Bug 792954. Add a WeakPtr implementation to use instead of nsISupportsWeakReference. r=joe,ehsan,Waldo

This patch also replaces the usage of nsISupportsWeakReference in RasterImage as an example.

--HG--
extra : rebase_source : ac6a039dcc3227a04ac4c2221f38856bb308c695

image/src/RasterImage.cpp

@@ -204,11 +204,10 @@ namespace image {
 static nsCOMPtr<nsIThread> sScaleWorkerThread = nullptr;
 
 #ifndef DEBUG
-NS_IMPL_ISUPPORTS3(RasterImage, imgIContainer, nsIProperties,
-                   nsISupportsWeakReference)
+NS_IMPL_ISUPPORTS2(RasterImage, imgIContainer, nsIProperties)
 #else
-NS_IMPL_ISUPPORTS4(RasterImage, imgIContainer, nsIProperties,
-                   imgIContainerDebug, nsISupportsWeakReference)
+NS_IMPL_ISUPPORTS3(RasterImage, imgIContainer, nsIProperties,
+                   imgIContainerDebug)
 #endif
 
 //******************************************************************************
@@ -2557,7 +2556,7 @@ RasterImage::RequestDecode()
   // RequestDecode() is an asynchronous function this works fine (though it's
   // a little slower).
   if (mInDecoder) {
-    nsRefPtr<imgDecodeRequestor> requestor = new imgDecodeRequestor(this);
+    nsRefPtr<imgDecodeRequestor> requestor = new imgDecodeRequestor(*this);
     return NS_DispatchToCurrentThread(requestor);
   }
 

image/src/RasterImage.h

@@ -33,6 +33,7 @@
 #include "mozilla/Telemetry.h"
 #include "mozilla/LinkedList.h"
 #include "mozilla/StaticPtr.h"
+#include "mozilla/WeakPtr.h"
 #ifdef DEBUG
   #include "imgIContainerDebug.h"
 #endif
@@ -136,7 +137,7 @@ class Decoder;
 
 class RasterImage : public Image
                   , public nsIProperties
-                  , public nsSupportsWeakReference
+                  , public SupportsWeakPtr<RasterImage>
 #ifdef DEBUG
                   , public imgIContainerDebug
 #endif
@@ -811,18 +812,17 @@ protected:
 class imgDecodeRequestor : public nsRunnable
 {
   public:
-    imgDecodeRequestor(imgIContainer *aContainer) {
-      mContainer = do_GetWeakReference(aContainer);
+    imgDecodeRequestor(RasterImage &aContainer) {
+      mContainer = aContainer.asWeakPtr();
     }
     NS_IMETHOD Run() {
-      nsCOMPtr<imgIContainer> con = do_QueryReferent(mContainer);
-      if (con)
-        con->RequestDecode();
+      if (mContainer)
+        mContainer->RequestDecode();
       return NS_OK;
     }
 
   private:
-    nsWeakPtr mContainer;
+    WeakPtr<RasterImage> mContainer;
 };
 
 } // namespace image

mfbt/WeakPtr.h

@@ -0,0 +1,139 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Weak pointer functionality, implemented as a mixin for use with any class. */
+
+/**
+ * SupportsWeakPtr lets you have a pointer to an object 'Foo' without affecting
+ * its lifetime. It works by creating a single shared reference counted object
+ * (WeakReference) that each WeakPtr will access 'Foo' through. This lets 'Foo'
+ * clear the pointer in the WeakReference without having to know about all of
+ * the WeakPtrs to it and allows the WeakReference to live beyond the lifetime
+ * of 'Foo'.
+ *
+ * The overhead of WeakPtr is that accesses to 'Foo' becomes an additional
+ * dereference, and an additional heap allocated pointer sized object shared
+ * between all of the WeakPtrs.
+ *
+ * Example of usage:
+ *
+ *   // To have a class C support weak pointers, inherit from SupportsWeakPtr<C>.
+ *   class C : public SupportsWeakPtr<C>
+ *   {
+ *    public:
+ *      int num;
+ *      void act();
+ *   };
+ *
+ *   C* ptr =  new C();
+ *
+ *   // Get weak pointers to ptr. The first time asWeakPtr is called
+ *   // a reference counted WeakReference object is created that
+ *   // can live beyond the lifetime of 'ptr'. The WeakReference
+ *   // object will be notified of 'ptr's destruction.
+ *   WeakPtr<C> weak = ptr->asWeakPtr();
+ *   WeakPtr<C> other = ptr->asWeakPtr();
+ *
+ *   // Test a weak pointer for validity before using it.
+ *   if (weak) {
+ *     weak->num = 17;
+ *     weak->act();
+ *   }
+ *
+ *   // Destroying the underlying object clears weak pointers to it.
+ *   delete ptr;
+ *
+ *   MOZ_ASSERT(!weak, "Deleting |ptr| clears weak pointers to it.");
+ *   MOZ_ASSERT(!other, "Deleting |ptr| clears all weak pointers to it.");
+ *
+ * WeakPtr is typesafe and may be used with any class. It is not required that
+ * the class be reference-counted or allocated in any particular way.
+ *
+ * The API was loosely inspired by Chromium's weak_ptr.h:
+ * http://src.chromium.org/svn/trunk/src/base/memory/weak_ptr.h
+ */
+
+#ifndef mozilla_WeakPtr_h_
+#define mozilla_WeakPtr_h_
+
+#include "mozilla/Assertions.h"
+#include "mozilla/NullPtr.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/TypeTraits.h"
+
+namespace mozilla {
+
+template <typename T> class WeakPtr;
+
+template <typename T>
+class SupportsWeakPtr
+{
+  public:
+    WeakPtr<T> asWeakPtr() {
+      if (!weakRef)
+        weakRef = new WeakReference(static_cast<T*>(this));
+      return WeakPtr<T>(weakRef);
+    }
+
+  protected:
+    ~SupportsWeakPtr() {
+      MOZ_STATIC_ASSERT((IsBaseOf<SupportsWeakPtr<T>, T>::value), "T must derive from SupportsWeakPtr<T>");
+      if (weakRef)
+        weakRef->detach();
+    }
+
+  private:
+    friend class WeakPtr<T>;
+
+    // This can live beyond the lifetime of the class derived from SupportsWeakPtr.
+    class WeakReference : public RefCounted<WeakReference>
+    {
+      public:
+        explicit WeakReference(T* ptr) : ptr(ptr) {}
+        T* get() const {
+          return ptr;
+        }
+
+      private:
+        friend class WeakPtr<T>;
+        friend class SupportsWeakPtr<T>;
+        void detach() {
+          ptr = nullptr;
+        }
+        T* ptr;
+    };
+
+    RefPtr<WeakReference> weakRef;
+};
+
+template <typename T>
+class WeakPtr
+{
+  public:
+    WeakPtr(const WeakPtr<T>& o) : ref(o.ref) {}
+    WeakPtr() : ref(nullptr) {}
+
+    operator T*() const {
+      return ref->get();
+    }
+    T& operator*() const {
+      return *ref->get();
+    }
+
+    T* operator->() const {
+      return ref->get();
+    }
+
+  private:
+    friend class SupportsWeakPtr<T>;
+
+    explicit WeakPtr(const RefPtr<typename SupportsWeakPtr<T>::WeakReference> &o) : ref(o) {}
+
+    RefPtr<typename SupportsWeakPtr<T>::WeakReference> ref;
+};
+
+} // namespace mozilla
+
+#endif /* ifdef mozilla_WeakPtr_h_ */

mfbt/exported_headers.mk

@@ -31,4 +31,5 @@ EXPORTS_mozilla += \
   TypeTraits.h \
   Types.h \
   Util.h \
+  WeakPtr.h \
   $(NULL)

mfbt/tests/Makefile.in

@@ -15,6 +15,7 @@ CPP_UNIT_TESTS = \
   TestCheckedInt.cpp \
   TestTypeTraits.cpp \
   TestSHA1.cpp \
+  TestWeakPtr.cpp \
   $(NULL)
 
 # in order to prevent rules.mk from trying to link to libraries that are

mfbt/tests/TestWeakPtr.cpp

@@ -0,0 +1,76 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/WeakPtr.h"
+
+using mozilla::SupportsWeakPtr;
+using mozilla::WeakPtr;
+
+// To have a class C support weak pointers, inherit from SupportsWeakPtr<C>.
+class C : public SupportsWeakPtr<C>
+{
+  public:
+    int num;
+    void act() {}
+};
+
+static void
+example()
+{
+
+  C* ptr =  new C();
+
+  // Get weak pointers to ptr. The first time asWeakPtr is called
+  // a reference counted WeakReference object is created that
+  // can live beyond the lifetime of 'ptr'. The WeakReference
+  // object will be notified of 'ptr's destruction.
+  WeakPtr<C> weak = ptr->asWeakPtr();
+  WeakPtr<C> other = ptr->asWeakPtr();
+
+  // Test a weak pointer for validity before using it.
+  if (weak) {
+    weak->num = 17;
+    weak->act();
+  }
+
+  // Destroying the underlying object clears weak pointers to it.
+  delete ptr;
+
+  MOZ_ASSERT(!weak, "Deleting |ptr| clears weak pointers to it.");
+  MOZ_ASSERT(!other, "Deleting |ptr| clears all weak pointers to it.");
+}
+
+struct A : public SupportsWeakPtr<A>
+{
+    int data;
+};
+
+
+int main()
+{
+
+  A* a = new A;
+
+  // a2 is unused to test the case when we haven't initialized
+  // the internal WeakReference pointer.
+  A* a2 = new A;
+
+  a->data = 5;
+  WeakPtr<A> ptr = a->asWeakPtr();
+  {
+      WeakPtr<A> ptr2 = a->asWeakPtr();
+      MOZ_ASSERT(ptr->data == 5);
+      WeakPtr<A> ptr3 = a->asWeakPtr();
+      MOZ_ASSERT(ptr->data == 5);
+  }
+
+  delete a;
+  MOZ_ASSERT(!ptr);
+
+  delete a2;
+
+  example();
+
+  return 0;
+}

xpcom/glue/nsWeakReference.h

@@ -9,6 +9,8 @@
 
 // nsWeakReference.h
 
+// See mfbt/WeakPtr.h for a more typesafe C++ implementation of weak references
+
 #include "nsIWeakReferenceUtils.h"
 
 class nsWeakReference;