diff -r 000000000000 -r 6474c204b198 gfx/2d/BorrowedContext.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gfx/2d/BorrowedContext.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,132 @@
+/* -*- Mode: C++; tab-width: 20; 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/. */
+
+#ifndef _MOZILLA_GFX_BORROWED_CONTEXT_H
+#define _MOZILLA_GFX_BORROWED_CONTEXT_H
+
+#include "2D.h"
+
+struct _cairo;
+typedef struct _cairo cairo_t;
+
+namespace mozilla {
+
+namespace gfx {
+
+/* This is a helper class that let's you borrow a cairo_t from a
+ * DrawTargetCairo. This is used for drawing themed widgets.
+ *
+ * Callers should check the cr member after constructing the object
+ * to see if it succeeded. The DrawTarget should not be used while
+ * the context is borrowed. */
+class BorrowedCairoContext
+{
+public:
+  BorrowedCairoContext()
+    : mCairo(nullptr)
+    , mDT(nullptr)
+  { }
+
+  BorrowedCairoContext(DrawTarget *aDT)
+    : mDT(aDT)
+  {
+    mCairo = BorrowCairoContextFromDrawTarget(aDT);
+  }
+
+  // We can optionally Init after construction in
+  // case we don't know what the DT will be at construction
+  // time.
+  cairo_t *Init(DrawTarget *aDT)
+  {
+    MOZ_ASSERT(!mDT, "Can't initialize twice!");
+    mDT = aDT;
+    return mCairo = BorrowCairoContextFromDrawTarget(aDT);
+  }
+
+  // The caller needs to call Finish if cr is non-null when
+  // they are done with the context. This is currently explicit
+  // instead of happening implicitly in the destructor to make
+  // what's happening in the caller more clear. It also
+  // let's you resume using the DrawTarget in the same scope.
+  void Finish()
+  {
+    if (mCairo) {
+      ReturnCairoContextToDrawTarget(mDT, mCairo);
+      mCairo = nullptr;
+    }
+  }
+
+  ~BorrowedCairoContext() {
+    MOZ_ASSERT(!mCairo);
+  }
+
+  cairo_t *mCairo;
+private:
+  static cairo_t* BorrowCairoContextFromDrawTarget(DrawTarget *aDT);
+  static void ReturnCairoContextToDrawTarget(DrawTarget *aDT, cairo_t *aCairo);
+  DrawTarget *mDT;
+};
+
+#ifdef XP_MACOSX
+/* This is a helper class that let's you borrow a CGContextRef from a
+ * DrawTargetCG. This is used for drawing themed widgets.
+ *
+ * Callers should check the cg member after constructing the object
+ * to see if it succeeded. The DrawTarget should not be used while
+ * the context is borrowed. */
+class BorrowedCGContext
+{
+public:
+  BorrowedCGContext()
+    : cg(nullptr)
+    , mDT(nullptr)
+  { }
+
+  BorrowedCGContext(DrawTarget *aDT)
+    : mDT(aDT)
+  {
+    cg = BorrowCGContextFromDrawTarget(aDT);
+  }
+
+  // We can optionally Init after construction in
+  // case we don't know what the DT will be at construction
+  // time.
+  CGContextRef Init(DrawTarget *aDT)
+  {
+    MOZ_ASSERT(!mDT, "Can't initialize twice!");
+    mDT = aDT;
+    cg = BorrowCGContextFromDrawTarget(aDT);
+    return cg;
+  }
+
+  // The caller needs to call Finish if cg is non-null when
+  // they are done with the context. This is currently explicit
+  // instead of happening implicitly in the destructor to make
+  // what's happening in the caller more clear. It also
+  // let's you resume using the DrawTarget in the same scope.
+  void Finish()
+  {
+    if (cg) {
+      ReturnCGContextToDrawTarget(mDT, cg);
+      cg = nullptr;
+    }
+  }
+
+  ~BorrowedCGContext() {
+    MOZ_ASSERT(!cg);
+  }
+
+  CGContextRef cg;
+private:
+  static CGContextRef BorrowCGContextFromDrawTarget(DrawTarget *aDT);
+  static void ReturnCGContextToDrawTarget(DrawTarget *aDT, CGContextRef cg);
+  DrawTarget *mDT;
+};
+#endif
+
+}
+}
+
+#endif // _MOZILLA_GFX_BORROWED_CONTEXT_H