Add Callback::IsCancelled

base/callback_internal.h

Index: base/callback_internal.h
diff --git a/base/callback_internal.h b/base/callback_internal.h
index 6aa5fa4e841657bcffd64fa541ecf92c8ce99677..6de18fd39d505b42ef4c07c906ae2c4086bf59e8 100644
--- a/base/callback_internal.h
+++ b/base/callback_internal.h
@@ -36,7 +36,8 @@ class BASE_EXPORT BindStateBase {
 
  protected:
   BindStateBase(InvokeFuncStorage polymorphic_invoke,
-                void (*destructor)(BindStateBase*));
+                void (*destructor)(BindStateBase*),
+                bool (*is_cancelled)(const BindStateBase*));
   ~BindStateBase() = default;
 
  private:
@@ -44,6 +45,10 @@ class BASE_EXPORT BindStateBase {
   template <CopyMode copy_mode>
   friend class CallbackBase;
 
+  bool IsCancelled() const {
+    return is_cancelled_(this);
+  }
+
   void AddRef();
   void Release();
 
@@ -57,6 +62,7 @@ class BASE_EXPORT BindStateBase {
 
   // Pointer to a function that will properly destroy |this|.
   void (*destructor_)(BindStateBase*);
+  bool (*is_cancelled_)(const BindStateBase*);
 
   DISALLOW_COPY_AND_ASSIGN(BindStateBase);
 };
@@ -75,6 +81,10 @@ class BASE_EXPORT CallbackBase<CopyMode::MoveOnly> {
   bool is_null() const { return bind_state_.get() == NULL; }
   explicit operator bool() const { return !is_null(); }
 
+  // Returns true if the callback invocation will be nop due to an cancellation.
+  // It's invalid to call this on uninitialized callback.
+  bool IsCancelled() const;
+
   // Returns the Callback into an uninitialized state.
   void Reset();