Supplement should have a Member to the corresponding Supplementable object (Part 1)

third_party/WebKit/Source/platform/Supplementable.h

Index: third_party/WebKit/Source/platform/Supplementable.h
diff --git a/third_party/WebKit/Source/platform/Supplementable.h b/third_party/WebKit/Source/platform/Supplementable.h
index 487873705cf0e96439468a31084aa8268612a03a..ba2aa152ff60f429702d2561f179145d764c2e67 100644
--- a/third_party/WebKit/Source/platform/Supplementable.h
+++ b/third_party/WebKit/Source/platform/Supplementable.h
@@ -93,6 +93,12 @@ class Supplementable;
 template<typename T>
 class Supplement : public GarbageCollectedMixin {
 public:
+    // TODO(haraken): Remove the default constructor.
+    // All Supplement objects should be instantiated with m_host.
+    Supplement() { }
+    explicit Supplement(T& host) : m_host(&host) { }
+    T* host() const { return m_host; }
+
     static void provideTo(Supplementable<T>& host, const char* key, Supplement<T>* supplement)
     {
         host.provideSupplement(key, supplement);
@@ -108,7 +114,13 @@ public:
         return host ? host->requireSupplement(key) : 0;
     }
 
-    DEFINE_INLINE_VIRTUAL_TRACE() { }
+    DEFINE_INLINE_VIRTUAL_TRACE()
+    {
+        visitor->trace(m_host);
+    }
+
+private:
+    WeakMember<T> m_host;

[sof, 2016/07/13 07:20:41]

Thinking about this some more, what value does it bring over status quo to have
a WeakMember<> here? i.e., could you clarify how you intend to make use of
|m_host| in supplement impls?

[haraken, 2016/07/13 07:24:26]

Many Supplement classes that inherit from DOMWindowProperty and
LocalFrameLifecycleObserver are inheriting from them just because they want to
get the host object. If Supplement provides the host object, we can remove the
inheritance.

It's nasty that we need to make many NavigatorXXX objects inherit from
DOMWindowProperty/LocalFrameLifecycleObserver. They should be able to just get
the host object from Supplement.

[sof, 2016/07/13 07:39:41]

ok, thanks for clarifying the purpose, i.e., |m_host| is not a lifetime-extender
for supplement, but a way to avoid keeping ad-hoc "parent pointers".

[haraken, 2016/07/13 07:42:12]

Yes, right.

 };
 
 // Supplementable<T> inherits from GarbageCollectedMixin virtually