implement Object::AddFinalizer

This allows one to tie the life cycle of one JavaScript object to
another. In effect, this allows for JavaScript-style closures.

Fixes: #508
PR_URL: #551
Reviewed-By: Kevin Eady <8634912+KevinEady@users.noreply.github.com>

Author: Gabriel Schulhof

doc/object.md

@@ -137,6 +137,40 @@ Returns a `bool` that is true if the `Napi::Object` is an instance created by th
 
 Note: This is equivalent to the JavaScript instanceof operator.
 
+### AddFinalizer()
+```cpp
+template <typename Finalizer, typename T>
+inline void AddFinalizer(Finalizer finalizeCallback, T* data);
+```
+
+- `[in] finalizeCallback`: The function to call when the object is garbage-collected.
+- `[in] data`: The data to associate with the object.
+
+Associates `data` with the object, calling `finalizeCallback` when the object is garbage-collected. `finalizeCallback`
+has the signature
+```cpp
+void finalizeCallback(Napi::Env env, T* data);
+```
+where `data` is the pointer that was passed into the call to `AddFinalizer()`.
+
+### AddFinalizer()
+```cpp
+template <typename Finalizer, typename T, typename Hint>
+inline void AddFinalizer(Finalizer finalizeCallback,
+                         T* data,
+                         Hint* finalizeHint);
+```
+
+- `[in] data`: The data to associate with the object.
+- `[in] finalizeCallback`: The function to call when the object is garbage-collected.
+
+Associates `data` with the object, calling `finalizeCallback` when the object is garbage-collected. An additional hint
+may be given. It will also be passed to `finalizeCallback`, which has the signature
+```cpp
+void finalizeCallback(Napi::Env env, T* data, Hint* hint);
+```
+where `data` and `hint` are the pointers that were passed into the call to `AddFinalizer()`.
+
 ### DefineProperty()
 
 ```cpp

napi-inl.h

@@ -24,16 +24,21 @@ namespace details {
 template <typename FreeType>
 static inline napi_status AttachData(napi_env env,
                                      napi_value obj,
-                                     FreeType* data) {
+                                     FreeType* data,
+                                     napi_finalize finalizer = nullptr,
+                                     void* hint = nullptr) {
   napi_value symbol, external;
   napi_status status = napi_create_symbol(env, nullptr, &symbol);
   if (status == napi_ok) {
+    if (finalizer == nullptr) {
+      finalizer = [](napi_env /*env*/, void* data, void* /*hint*/) {
+        delete static_cast<FreeType*>(data);
+      };
+    }
     status = napi_create_external(env,
                               data,
-                              [](napi_env /*env*/, void* data, void* /*hint*/) {
-                                delete static_cast<FreeType*>(data);
-                              },
-                              nullptr,
+                              finalizer,
+                              hint,
                               &external);
     if (status == napi_ok) {
       napi_property_descriptor desc = {
@@ -1170,6 +1175,40 @@ inline bool Object::InstanceOf(const Function& constructor) const {
   return result;
 }
 
+template <typename Finalizer, typename T>
+inline void Object::AddFinalizer(Finalizer finalizeCallback, T* data) {
+  details::FinalizeData<T, Finalizer>* finalizeData =
+    new details::FinalizeData<T, Finalizer>({ finalizeCallback, nullptr });
+  napi_status status =
+      details::AttachData(_env,
+                          *this,
+                          data,
+                          details::FinalizeData<T, Finalizer>::Wrapper,
+                          finalizeData);
+  if (status != napi_ok) {
+    delete finalizeData;
+    NAPI_THROW_IF_FAILED_VOID(_env, status);
+  }
+}
+
+template <typename Finalizer, typename T, typename Hint>
+inline void Object::AddFinalizer(Finalizer finalizeCallback,
+                                 T* data,
+                                 Hint* finalizeHint) {
+  details::FinalizeData<T, Finalizer, Hint>* finalizeData =
+    new details::FinalizeData<T, Finalizer, Hint>({ finalizeCallback, finalizeHint });
+  napi_status status =
+      details::AttachData(_env,
+                          *this,
+                          data,
+                          details::FinalizeData<T, Finalizer, Hint>::WrapperWithHint,
+                          finalizeData);
+  if (status != napi_ok) {
+    delete finalizeData;
+    NAPI_THROW_IF_FAILED_VOID(_env, status);
+  }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // External class
 ////////////////////////////////////////////////////////////////////////////////

napi.h

@@ -709,6 +709,14 @@ namespace Napi {
     bool InstanceOf(
       const Function& constructor ///< Constructor function
     ) const;
+
+    template <typename Finalizer, typename T>
+    inline void AddFinalizer(Finalizer finalizeCallback, T* data);
+
+    template <typename Finalizer, typename T, typename Hint>
+    inline void AddFinalizer(Finalizer finalizeCallback,
+                             T* data,
+                             Hint* finalizeHint);
   };
 
   template <typename T>

package.json

@@ -55,7 +55,17 @@
   },
   "directories": {},
   "homepage": "https://github.com/nodejs/node-addon-api",
-  "keywords": ["n-api", "napi", "addon", "native", "bindings", "c", "c++", "nan", "node-addon-api"],
+  "keywords": [
+    "n-api",
+    "napi",
+    "addon",
+    "native",
+    "bindings",
+    "c",
+    "c++",
+    "nan",
+    "node-addon-api"
+  ],
   "license": "MIT",
   "main": "index.js",
   "name": "node-addon-api",

test/binding.gyp

@@ -27,6 +27,7 @@
         'memory_management.cc',
         'name.cc',
         'object/delete_property.cc',
+        'object/finalizer.cc',
         'object/get_property.cc',
         'object/has_own_property.cc',
         'object/has_property.cc',

test/index.js

@@ -30,6 +30,7 @@ let testModules = [
   'memory_management',
   'name',
   'object/delete_property',
+  'object/finalizer',
   'object/get_property',
   'object/has_own_property',
   'object/has_property',

test/object/finalizer.cc

@@ -0,0 +1,29 @@
+#include "napi.h"
+
+using namespace Napi;
+
+static int dummy;
+
+Value AddFinalizer(const CallbackInfo& info) {
+  ObjectReference* ref = new ObjectReference;
+  *ref = Persistent(Object::New(info.Env()));
+  info[0]
+    .As<Object>()
+    .AddFinalizer([](Napi::Env /*env*/, ObjectReference* ref) {
+      ref->Set("finalizerCalled", true);
+      delete ref;
+    }, ref);
+  return ref->Value();
+}
+
+Value AddFinalizerWithHint(const CallbackInfo& info) {
+  ObjectReference* ref = new ObjectReference;
+  *ref = Persistent(Object::New(info.Env()));
+  info[0]
+    .As<Object>()
+    .AddFinalizer([](Napi::Env /*env*/, ObjectReference* ref, int* dummy_p) {
+      ref->Set("finalizerCalledWithCorrectHint", dummy_p == &dummy);
+      delete ref;
+    }, ref, &dummy);
+  return ref->Value();
+}

test/object/finalizer.js

@@ -0,0 +1,21 @@
+'use strict';
+
+const buildType = process.config.target_defaults.default_configuration;
+const assert = require('assert');
+
+test(require(`../build/${buildType}/binding.node`));
+test(require(`../build/${buildType}/binding_noexcept.node`));
+
+function createWeakRef(binding, bindingToTest) {
+  return binding.object[bindingToTest]({});
+}
+
+function test(binding) {
+  const obj1 = createWeakRef(binding, 'addFinalizer');
+  global.gc();
+  assert.deepStrictEqual(obj1, { finalizerCalled: true });
+
+  const obj2 = createWeakRef(binding, 'addFinalizerWithHint');
+  global.gc();
+  assert.deepStrictEqual(obj2, { finalizerCalledWithCorrectHint: true });
+}

test/object/object.cc

@@ -32,6 +32,10 @@ Value HasPropertyWithNapiWrapperValue(const CallbackInfo& info);
 Value HasPropertyWithCStyleString(const CallbackInfo& info);
 Value HasPropertyWithCppStyleString(const CallbackInfo& info);
 
+// Native wrappers for testing Object::AddFinalizer()
+Value AddFinalizer(const CallbackInfo& info);
+Value AddFinalizerWithHint(const CallbackInfo& info);
+
 static bool testValue = true;
 // Used to test void* Data() integrity
 struct UserDataHolder {
@@ -201,5 +205,8 @@ Object InitObject(Env env) {
 
   exports["createObjectUsingMagic"] = Function::New(env, CreateObjectUsingMagic);
 
+  exports["addFinalizer"] = Function::New(env, AddFinalizer);
+  exports["addFinalizerWithHint"] = Function::New(env, AddFinalizerWithHint);
+
   return exports;
 }