Bug 790673. Add BorrowedCGContext. r=bas

This can be used to safely get at the underlying CGContext.

gfx/2d/2D.h

@@ -40,6 +40,9 @@ struct IDWriteRenderingParams;
 class GrContext;
 struct GrGLInterface;
 
+struct CGContext;
+typedef struct CGContext *CGContextRef;
+
 namespace mozilla {
 
 namespace gfx {
@@ -993,6 +996,46 @@ private:
   static DrawEventRecorder *mRecorder;
 };
 
+#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(DrawTarget *aDT) : mDT(aDT)
+  {
+    cg = BorrowCGContextFromDrawTarget(aDT);
+  }
+
+  // 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
+
 }
 }
 

gfx/2d/DrawTargetCG.cpp

@@ -1166,6 +1166,33 @@ DrawTargetCG::MarkChanged()
   }
 }
 
+CGContextRef
+BorrowedCGContext::BorrowCGContextFromDrawTarget(DrawTarget *aDT)
+{
+  if (aDT->GetType() == BACKEND_COREGRAPHICS || aDT->GetType() == BACKEND_COREGRAPHICS_ACCELERATED) {
+    DrawTargetCG* cgDT = static_cast<DrawTargetCG*>(aDT);
+    cgDT->MarkChanged();
+
+    // swap out the context
+    CGContextRef cg = cgDT->mCg;
+    cgDT->mCg = nullptr;
+
+    // save the state to make it easier for callers to avoid mucking with things
+    CGContextSaveGState(cg);
+
+    return cg;
+  }
+  return nullptr;
+}
+
+void
+BorrowedCGContext::ReturnCGContextToDrawTarget(DrawTarget *aDT, CGContextRef cg)
+{
+  DrawTargetCG* cgDT = static_cast<DrawTargetCG*>(aDT);
+
+  CGContextRestoreGState(cg);
+  cgDT->mCg = cg;
+}
 
 
 }

gfx/2d/DrawTargetCG.h

@@ -85,6 +85,7 @@ SetStrokeOptions(CGContextRef cg, const StrokeOptions &aStrokeOptions)
 class DrawTargetCG : public DrawTarget
 {
 public:
+  friend BorrowedCGContext;
   DrawTargetCG();
   virtual ~DrawTargetCG();