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// Copyright 2015 The Cobalt Authors. All Rights Reserved.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
#include <algorithm>
#include <memory>
#include "base/basictypes.h"
#include "base/logging.h"
#include "cobalt/script/tracer.h"
namespace cobalt {
namespace script {
template <typename T>
class Handle;
class Wrappable;
// ScriptValue is a wrapper around raw JavaScript values that are being passed
// into Cobalt. These are values that do not correspond to Cobalt Wrappable
// objects. Specifically, this could include objects that implement the
// EventListener interface, callback functions, promises, or any Javascript
// value at all. As long as Cobalt maintains a handle to such an value, it
// should not be garbage collected, if it is a garbage-collectible thing (GC
// thing). Web API implementations should hold on to a reference to a
// ScriptValue implementation by constructing a ScriptValue::Reference object.
// The Reference class takes as a constructor parameter a pointer to the
// Wrappable that is holding onto the ScriptValue. This ensures that the
// JavaScript engine's garbage collection is aware of the relationship between
// the ScriptValue and the Wrappable's corresponding JavaScript wrapper. This
// will ensure that the garbage collector can detect when these values are
// detached from the rest of the graph of JavaScript GC things, and can safely
// be garbage collected.
template <class T>
class ScriptValue {
// This Deleter is meant to be used only when instantiating std::unique_ptr.
// struct Deleter {
// Deleter() = default;
// Deleter(Deleter&) = default;
// Deleter(const Deleter&) = default;
// Deleter(Deleter&&) = default;
// inline void operator()(ScriptValue* value) const { delete value; };
// };
// The Reference class maintains the ownership relationship between a
// Wrappable and the JavaScript value wrapped by a ScriptValue. This is an
// RAII object in that creation of a Reference instance will mark the
// underlying value as owned by this wrappable, and the underlying value
// will be unmarked when this Reference is destructed. The lifetime of a
// Reference must be at least as long as the Wrappable that has been passed
// into the constructor.
class Reference {
Reference(Wrappable* wrappable, const ScriptValue& script_value)
: owner_(wrappable), referenced_value_(script_value.MakeCopy()) {
Reference(Wrappable* wrappable, const Handle<T>& local)
: owner_(wrappable),
referenced_value_(local.GetScriptValue()->MakeCopy()) {
const T& value() const { return *(referenced_value_->GetValue()); }
// Return the referenced ScriptValue. This ScriptValue can be passed back
// into the JavaScript bindings layer where the referenced JavaScript
// value can be extracted from the ScriptValue.
const ScriptValue<T>& referenced_value() const {
return *(referenced_value_.get());
~Reference() { referenced_value_->DeregisterOwner(owner_); }
Wrappable* const owner_;
// std::unique_ptr<ScriptValue, Deleter> referenced_value_;
std::unique_ptr<ScriptValue> referenced_value_;
// Return true if and only if |other| refers to the same underlying
// JavaScript value.
virtual bool EqualTo(const ScriptValue& other) const = 0;
// Returns true if this ScriptValue is referring to a NULL JavaScript value.
bool IsNull() const { return GetValue() == NULL; }
// Creates a new ScriptValue that contains a weak reference to the same
// underlying JavaScript value. Note that this will not prevent the value
// from being garbage collected, one must create a Reference to do that.
std::unique_ptr<ScriptValue> MakeWeakCopy() const {
return std::move(MakeCopy());
virtual ~ScriptValue() {}
// Register this Wrappable as owning a handle to the underlying JavaScript
// value.
virtual void RegisterOwner(Wrappable* owner) = 0;
virtual void DeregisterOwner(Wrappable* owner) = 0;
// Prevent/Allow garbage collection of the underlying ScriptValue. Calls
// must be balanced and are not idempotent. While the number of calls to
// |Prevent| are greater than the number of calls to |Allow|, the underlying
// value will never be garbage collected.
virtual void PreventGarbageCollection() = 0;
virtual void AllowGarbageCollection() = 0;
// Return a pointer to the value that wraps the underlying JavaScript
// value.
virtual T* GetValue() = 0;
virtual const T* GetValue() const = 0;
// Make a new ScriptValue instance that holds a handle to the same
// underlying JavaScript value. This should not be called for a ScriptValue
// that has a NULL script value (that is, GetScriptValue() returns NULL).
virtual std::unique_ptr<ScriptValue> MakeCopy() const = 0;
int reference_count_ = 0;
template <typename F>
friend class Handle;
friend class Reference;
// A handle that references a |ScriptValue|, and manages its garbage
// collection state via reference counting. This is the preferred type for
// receiving, returning, and manipulating |ScriptValue|s.
template <typename T>
class Handle {
// This should only be used by internals, next to where engine specific
// |ScriptValue| implementations are constructed. Calling this from common
// code is a usage error. If you want a new |Local<T>|, then you should
// build it from an existing reference, rather than working directly with
// the |ScriptValue|.
explicit Handle(ScriptValue<T>* script_value) : script_value_(script_value) {
DCHECK_EQ(script_value_->reference_count_, 0);
// Intentionally not explicit because bindings' usage of ToJSValue relies on
// implicit conversion.
Handle(const ScriptValue<T>* script_value) // NOLINT
: Handle(script_value->MakeWeakCopy().release()) {}
Handle(const ScriptValue<T>& script_value) // NOLINT
: Handle(script_value.MakeWeakCopy().release()) {}
Handle(const typename ScriptValue<T>::Reference& reference) // NOLINT
: Handle(reference.referenced_value().MakeWeakCopy().release()) {}
Handle(const Handle& other) : script_value_(other.script_value_) {
// We need the default constructor for nullable ScriptValue.
Handle() = default;
Handle& operator=(const Handle& other) {
// Increment |other|'s value first to allow for self assignment.
if (other.script_value_) {
script_value_ = other.script_value_;
return *this;
Handle(Handle&& other) : script_value_(other.script_value_) {
other.script_value_ = nullptr;
Handle& operator=(Handle&& other) {
std::swap(script_value_, other.script_value_);
return *this;
bool IsEmpty() const { return script_value_ == nullptr; }
~Handle() { Clear(); }
T* operator*() { return script_value_->GetValue(); }
T* operator->() const { return script_value_->GetValue(); }
// These are primarily exposed for internals. In most cases you don't need
// to work with the ScriptValue directly.
ScriptValue<T>* GetScriptValue() { return script_value_; }
const ScriptValue<T>* GetScriptValue() const { return script_value_; }
void Clear() {
if (script_value_) {
if (script_value_->reference_count_ == 0) {
delete script_value_;
script_value_ = nullptr;
ScriptValue<T>* script_value_ = nullptr;
} // namespace script
} // namespace cobalt