Env env = info.Env();
    return Napi::Number::New(env, this->_value);
}

Napi::Value Example::SetValue(const Napi::CallbackInfo &info){
    Napi::Env env = info.Env();
    // ...
    Napi::Number value = info[0].As<Napi::Number>();
    this->_value = value.DoubleValue();
    return this->GetValue(info);
}

// Initialize native add-on
Napi::Object Init (Napi::Env env, Napi::Object exports) {
    Example::Init(env, exports);
    return exports;
}

// Register and initialize native add-on
NODE_API_MODULE(NODE_GYP_MODULE_NAME, Init)

The above code can be used from JavaScript as follows:

'use strict'

const { Example } = require('bindings')('addon')

const example = new Example(11)
console.log(example.GetValue())
// It prints 11
example.SetValue(19)
console.log(example.GetValue());
// It prints 19

At initialization time, the Napi::ObjectWrap::DefineClass() method must be used to hook up the accessor and method callbacks. It takes a list of property descriptors, which can be constructed via the various static methods on the base class.

When JavaScript code invokes the constructor, the constructor callback will create a new C++ instance and "wrap" it into the newly created JavaScript object.

When JavaScript code invokes a method or a property accessor on the class the corresponding C++ callback function will be executed.

For a wrapped object it could be difficult to distinguish between a function called on a class prototype and a function called on instance of a class. Therefore it is good practice to save a persistent reference to the class constructor. This allows the two cases to be distinguished from each other by checking the this object against the class constructor.

Methods

Constructor

Creates a new instance of a JavaScript object that wraps native instance.

Napi::ObjectWrap(const Napi::CallbackInfo& callbackInfo);

• [in] callbackInfo: The object representing the components of the JavaScript request being made.

Unwrap

Retrieves a native instance wrapped in a JavaScript object.

static T* Napi::ObjectWrap::Unwrap(Napi::Object wrapper);

• [in] wrapper: The JavaScript object that wraps the native instance.

Returns a native instace wrapped in a JavaScript object. Given the Napi:Object, this allows a method to get a pointer to the wrapped C++ object and then reference fields, call methods, etc. within that class. In many cases calling Unwrap is not required, as methods can use the this field for ObjectWrap when running in a method on a class that extends ObjectWrap.

DefineClass

Defnines a JavaScript class with constructor, static and instance properties and methods.

static Napi::Function Napi::ObjectWrap::DefineClass(Napi::Env env,
                                const char* utf8name,
                                const std::initializer_list<PropertyDescriptor>& properties,
                                void* data = nullptr);

• [in] env: The environment in which to construct a JavaScript class.
• [in] utf8name: Null-terminated string that represents the name of the JavaScript constructor function.
• [in] properties: Initializer list of class property descriptor describing static and instance properties and methods of the class. See: Class propertry and descriptor.
• [in] data: User-provided data passed to the constructor callback as data property of the Napi::CallbackInfo.

Returns a Napi::Function representing the constructor function for the class.

DefineClass

Defnines a JavaScript class with constructor, static and instance properties and methods.

static Napi::Function Napi::ObjectWrap::DefineClass(Napi::Env env,
                            const char* utf8name,
                            const std::vector<PropertyDescriptor>& properties,
                            void* data = nullptr);

• [in] env: The environment in which to construct a JavaScript class.
• [in] utf8name: Null-terminated string that represents the name of the JavaScript constructor function.
• [in] properties: Vector of class property descriptor describing static and instance properties and methods of the class. See: Class propertry and descriptor.
• [in] data: User-provided data passed to the constructor callback as data property of the Napi::CallbackInfo.

Returns a Napi::Function representing the constructor function for the class.

StaticMethod

Creates property descriptor that represents a static method of a JavaScript class.

static Napi::PropertyDescriptor Napi::ObjectWrap::StaticMethod(const char* utf8name,
                                       StaticVoidMethodCallback method,
                                       napi_property_attributes attributes = napi_default,
                                       void* data = nullptr);

• [in] utf8name: Null-terminated string that represents the name of a static method for the class.
• [in] method: The native function that represents a static method of a JavaScript class.
• [in] attributes: The attributes associated with a particular property. One or more of napi_property_attributes.
• [in] data: User-provided data passed into method when it is invoked.

Returns Napi::PropertyDescriptor object that represents the static method of a JavaScript class.

StaticMethod

Creates property descriptor that represents a static method of a JavaScript class.

static Napi::PropertyDescriptor Napi::ObjectWrap::StaticMethod(const char* utf8name,
                                       StaticMethodCallback method,
                                       napi_property_attributes attributes = napi_default,
                                       void* data = nullptr);

• [in] utf8name: Null-terminated string that represents the name of a static method for the class.
• [in] method: The native function that represents a static method of a JavaScript class.
• [in] attributes: The attributes associated with a particular property. One or more of napi_property_attributes.
• [in] data: User-provided data passed into method when it is invoked.

Returns Napi::PropertyDescriptor object that represents a static method of a JavaScript class.

StaticMethod

Creates property descriptor that represents a static method of a JavaScript class.

static Napi::PropertyDescriptor Napi::ObjectWrap::StaticMethod(Symbol name,
                                       StaticVoidMethodCallback method,
                                       napi_property_attributes attributes = napi_default,
                                       void* data = nullptr);

• [in] name: Napi:Symbol that represents the name of a static method for the class.
• [in] method: The native function that represents a static method of a JavaScript class.
• [in] attributes: The attributes associated with a particular property. One or more of napi_property_attributes.
• [in] data: User-provided data passed into method when it is invoked.

Returns Napi::PropertyDescriptor object that represents the static method of a JavaScript class.

StaticMethod

Creates property descriptor that represents a static method of a JavaScript class.

static Napi::PropertyDescriptor Napi::ObjectWrap::StaticMethod(Symbol name,
                                       StaticMethodCallback method,
                                       napi_property_attributes attributes = napi_default,
                                       void* data = nullptr);

method for the class.

• [in] name: Napi:Symbol that represents the name of a static.
• [in] method: The native function that represents a static method of a JavaScript class.
• [in] attributes: The attributes associated with a particular property. One or more of napi_property_attributes.
• [in] data: User-provided data passed into method when it is invoked.

Returns Napi::PropertyDescriptor object that represents a static method of a JavaScript class.

StaticAccessor

Creates property descriptor that represents a static accessor property of a JavaScript class.

static Napi::PropertyDescriptor Napi::ObjectWrap::StaticAccessor(const char* utf8name,
                                         StaticGetterCallback getter,
                                         StaticSetterCallback setter,
                                         napi_property_attributes attributes = napi_default,
                                         void* data = nullptr);

• [in] utf8name: Null-terminated string that represents the name of a static accessor property for the class.
• [in] getter: The native function to call when a get access to the property of a JavaScript class is performed.
• [in] setter: The native function to call when a set access to the property of a JavaScript class is performed.
• [in] attributes: The attributes associated with a particular property. One or more of napi_property_attributes.
• [in] data: User-provided data passed into getter or setter when is invoked.

Returns Napi::PropertyDescriptor object that represents a static accessor property of a JavaScript class.

StaticAccessor

Creates property descriptor that represents a static accessor property of a JavaScript class.

static Napi::PropertyDescriptor Napi::ObjectWrap::StaticAccessor(Symbol name,
                                         StaticGetterCallback getter,
                                         StaticSetterCallback setter,
                                         napi_property_attributes attributes = napi_default,
                                         void* data = nullptr);

• [in] name: Napi:Symbol that represents the name of a static accessor.
• [in] getter: The native function to call when a get access to the property of a JavaScript class is performed.
• [in] setter: The native function to call when a set access to the property of a JavaScript class is performed.
• [in] attributes: The attributes associated with a particular property. One or more of napi_property_attributes.
• [in] data: User-provided data passed into getter or setter when is invoked.

Returns Napi::PropertyDescriptor object that represents a static accessor property of a JavaScript class.

InstanceMethod

Creates property descriptor that represents an instance method of a JavaScript class.

static Napi::PropertyDescriptor Napi::ObjectWrap::InstanceMethod(const char* utf8name,
                                         InstanceVoidMethodCallback method,
                                         napi_property_attributes attributes = napi_default,
                                         void* data = nullptr);

• [in] utf8name: Null-terminated string that represents the name of an instance method for the class.
• [in] method: The native function that represents an instance method of a JavaScript class.
• [in] attributes: The attributes associated with a particular property. One or more of napi_property_attributes.
• [in] data: User-provided data passed into method when it is invoked.

Returns Napi::PropertyDescriptor object that represents an instance method of a JavaScript class.

InstanceMethod

Creates property descriptor that represents an instance method of a JavaScript class.

static Napi::PropertyDescriptor Napi::ObjectWrap::InstanceMethod(const char* utf8name,
                                         InstanceMethodCallback method,
                                         napi_property_attributes attributes = napi_default,
                                         void* data = nullptr);

• [in] utf8name: Null-terminated string that represents the name of an instance method for the class.
• [in] method: The native function that represents an instance method of a JavaScript class.
• [in] attributes: The attributes associated with a particular property. One or more of napi_property_attributes.
• [in] data: User-provided data passed into method when it is invoked.

Returns Napi::PropertyDescriptor object that represents an instance method of a JavaScript class.

InstanceMethod

Creates property descriptor that represents an instance method of a JavaScript class.

static Napi::PropertyDescriptor Napi::ObjectWrap::InstanceMethod(Napi::Symbol name,
                                         InstanceVoidMethodCallback method,
                                         napi_property_attributes attributes = napi_default,
                                         void* data = nullptr);

• [in] name: The Napi::Symbol object whose value is used to identify the instance method for the class.
• [in] method: The native function that represents an instance method of a JavaScript class.
• [in] attributes: The attributes associated with a particular property. One or more of napi_property_attributes.
• [in] data: User-provided data passed into method when it is invoked.

Returns Napi::PropertyDescriptor object that represents an instance method of a JavaScript class.

InstanceMethod

Creates property descriptor that represents an instance method of a JavaScript class.

static Napi::PropertyDescriptor Napi::ObjectWrap::InstanceMethod(Napi::Symbol name,
                                         InstanceMethodCallback method,
                                         napi_property_attributes attributes = napi_default,
                                         void* data = nullptr);

• [in] name: The Napi::Symbol object whose value is used to identify the instance method for the class.
• [in] method: The native function that represents an instance method of a JavaScript class.
• [in] attributes: The attributes associated with a particular property. One or more of napi_property_attributes.
• [in] data: User-provided data passed into method when it is invoked.

Returns Napi::PropertyDescriptor object that represents an instance method of a JavaScript class.

InstanceAccessor

Creates property descriptor that represents an instance accessor property of a JavaScript class.

static Napi::PropertyDescriptor Napi::ObjectWrap::InstanceAccessor(const char* utf8name,
                                           InstanceGetterCallback getter,
                                           InstanceSetterCallback setter,
                                           napi_property_attributes attributes = napi_default,
                                           void* data = nullptr);

• [in] utf8name: Null-terminated string that represents the name of an instance accessor property for the class.
• [in] getter: The native function to call when a get access to the property of a JavaScript class is performed.
• [in] setter: The native function to call when a set access to the property of a JavaScript class is performed.
• [in] attributes: The attributes associated with the particular property. One or more of napi_property_attributes.
• [in] data: User-provided data passed into getter or setter when this is invoked.

Returns Napi::PropertyDescriptor object that represents an instance accessor property of a JavaScript class.

InstanceAccessor

Creates property descriptor that represents an instance accessor property of a JavaScript class.

static Napi::PropertyDescriptor Napi::ObjectWrap::InstanceAccessor(Symbol name,
                                           InstanceGetterCallback getter,
                                           InstanceSetterCallback setter,
                                           napi_property_attributes attributes = napi_default,
                                           void* data = nullptr);

• [in] name: The Napi::Symbol object whose value is used to identify the instance accessor.
• [in] getter: The native function to call when a get access to the property of a JavaScript class is performed.
• [in] setter: The native function to call when a set access to the property of a JavaScript class is performed.
• [in] attributes: The attributes associated with the particular property. One or more of napi_property_attributes.
• [in] data: User-provided data passed into getter or setter when this is invoked.

Returns Napi::PropertyDescriptor object that represents an instance accessor property of a JavaScript class.

StaticValue

Creates property descriptor that represents an static value property of a JavaScript class.

static Napi::PropertyDescriptor Napi::ObjectWrap::StaticValue(const char* utf8name,
                                      Napi::Value value,
                                      napi_property_attributes attributes = napi_default);

• [in] utf8name: Null-terminated string that represents the name of the static property.
• [in] value: The value that's retrieved by a get access of the property.
• [in] attributes: The attributes to be associated with the property in addition to the napi_static attribute. One or more of napi_property_attributes.

Returns Napi::PropertyDescriptor object that represents an static value property of a JavaScript class

StaticValue

Creates property descriptor that represents an static value property of a JavaScript class.

static Napi::PropertyDescriptor Napi::ObjectWrap::StaticValue(Symbol name,
                                      Napi::Value value,
                                      napi_property_attributes attributes = napi_default);

• [in] name: The Napi::Symbol object whose value is used to identify the name of the static property.
• [in] value: The value that's retrieved by a get access of the property.
• [in] attributes: The attributes to be associated with the property in addition to the napi_static attribute. One or more of napi_property_attributes.

Returns Napi::PropertyDescriptor object that represents an static value property of a JavaScript class

InstanceValue

Creates property descriptor that represents an instance value property of a JavaScript class.

static Napi::PropertyDescriptor Napi::ObjectWrap::InstanceValue(const char* utf8name,
                                        Napi::Value value,
                                        napi_property_attributes attributes = napi_default);

• [in] utf8name: Null-terminated string that represents the name of the property.
• [in] value: The value that's retrieved by a get access of the property.
• [in] attributes: The attributes to be associated with the property. One or more of napi_property_attributes.

Returns Napi::PropertyDescriptor object that represents an instance value property of a JavaScript class.

InstanceValue

Creates property descriptor that represents an instance value property of a JavaScript class.

static Napi::PropertyDescriptor Napi::ObjectWrap::InstanceValue(Symbol name,
                                        Napi::Value value,
                                        napi_property_attributes attributes = napi_default);

• [in] name: The Napi::Symbol object whose value is used to identify the name of the property.
• [in] value: The value that's retrieved by a get access of the property.
• [in] attributes: The attributes to be associated with the property. One or more of napi_property_attributes.

Returns Napi::PropertyDescriptor object that represents an instance value