Fabric: Introducing ContextContainer::update()

Summary:
The new method allows updating a ContextContainer using another one.
We will need that in the coming diffs.

Changelog: [Internal] Introducing `ContextContainer::update`

Reviewed By: sammy-SC

Differential Revision: D17960238

fbshipit-source-id: 187dcecbeee8c3de082575e53d5430d91b5338c5

ReactCommon/utils/ContextContainer.h

@@ -62,6 +62,25 @@ class ContextContainer final {
 #endif
   }
 
+  /*
+   * Updates the container with values from a given container.
+   * Values with keys that already exist in the container will be replaced with
+   * values from the given container.
+   */
+  void update(ContextContainer const &contextContainer) const {
+    std::unique_lock<better::shared_mutex> lock(mutex_);
+
+    for (auto const &pair : contextContainer.instances_) {
+      instances_.erase(pair.first);
+      instances_.insert(pair);
+#ifndef NDEBUG
+      typeNames_.erase(pair.first);
+      typeNames_.insert(
+          {pair.first, contextContainer.typeNames_.at(pair.first)});
+#endif
+    }
+  }
+
   /*
    * Returns a previously registered instance of the particular type `T`
    * for `key`.