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keksAccountGUI/node_modulesOLD/nan/doc/object_wrappers.md

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2019-08-11 18:48:02 +00:00
## Object Wrappers
The `ObjectWrap` class can be used to make wrapped C++ objects and a factory of wrapped objects.
- <a href="#api_nan_object_wrap"><b><code>Nan::ObjectWrap</code></b></a>
<a name="api_nan_object_wrap"></a>
### Nan::ObjectWrap()
A reimplementation of `node::ObjectWrap` that adds some API not present in older versions of Node. Should be preferred over `node::ObjectWrap` in all cases for consistency.
Definition:
```c++
class ObjectWrap {
public:
ObjectWrap();
virtual ~ObjectWrap();
template <class T>
static inline T* Unwrap(v8::Local<v8::Object> handle);
inline v8::Local<v8::Object> handle();
inline Nan::Persistent<v8::Object>& persistent();
protected:
inline void Wrap(v8::Local<v8::Object> handle);
inline void MakeWeak();
/* Ref() marks the object as being attached to an event loop.
* Refed objects will not be garbage collected, even if
* all references are lost.
*/
virtual void Ref();
/* Unref() marks an object as detached from the event loop. This is its
* default state. When an object with a "weak" reference changes from
* attached to detached state it will be freed. Be careful not to access
* the object after making this call as it might be gone!
* (A "weak reference" means an object that only has a
* persistant handle.)
*
* DO NOT CALL THIS FROM DESTRUCTOR
*/
virtual void Unref();
int refs_; // ro
};
```
See the Node documentation on [Wrapping C++ Objects](https://nodejs.org/api/addons.html#addons_wrapping_c_objects) for more details.
### This vs. Holder
When calling `Unwrap`, it is important that the argument is indeed some JavaScript object which got wrapped by a `Wrap` call for this class or any derived class.
The `Signature` installed by [`Nan::SetPrototypeMethod()`](methods.md#api_nan_set_prototype_method) does ensure that `info.Holder()` is just such an instance.
In Node 0.12 and later, `info.This()` will also be of such a type, since otherwise the invocation will get rejected.
However, in Node 0.10 and before it was possible to invoke a method on a JavaScript object which just had the extension type in its prototype chain.
In such a situation, calling `Unwrap` on `info.This()` will likely lead to a failed assertion causing a crash, but could lead to even more serious corruption.
On the other hand, calling `Unwrap` in an [accessor](methods.md#api_nan_set_accessor) should not use `Holder()` if the accessor is defined on the prototype.
So either define your accessors on the instance template,
or use `This()` after verifying that it is indeed a valid object.
### Examples
#### Basic
```c++
class MyObject : public Nan::ObjectWrap {
public:
static NAN_MODULE_INIT(Init) {
v8::Local<v8::FunctionTemplate> tpl = Nan::New<v8::FunctionTemplate>(New);
tpl->SetClassName(Nan::New("MyObject").ToLocalChecked());
tpl->InstanceTemplate()->SetInternalFieldCount(1);
Nan::SetPrototypeMethod(tpl, "getHandle", GetHandle);
Nan::SetPrototypeMethod(tpl, "getValue", GetValue);
constructor().Reset(Nan::GetFunction(tpl).ToLocalChecked());
Nan::Set(target, Nan::New("MyObject").ToLocalChecked(),
Nan::GetFunction(tpl).ToLocalChecked());
}
private:
explicit MyObject(double value = 0) : value_(value) {}
~MyObject() {}
static NAN_METHOD(New) {
if (info.IsConstructCall()) {
double value = info[0]->IsUndefined() ? 0 : Nan::To<double>(info[0]).FromJust();
MyObject *obj = new MyObject(value);
obj->Wrap(info.This());
info.GetReturnValue().Set(info.This());
} else {
const int argc = 1;
v8::Local<v8::Value> argv[argc] = {info[0]};
v8::Local<v8::Function> cons = Nan::New(constructor());
info.GetReturnValue().Set(Nan::NewInstance(cons, argc, argv).ToLocalChecked());
}
}
static NAN_METHOD(GetHandle) {
MyObject* obj = Nan::ObjectWrap::Unwrap<MyObject>(info.Holder());
info.GetReturnValue().Set(obj->handle());
}
static NAN_METHOD(GetValue) {
MyObject* obj = Nan::ObjectWrap::Unwrap<MyObject>(info.Holder());
info.GetReturnValue().Set(obj->value_);
}
static inline Nan::Persistent<v8::Function> & constructor() {
static Nan::Persistent<v8::Function> my_constructor;
return my_constructor;
}
double value_;
};
NODE_MODULE(objectwrapper, MyObject::Init)
```
To use in Javascript:
```Javascript
var objectwrapper = require('bindings')('objectwrapper');
var obj = new objectwrapper.MyObject(5);
console.log('Should be 5: ' + obj.getValue());
```
#### Factory of wrapped objects
```c++
class MyFactoryObject : public Nan::ObjectWrap {
public:
static NAN_MODULE_INIT(Init) {
v8::Local<v8::FunctionTemplate> tpl = Nan::New<v8::FunctionTemplate>(New);
tpl->InstanceTemplate()->SetInternalFieldCount(1);
Nan::SetPrototypeMethod(tpl, "getValue", GetValue);
constructor().Reset(Nan::GetFunction(tpl).ToLocalChecked());
}
static NAN_METHOD(NewInstance) {
v8::Local<v8::Function> cons = Nan::New(constructor());
double value = info[0]->IsNumber() ? Nan::To<double>(info[0]).FromJust() : 0;
const int argc = 1;
v8::Local<v8::Value> argv[1] = {Nan::New(value)};
info.GetReturnValue().Set(Nan::NewInstance(cons, argc, argv).ToLocalChecked());
}
// Needed for the next example:
inline double value() const {
return value_;
}
private:
explicit MyFactoryObject(double value = 0) : value_(value) {}
~MyFactoryObject() {}
static NAN_METHOD(New) {
if (info.IsConstructCall()) {
double value = info[0]->IsNumber() ? Nan::To<double>(info[0]).FromJust() : 0;
MyFactoryObject * obj = new MyFactoryObject(value);
obj->Wrap(info.This());
info.GetReturnValue().Set(info.This());
} else {
const int argc = 1;
v8::Local<v8::Value> argv[argc] = {info[0]};
v8::Local<v8::Function> cons = Nan::New(constructor());
info.GetReturnValue().Set(Nan::NewInstance(cons, argc, argv).ToLocalChecked());
}
}
static NAN_METHOD(GetValue) {
MyFactoryObject* obj = ObjectWrap::Unwrap<MyFactoryObject>(info.Holder());
info.GetReturnValue().Set(obj->value_);
}
static inline Nan::Persistent<v8::Function> & constructor() {
static Nan::Persistent<v8::Function> my_constructor;
return my_constructor;
}
double value_;
};
NAN_MODULE_INIT(Init) {
MyFactoryObject::Init(target);
Nan::Set(target,
Nan::New<v8::String>("newFactoryObjectInstance").ToLocalChecked(),
Nan::GetFunction(
Nan::New<v8::FunctionTemplate>(MyFactoryObject::NewInstance)).ToLocalChecked()
);
}
NODE_MODULE(wrappedobjectfactory, Init)
```
To use in Javascript:
```Javascript
var wrappedobjectfactory = require('bindings')('wrappedobjectfactory');
var obj = wrappedobjectfactory.newFactoryObjectInstance(10);
console.log('Should be 10: ' + obj.getValue());
```
#### Passing wrapped objects around
Use the `MyFactoryObject` class above along with the following:
```c++
static NAN_METHOD(Sum) {
Nan::MaybeLocal<v8::Object> maybe1 = Nan::To<v8::Object>(info[0]);
Nan::MaybeLocal<v8::Object> maybe2 = Nan::To<v8::Object>(info[1]);
// Quick check:
if (maybe1.IsEmpty() || maybe2.IsEmpty()) {
// return value is undefined by default
return;
}
MyFactoryObject* obj1 =
Nan::ObjectWrap::Unwrap<MyFactoryObject>(maybe1.ToLocalChecked());
MyFactoryObject* obj2 =
Nan::ObjectWrap::Unwrap<MyFactoryObject>(maybe2.ToLocalChecked());
info.GetReturnValue().Set(Nan::New<v8::Number>(obj1->value() + obj2->value()));
}
NAN_MODULE_INIT(Init) {
MyFactoryObject::Init(target);
Nan::Set(target,
Nan::New<v8::String>("newFactoryObjectInstance").ToLocalChecked(),
Nan::GetFunction(
Nan::New<v8::FunctionTemplate>(MyFactoryObject::NewInstance)).ToLocalChecked()
);
Nan::Set(target,
Nan::New<v8::String>("sum").ToLocalChecked(),
Nan::GetFunction(Nan::New<v8::FunctionTemplate>(Sum)).ToLocalChecked()
);
}
NODE_MODULE(myaddon, Init)
```
To use in Javascript:
```Javascript
var myaddon = require('bindings')('myaddon');
var obj1 = myaddon.newFactoryObjectInstance(5);
var obj2 = myaddon.newFactoryObjectInstance(10);
console.log('sum of object values: ' + myaddon.sum(obj1, obj2));
```