doc: update context-aware section of addon doc

Gabriel Schulhof · MylesBorins · commit 979fb155ff7c · 2020-03-31T18:09:00.000-04:00
Replace the portion of the context-aware addon example that advocates for managing the life cycle of the per-addon-instance data using a weak reference with code that illustrates how to manage its life cycle using an environment cleanup hook. Signed-off-by: Gabriel Schulhof <gabriel.schulhof@intel.com> PR-URL: #28659 Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
diff --git a/doc/api/addons.md b/doc/api/addons.md
@@ -155,18 +155,25 @@ they were created.
 
 The context-aware addon can be structured to avoid global static data by
 performing the following steps:
-
-* defining a class which will hold per-addon-instance data. Such
-a class should include a `v8::Global<v8::Object>` which will hold a weak
-reference to the addon's `exports` object. The callback associated with the weak
-reference will then destroy the instance of the class.
-* constructing an instance of this class in the addon initializer such that the
-`v8::Global<v8::Object>` is set to the `exports` object.
-* storing the instance of the class in a `v8::External`, and
-* passing the `v8::External` to all methods exposed to JavaScript by passing it
-to the `v8::FunctionTemplate` constructor which creates the native-backed
-JavaScript functions. The `v8::FunctionTemplate` constructor's third parameter
-accepts the `v8::External`.
+* Define a class which will hold per-addon-instance data and which has a static
+member of the form
+    ```C++
+    static void DeleteInstance(void* data) {
+      // Cast `data` to an instance of the class and delete it.
+    }
+    ```
+* Heap-allocate an instance of this class in the addon initializer. This can be
+accomplished using the `new` keyword.
+* Call `node::AddEnvironmentCleanupHook()`, passing it the above-created
+instance and a pointer to `DeleteInstance()`. This will ensure the instance is
+deleted when the environment is torn down.
+* Store the instance of the class in a `v8::External`, and
+* Pass the `v8::External` to all methods exposed to JavaScript by passing it
+to `v8::FunctionTemplate::New()` or `v8::Function::New()` which creates the
+native-backed JavaScript functions. The third parameter of
+`v8::FunctionTemplate::New()` or `v8::Function::New()`  accepts the
+`v8::External` and makes it available in the native callback using the
+`v8::FunctionCallbackInfo::Data()` method.
 
 This will ensure that the per-addon-instance data reaches each binding that can
 be called from JavaScript. The per-addon-instance data must also be passed into
@@ -181,25 +188,18 @@ using namespace v8;
 
 class AddonData {
  public:
-  AddonData(Isolate* isolate, Local<Object> exports):
+  explicit AddonData(Isolate* isolate):
       call_count(0) {
-    // Link the existence of this object instance to the existence of exports.
-    exports_.Reset(isolate, exports);
-    exports_.SetWeak(this, DeleteMe, WeakCallbackType::kParameter);
+    // Ensure this per-addon-instance data is deleted at environment cleanup.
+    node::AddEnvironmentCleanupHook(isolate, DeleteInstance, this);
   }
 
   // Per-addon data.
   int call_count;
 
- private:
-  // Method to call when "exports" is about to be garbage-collected.
-  static void DeleteMe(const WeakCallbackInfo<AddonData>& info) {
-    delete info.GetParameter();
+  static void DeleteInstance(void* data) {
+    delete static_cast<AddonData*>(data);
   }
-
-  // Weak handle to the "exports" object. An instance of this class will be
-  // destroyed along with the exports object to which it is weakly bound.
-  v8::Global<v8::Object> exports_;
 };
 
 static void Method(const v8::FunctionCallbackInfo<v8::Value>& info) {
@@ -214,14 +214,17 @@ static void Method(const v8::FunctionCallbackInfo<v8::Value>& info) {
 NODE_MODULE_INIT(/* exports, module, context */) {
   Isolate* isolate = context->GetIsolate();
 
-  // Create a new instance of AddonData for this instance of the addon.
-  AddonData* data = new AddonData(isolate, exports);
-  // Wrap the data in a v8::External so we can pass it to the method we expose.
+  // Create a new instance of `AddonData` for this instance of the addon and
+  // tie its life cycle to that of the Node.js environment.
+  AddonData* data = new AddonData(isolate);
+
+  // Wrap the data in a `v8::External` so we can pass it to the method we
+  // expose.
   Local<External> external = External::New(isolate, data);
 
-  // Expose the method "Method" to JavaScript, and make sure it receives the
+  // Expose the method `Method` to JavaScript, and make sure it receives the
   // per-addon-instance data we created above by passing `external` as the
-  // third parameter to the FunctionTemplate constructor.
+  // third parameter to the `FunctionTemplate` constructor.
   exports->Set(context,
                String::NewFromUtf8(isolate, "method", NewStringType::kNormal)
                   .ToLocalChecked(),
