WIP for display item list backed by ListContainer

Source/platform/graphics/paint/DisplayItem.h

 // Copyright 2014 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 DisplayItem_h
 #define DisplayItem_h
 
 #include "platform/PlatformExport.h"
 #include "platform/graphics/paint/DisplayItemClient.h"
 #include "wtf/Assertions.h"
 #include "wtf/PassOwnPtr.h"
 
 #ifndef NDEBUG
 #include "wtf/text/StringBuilder.h"
 #include "wtf/text/WTFString.h"
 #endif
 
 
 namespace blink {
 
 class GraphicsContext;
 class WebDisplayItemList;
 
 class PLATFORM_EXPORT DisplayItem {
 public:
+    DisplayItem()
+        : m_client(nullptr)
+        , m_scopeContainer(nullptr)
+        , m_scopeId(0)
+        , m_type(UninitializedType)
+        , m_skippedCache(false)
+#ifndef NDEBUG
+        , m_clientDebugString("invalid")
+#endif
+    { }
+
     enum {
         // Must be kept in sync with core/layout/PaintPhase.h.
         PaintPhaseMax = 12,
     };
 
     // A display item type uniquely identifies a display item of a client.
     // Some display item types can be categorized using the following directives:
     // - In enum Type:
     //   - enum value <Category>First;
     //   - enum values of the category, first of which should equal <Category>First;
@@ skipping 130 matching lines @@
         BeginSubtreeFirst,
         BeginSubtreePaintPhaseFirst = BeginSubtreeFirst,
         BeginSubtreePaintPhaseLast = BeginSubtreePaintPhaseFirst + PaintPhaseMax,
         BeginSubtreeLast = BeginSubtreePaintPhaseLast,
 
         EndSubtreeFirst,
         EndSubtreePaintPhaseFirst = EndSubtreeFirst,
         EndSubtreePaintPhaseLast = EndSubtreePaintPhaseFirst + PaintPhaseMax,
         EndSubtreeLast = EndSubtreePaintPhaseLast,
 
-        TypeLast = EndSubtreeLast
+        UninitializedType,
+        TypeLast = UninitializedType
     };
 
     struct Id {
         Id(DisplayItemClient c, Type t) : client(c), type(t), scopeId(0), scopeContainer(nullptr)
         {
             ASSERT(c);
         }
 
         bool operator==(const Id& other) const
         {
@@ skipping 117 matching lines @@
     virtual bool drawsContent() const { return false; }
 
 #ifndef NDEBUG
     static WTF::String typeAsDebugString(DisplayItem::Type);
     const WTF::String& clientDebugString() const { return m_clientDebugString; }
     WTF::String asDebugString() const;
     virtual void dumpPropertiesAsDebugString(WTF::StringBuilder&) const;
 #endif
 
 protected:
-    DisplayItem(const DisplayItemClientWrapper& client, Type type)
-        : m_client(client.displayItemClient())
-        , m_scopeContainer(nullptr)
-        , m_scopeId(0)
-        , m_type(type)
-        , m_skippedCache(false)
+    // TODO(pdr): friend just the function DisplayItemList::createAndAppend(...).
+    friend class DisplayItemList;
+    void setClientAndType(const DisplayItemClientWrapper& clientWrapper, Type type)
+    {
+        // Clients should never change once set.
+        ASSERT(m_client == nullptr);
+        m_client = clientWrapper.displayItemClient();
+
+        // Types should never change once set.
+        ASSERT(m_type = UninitializedType);
+        m_type = type;
+
 #ifndef NDEBUG
-        , m_clientDebugString(client.debugName())
+        m_clientDebugString = clientWrapper.debugName();
 #endif
-    { }
+    }
 
 private:
-    const DisplayItemClient m_client;
+    DisplayItemClient m_client;
     DisplayItemClient m_scopeContainer;
     int m_scopeId;
     static_assert(TypeLast < (1 << 16), "DisplayItem::Type should fit in 16 bits");
-    const Type m_type : 16;
+    Type m_type : 16;
     unsigned m_skippedCache : 1;
 
 #ifndef NDEBUG
     WTF::String m_clientDebugString;
 #endif
 };
 
 class PLATFORM_EXPORT PairedBeginDisplayItem : public DisplayItem {
 protected:
-    PairedBeginDisplayItem(const DisplayItemClientWrapper& client, Type type) : DisplayItem(client, type) { }
+    PairedBeginDisplayItem() { }
 
 private:
     virtual bool isBegin() const override final { return true; }
 };
 
 class PLATFORM_EXPORT PairedEndDisplayItem : public DisplayItem {
 protected:
-    PairedEndDisplayItem(const DisplayItemClientWrapper& client, Type type) : DisplayItem(client, type) { }
+    PairedEndDisplayItem() { }
 
 #if ENABLE(ASSERT)
     virtual bool isEndAndPairedWith(DisplayItem::Type otherType) const override = 0;
 #endif
 
 private:
     virtual bool isEnd() const override final { return true; }
 };
 
 } // namespace blink
 
 #endif // DisplayItem_h