Bug 1823215 - Extract ElementOrArray<T> from CanvasRenderingContext2D into gfxUtils.h so it can also be used elsewhere in gfx/thebes. r=gfx-reviewers,lsalzman

Differential Revision: https://phabricator.services.mozilla.com/D172938

dom/canvas/CanvasRenderingContext2D.h

@@ -24,6 +24,7 @@
 #include "FilterDescription.h"
 #include "gfx2DGlue.h"
 #include "gfxFontConstants.h"
+#include "gfxUtils.h"
 #include "nsICanvasRenderingContextInternal.h"
 #include "nsColor.h"
 #include "nsIFrame.h"
@@ -78,106 +79,6 @@ struct DOMMatrix2DInit;
 class CanvasRenderingContext2D : public nsICanvasRenderingContextInternal,
                                  public nsWrapperCache,
                                  public BasicRenderingContext2D {
-  // Container for either a single element of type T, or an nsTArray<T>.
-  // Provides a minimal subset of nsTArray's API, just enough to support use
-  // by ContextState for the clipsAndTransforms list.
-  // Using this instead of a simple nsTArray avoids an extra allocation in the
-  // common case where no more than one element is ever added to the list.
-  // Unlike an AutoTArray<..., 1>, this class is memmovable and therefore can
-  // be used in ContextState without breaking its movability.
-  template <typename T>
-  class ElementOrArray {
-    union {
-      T mElement;
-      nsTArray<T> mArray;
-    };
-    enum class Tag : uint8_t {
-      Element,
-      Array,
-    } mTag;
-
-   public:
-    // Construct as an empty array.
-    ElementOrArray() : mTag(Tag::Array) { new (&mArray) nsTArray<T>(); }
-
-    // For now, don't support copy/move.
-    ElementOrArray(const ElementOrArray&) = delete;
-    ElementOrArray(ElementOrArray&&) = delete;
-
-    ElementOrArray& operator=(const ElementOrArray&) = delete;
-    ElementOrArray& operator=(ElementOrArray&&) = delete;
-
-    // Destroy the appropriate variant.
-    ~ElementOrArray() {
-      switch (mTag) {
-        case Tag::Element:
-          mElement.~T();
-          break;
-        case Tag::Array:
-          mArray.~nsTArray();
-          break;
-      }
-    }
-
-    size_t Length() const { return mTag == Tag::Element ? 1 : mArray.Length(); }
-
-    T* AppendElement(const T& aElement) {
-      switch (mTag) {
-        case Tag::Element: {
-          // Move the existing element into an array, then append the new one.
-          T temp = std::move(mElement);
-          mElement.~T();
-          mTag = Tag::Array;
-          new (&mArray) nsTArray<T>();
-          mArray.AppendElement(std::move(temp));
-          return mArray.AppendElement(aElement);
-        }
-        case Tag::Array: {
-          // If currently empty, just store the element directly.
-          if (mArray.IsEmpty()) {
-            mArray.~nsTArray();
-            mTag = Tag::Element;
-            new (&mElement) T(aElement);
-            return &mElement;
-          }
-          // Otherwise, append it to the array.
-          return mArray.AppendElement(aElement);
-        }
-        default:
-          MOZ_MAKE_COMPILER_ASSUME_IS_UNREACHABLE("invalid tag");
-      }
-    }
-
-    const T& LastElement() const {
-      return mTag == Tag::Element ? mElement : mArray.LastElement();
-    }
-
-    T& LastElement() {
-      return mTag == Tag::Element ? mElement : mArray.LastElement();
-    }
-
-    bool IsEmpty() const {
-      return mTag == Tag::Element ? false : mArray.IsEmpty();
-    }
-
-    // Simple iterators to support range-for loops.
-    const T* begin() const {
-      return mTag == Tag::Array ? mArray.IsEmpty() ? nullptr : &*mArray.begin()
-                                : &mElement;
-    }
-    T* begin() {
-      return mTag == Tag::Array ? mArray.IsEmpty() ? nullptr : &*mArray.begin()
-                                : &mElement;
-    }
-
-    const T* end() const {
-      return mTag == Tag::Array ? begin() + mArray.Length() : &mElement + 1;
-    }
-    T* end() {
-      return mTag == Tag::Array ? begin() + mArray.Length() : &mElement + 1;
-    }
-  };
-
  protected:
   virtual ~CanvasRenderingContext2D();
 

gfx/thebes/gfxUtils.h

@@ -417,6 +417,106 @@ class gfxUtils {
 
 namespace mozilla {
 
+// Container for either a single element of type T, or an nsTArray<T>.
+// Provides a minimal subset of nsTArray's API, just enough to support use
+// by ContextState for the clipsAndTransforms list.
+// Using this instead of a simple nsTArray avoids an extra allocation in the
+// common case where no more than one element is ever added to the list.
+// Unlike an AutoTArray<..., 1>, this class is memmovable and therefore can
+// be used in ContextState without breaking its movability.
+template <typename T>
+class ElementOrArray {
+  union {
+    T mElement;
+    nsTArray<T> mArray;
+  };
+  enum class Tag : uint8_t {
+    Element,
+    Array,
+  } mTag;
+
+ public:
+  // Construct as an empty array.
+  ElementOrArray() : mTag(Tag::Array) { new (&mArray) nsTArray<T>(); }
+
+  // For now, don't support copy/move.
+  ElementOrArray(const ElementOrArray&) = delete;
+  ElementOrArray(ElementOrArray&&) = delete;
+
+  ElementOrArray& operator=(const ElementOrArray&) = delete;
+  ElementOrArray& operator=(ElementOrArray&&) = delete;
+
+  // Destroy the appropriate variant.
+  ~ElementOrArray() {
+    switch (mTag) {
+      case Tag::Element:
+        mElement.~T();
+        break;
+      case Tag::Array:
+        mArray.~nsTArray();
+        break;
+    }
+  }
+
+  size_t Length() const { return mTag == Tag::Element ? 1 : mArray.Length(); }
+
+  T* AppendElement(const T& aElement) {
+    switch (mTag) {
+      case Tag::Element: {
+        // Move the existing element into an array, then append the new one.
+        T temp = std::move(mElement);
+        mElement.~T();
+        mTag = Tag::Array;
+        new (&mArray) nsTArray<T>();
+        mArray.AppendElement(std::move(temp));
+        return mArray.AppendElement(aElement);
+      }
+      case Tag::Array: {
+        // If currently empty, just store the element directly.
+        if (mArray.IsEmpty()) {
+          mArray.~nsTArray();
+          mTag = Tag::Element;
+          new (&mElement) T(aElement);
+          return &mElement;
+        }
+        // Otherwise, append it to the array.
+        return mArray.AppendElement(aElement);
+      }
+      default:
+        MOZ_MAKE_COMPILER_ASSUME_IS_UNREACHABLE("invalid tag");
+    }
+  }
+
+  const T& LastElement() const {
+    return mTag == Tag::Element ? mElement : mArray.LastElement();
+  }
+
+  T& LastElement() {
+    return mTag == Tag::Element ? mElement : mArray.LastElement();
+  }
+
+  bool IsEmpty() const {
+    return mTag == Tag::Element ? false : mArray.IsEmpty();
+  }
+
+  // Simple iterators to support range-for loops.
+  const T* begin() const {
+    return mTag == Tag::Array ? mArray.IsEmpty() ? nullptr : &*mArray.begin()
+                              : &mElement;
+  }
+  T* begin() {
+    return mTag == Tag::Array ? mArray.IsEmpty() ? nullptr : &*mArray.begin()
+                              : &mElement;
+  }
+
+  const T* end() const {
+    return mTag == Tag::Array ? begin() + mArray.Length() : &mElement + 1;
+  }
+  T* end() {
+    return mTag == Tag::Array ? begin() + mArray.Length() : &mElement + 1;
+  }
+};
+
 struct StyleAbsoluteColor;
 
 namespace gfx {