Delay instantiating InputDeviceCapabilities until accessed.

third_party/WebKit/Source/core/input/InputDeviceCapabilities.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 InputDeviceCapabilities_h
 #define InputDeviceCapabilities_h
 
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "core/CoreExport.h"
 #include "core/input/InputDeviceCapabilitiesInit.h"
 
 namespace blink {
 
+class DOMWindow;
+
 class CORE_EXPORT InputDeviceCapabilities final
     : public GarbageCollected<InputDeviceCapabilities>,
       public ScriptWrappable {
   DEFINE_WRAPPERTYPEINFO();
 
  public:
-  static InputDeviceCapabilities* create(bool firesTouchEvents) {
-    return new InputDeviceCapabilities(firesTouchEvents);
+  enum Descriptor {
+    // |Null| represents a null InputDeviceCapabilities*.
+    Null = 0x0,
+    // |None| represents an empty InputDeviceCapabilities instance with
+    // no capabilities.
+    None = 0x1,
+    FiresTouchEvents = 0x2,
+  };
+
+  static InputDeviceCapabilities* create(Descriptor descriptor) {
+    return new InputDeviceCapabilities(descriptor);
   }
 
   static InputDeviceCapabilities* create(
       const InputDeviceCapabilitiesInit& initializer) {
     return new InputDeviceCapabilities(initializer);
   }
 
   bool firesTouchEvents() const { return m_firesTouchEvents; }
 
   DEFINE_INLINE_TRACE() {}
 
  private:
-  InputDeviceCapabilities(bool firesTouchEvents);
-  InputDeviceCapabilities(const InputDeviceCapabilitiesInit&);
+  explicit InputDeviceCapabilities(Descriptor);
+  explicit InputDeviceCapabilities(const InputDeviceCapabilitiesInit&);
 
   // Whether this device dispatches touch events. This mainly lets developers
   // avoid handling both touch and mouse events dispatched for a single user
   // action.
   bool m_firesTouchEvents;
 };
 
-// Grouping constant-valued InputDeviceCapabilities objects together,
-// which is kept and used by each 'view' (DOMWindow) that dispatches
-// events parameterized over InputDeviceCapabilities.
-//
-// TODO(sof): lazily instantiate InputDeviceCapabilities instances upon
-// UIEvent access instead. This would allow internal tracking of such
-// capabilities by value.
+// To be able to delay the creation of InputDeviceCapabilities instances
+// until requested from script, we characterize the capabilities by a
+// descriptor and propagate its value internally.
+class InputDeviceCapabilitiesValue final {
+  DISALLOW_NEW();
+
+ public:
+  InputDeviceCapabilitiesValue()
+      : m_descriptor(InputDeviceCapabilities::Null) {}
+
+  InputDeviceCapabilitiesValue(const InputDeviceCapabilities::Descriptor desc)
+      : m_descriptor(desc) {}
+
+  // Provided for convenience while InputDeviceCapabilities only supports
+  // a single capability.
+  explicit InputDeviceCapabilitiesValue(bool firesTouchEvents)
+      : m_descriptor(firesTouchEvents
+                         ? InputDeviceCapabilities::FiresTouchEvents
+                         : InputDeviceCapabilities::None) {}
+
+  explicit InputDeviceCapabilitiesValue(InputDeviceCapabilities* capabilities)
+      : m_descriptor(InputDeviceCapabilities::Null),
+        m_capabilities(capabilities) {}
+
+  InputDeviceCapabilitiesValue(const InputDeviceCapabilitiesValue& other)
+      : m_descriptor(other.m_descriptor),
+        m_capabilities(other.m_capabilities) {}
+
+  InputDeviceCapabilities* getCapabilities(DOMWindow*);
+
+  DEFINE_INLINE_TRACE() { visitor->trace(m_capabilities); }
+
+ private:
+  InputDeviceCapabilities::Descriptor m_descriptor;
+  Member<InputDeviceCapabilities> m_capabilities;
+};
+
+// Grouping a DOMWindow's InputDeviceCapabilities objects together.
 class InputDeviceCapabilitiesConstants final
     : public GarbageCollected<InputDeviceCapabilitiesConstants> {
  public:
   // Returns an InputDeviceCapabilities which has
   // |firesTouchEvents| set to value of |firesTouch|.
-  InputDeviceCapabilities* firesTouchEvents(bool firesTouch);
+  InputDeviceCapabilities* get(InputDeviceCapabilities::Descriptor);
 
   DEFINE_INLINE_TRACE() {
     visitor->trace(m_firesTouchEvents);
     visitor->trace(m_doesntFireTouchEvents);
   }
 
  private:
   Member<InputDeviceCapabilities> m_firesTouchEvents;
   Member<InputDeviceCapabilities> m_doesntFireTouchEvents;
 };
 
 }  // namespace blink
 
 #endif  // InputDeviceCapabilities_h