Factor PaintArtifact out of DisplayItemList.

third_party/WebKit/Source/platform/graphics/paint/DisplayItems.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef DisplayItems_h
+#define DisplayItems_h
+
+#include "platform/graphics/ContiguousContainer.h"
+#include "platform/graphics/paint/DisplayItem.h"
+#include "platform/graphics/paint/Transform3DDisplayItem.h"
+#include "wtf/Alignment.h"
+#include "wtf/Assertions.h"
+
+#ifndef NDEBUG
+#include "wtf/text/WTFString.h"
+#endif
+
+namespace blink {
+
+// kDisplayItemAlignment must be a multiple of alignof(derived display item) for
+// each derived display item; the ideal value is the least common multiple.
+// Currently the limiting factor is TransformtionMatrix (in

[wkorman, 2015/10/09 04:45:50]

Transformtion -> Transformation

+// BeginTransform3DDisplayItem), which requests 16-byte alignment.
+static const size_t kDisplayItemAlignment = WTF_ALIGN_OF(BeginTransform3DDisplayItem);
+static const size_t kMaximumDisplayItemSize = sizeof(BeginTransform3DDisplayItem);
+
+// A container for a list of display items.
+class DisplayItems : public ContiguousContainer<DisplayItem, kDisplayItemAlignment> {

[pdr., 2015/10/09 03:57:51]

What do you think about expanding this class a bit to include all individual
display-listy things?

I think it's reasonable to store the scope stack here since it's implicit in the
current state of the list. So long as we don't regress back to having a single
class that both stores and manages the details of two lists. Doing this would
involve moving processNewItem/beginScope/beginSkippingCache/etc here and
disentangling the debug map spaghetti code.

If you don't agree about going whole hog on display-listy things, an alternative
is to just move processNewItem here and pass in the scope/skipcache/etc. Doing
this would involve moving processNewItem here and disentangling the debug map
spaghetti code.


There's a common theme in these suggestions about that debug map spaghetti..

[pdr., 2015/10/13 18:30:32]

This is kind of high-level but I'd like your thoughts.

+public:
+    DisplayItems(size_t initialSizeBytes)
+        : ContiguousContainer(kMaximumDisplayItemSize, initialSizeBytes) {}
+
+    DisplayItem& appendByMoving(DisplayItem& item)
+    {
+#ifndef NDEBUG
+        WTF::String originalDebugString = item.asDebugString();
+#endif
+        ASSERT(item.isValid());
+        DisplayItem& result = ContiguousContainer::appendByMoving(item, item.derivedSize());
+        // ContiguousContainer::appendByMoving() called in-place constructor on item, which invalidated it.
+        ASSERT(!item.isValid());
+#ifndef NDEBUG
+        // Save original debug string in the old item to help debugging.
+        item.setClientDebugString(originalDebugString);
+#endif
+        return result;
+    }
+};
+
+} // namespace blink
+
+#endif // DisplayItems_h