Bug 740200 - BorrowedContext support for cairo; r=mattwoodrow

gfx/2d/2D.h

@@ -27,6 +27,9 @@
 #include <string>
 #endif
 
+struct _cairo;
+typedef struct _cairo cairo_t;
+
 struct _cairo_surface;
 typedef _cairo_surface cairo_surface_t;
 
@@ -1017,6 +1020,60 @@ private:
   static DrawEventRecorder *mRecorder;
 };
 
+/* 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.

gfx/2d/DrawTargetCairo.cpp

@@ -1096,5 +1096,38 @@ DrawTargetCairo::SetTransform(const Matrix& aTransform)
   cairo_set_matrix(mContext, &mat);
 }
 
+cairo_t*
+BorrowedCairoContext::BorrowCairoContextFromDrawTarget(DrawTarget* aDT)
+{
+  if (aDT->GetType() != BACKEND_CAIRO || aDT->IsDualDrawTarget()) {
+    return nullptr;
+  }
+  DrawTargetCairo* cairoDT = static_cast<DrawTargetCairo*>(aDT);
+
+  cairoDT->WillChange();
+
+  // save the state to make it easier for callers to avoid mucking with things
+  cairo_save(cairoDT->mContext);
+
+  // Neuter the DrawTarget while the context is being borrowed
+  cairo_t* cairo = cairoDT->mContext;
+  cairoDT->mContext = nullptr;
+
+  return cairo;
+}
+
+void
+BorrowedCairoContext::ReturnCairoContextToDrawTarget(DrawTarget* aDT,
+                                                     cairo_t* aCairo)
+{
+  if (aDT->GetType() != BACKEND_CAIRO || aDT->IsDualDrawTarget()) {
+    return;
+  }
+  DrawTargetCairo* cairoDT = static_cast<DrawTargetCairo*>(aDT);
+
+  cairo_restore(aCairo);
+  cairoDT->mContext = aCairo;
+}
+
 }
 }

gfx/2d/DrawTargetCairo.h

@@ -51,6 +51,8 @@ class GradientStopsCairo : public GradientStops
 class DrawTargetCairo : public DrawTarget
 {
 public:
+  friend BorrowedCairoContext;
+
   DrawTargetCairo();
   virtual ~DrawTargetCairo();