object_lifetime_management.md

Object lifetime management

A handle may be created when any new node-addon-api Value and its subclasses is created or returned.

As the methods and classes within the node-addon-api are used, handles to objects in the heap for the underlying VM may be created. A handle may be created when any new node-addon-api Value or one of its subclasses is created or returned. These handles must hold the objects 'live' until they are no longer required by the native code, otherwise the objects could be collected by the garbage collector before the native code was finished using them.

As handles are created they are associated with a 'scope'. The lifespan for the default scope is tied to the lifespan of the native method call. The result is that, by default, handles remain valid and the objects associated with these handles will be held live for the lifespan of the native method call.

In many cases, however, it is necessary that the handles remain valid for either a shorter or longer lifespan than that of the native method. The sections which follow describe the node-addon-api classes and methods that than can be used to change the handle lifespan from the default.

Making handle lifespan shorter than that of the native method

It is often necessary to make the lifespan of handles shorter than the lifespan of a native method. For example, consider a native method that has a loop which creates a number of values and does something with each of the values, one at a time:

for (int i = 0; i < LOOP_MAX; i++) {
  std::string name = std::string("inner-scope") + std::to_string(i);
  Napi::Value newValue = Napi::String::New(info.Env(), name.c_str());
  // do something with newValue
};

This would result in a large number of handles being created, consuming substantial resources. In addition, even though the native code could only use the most recently created value, all of the previously created values would also be kept alive since they all share the same scope.

To handle this case, node-addon-api provides the ability to establish a new 'scope' to which newly created handles will be associated. Once those handles are no longer required, the scope can be deleted and any handles associated with the scope are invalidated. The Napi::HandleScope and Napi::EscapableHandleScope classes are provided by node-addon-api for creating additional scopes.

node-addon-api only supports a single nested hierarchy of scopes. There is only one active scope at any time, and all new handles will be associated with that scope while it is active. Scopes must be deleted in the reverse order from which they are opened. In addition, all scopes created within a native method must be deleted before returning from that method. Since Napi::HandleScopes are typically stack allocated the compiler will take care of deletion, however, care must be taken to create the scope in the right place such that you achieve the desired lifetime.

Taking the earlier example, creating a Napi::HandleScope in the innner loop would ensure that at most a single new value is held alive throughout the execution of the loop:

for (int i = 0; i < LOOP_MAX; i++) {
  Napi::HandleScope scope(info.Env());
  std::string name = std::string("inner-scope") + std::to_string(i);
  Napi::Value newValue = Napi::String::New(info.Env(), name.c_str());
  // do something with neValue
};

When nesting scopes, there are cases where a handle from an inner scope needs to live beyond the lifespan of that scope. node-addon-api provides the Napi::EscapableHandleScope with the Escape method in order to support this case. An escapable scope allows one object to be 'promoted' so that it 'escapes' the current scope and the lifespan of the handle changes from the current scope to that of the outer scope. The Escape method can only be called once for a given Napi::EscapableHandleScope.