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// Copyright (C) 2014 The Android Open Source Project
// 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 <list>
#include <string>
#include <base/bind.h>
#include <base/location.h>
#include <base/logging.h>
#include <base/time/time.h>
#include <brillo/message_loops/message_loop.h>
#include <gtest/gtest_prod.h> // for FRIEND_TEST
namespace chromeos_update_manager {
// The VariableMode specifies important behavior of the variable in terms of
// whether, how and when the value of the variable changes.
enum VariableMode {
// Const variables never changes during the life of a policy request, so the
// EvaluationContext caches the value even between different evaluations of
// the same policy request.
// Poll variables, or synchronous variables, represent a variable with a value
// that can be queried at any time, but it is not known when the value
// changes on the source of information. In order to detect if the value of
// the variable changes, it has to be queried again.
// Async variables are able to produce a signal or callback whenever the
// value changes. This means that it's not required to poll the value to
// detect when it changes, instead, you should register an observer to get
// a notification when that happens.
// This class is a base class with the common functionality that doesn't
// depend on the variable's type, implemented by all the variables.
class BaseVariable {
// Interface for observing changes on variable value.
class ObserverInterface {
virtual ~ObserverInterface() {}
// Called when the value on the variable changes.
virtual void ValueChanged(BaseVariable* variable) = 0;
virtual ~BaseVariable() {
if (!observer_list_.empty()) {
LOG(WARNING) << "Variable " << name_ << " deleted with "
<< observer_list_.size() << " observers.";
DCHECK(observer_list_.empty()) << "Don't destroy the variable without "
"removing the observers.";
// Returns the variable name as a string.
const std::string& GetName() const {
return name_;
// Returns the variable mode.
VariableMode GetMode() const {
return mode_;
// For VariableModePoll variables, it returns the polling interval of this
// variable. In other case, it returns 0.
base::TimeDelta GetPollInterval() const {
return poll_interval_;
// Adds and removes observers for value changes on the variable. This only
// works for kVariableAsync variables since the other modes don't track value
// changes. Adding the same observer twice has no effect.
virtual void AddObserver(BaseVariable::ObserverInterface* observer) {
if (std::find(observer_list_.begin(), observer_list_.end(), observer) ==
observer_list_.end()) {
virtual void RemoveObserver(BaseVariable::ObserverInterface* observer) {
// Creates a BaseVariable using the default polling interval (5 minutes).
BaseVariable(const std::string& name, VariableMode mode)
: BaseVariable(name, mode,
base::TimeDelta::FromMinutes(kDefaultPollMinutes)) {}
// Creates a BaseVariable with mode kVariableModePoll and the provided
// polling interval.
BaseVariable(const std::string& name, base::TimeDelta poll_interval)
: BaseVariable(name, kVariableModePoll, poll_interval) {}
// Reset the poll interval on a polling variable to the given one.
void SetPollInterval(base::TimeDelta poll_interval) {
DCHECK_EQ(kVariableModePoll, mode_) << "Can't set the poll_interval on a "
<< mode_ << " variable";
poll_interval_ = poll_interval;
// Calls ValueChanged on all the observers.
void NotifyValueChanged() {
// Fire all the observer methods from the main loop as single call. In order
// to avoid scheduling these callbacks when it is not needed, we check
// first the list of observers.
if (!observer_list_.empty()) {
friend class UmEvaluationContextTest;
FRIEND_TEST(UmBaseVariableTest, RepeatedObserverTest);
FRIEND_TEST(UmBaseVariableTest, NotifyValueChangedTest);
FRIEND_TEST(UmBaseVariableTest, NotifyValueRemovesObserversTest);
BaseVariable(const std::string& name, VariableMode mode,
base::TimeDelta poll_interval)
: name_(name), mode_(mode),
poll_interval_(mode == kVariableModePoll ?
poll_interval : base::TimeDelta()) {}
void OnValueChangedNotification() {
// A ValueChanged() method can change the list of observers, for example
// removing itself and invalidating the iterator, so we create a snapshot
// of the observers first. Also, to support the case when *another* observer
// is removed, we check for them.
std::list<BaseVariable::ObserverInterface*> observer_list_copy(
for (auto& observer : observer_list_copy) {
if (std::find(observer_list_.begin(), observer_list_.end(), observer) !=
observer_list_.end()) {
// The default PollInterval in minutes.
static constexpr int kDefaultPollMinutes = 5;
// The variable's name as a string.
const std::string name_;
// The variable's mode.
const VariableMode mode_;
// The variable's polling interval for VariableModePoll variable and 0 for
// other modes.
base::TimeDelta poll_interval_;
// The list of value changes observers.
std::list<BaseVariable::ObserverInterface*> observer_list_;
// Interface to an Update Manager variable of a given type. Implementation
// internals are hidden as protected members, since policies should not be
// using them directly.
template<typename T>
class Variable : public BaseVariable {
~Variable() override {}
// Only allow to get values through the EvaluationContext class and not
// directly from the variable.
friend class EvaluationContext;
// Needed to be able to verify variable contents during unit testing.
friend class UmTestUtils;
FRIEND_TEST(UmRealRandomProviderTest, GetRandomValues);
Variable(const std::string& name, VariableMode mode)
: BaseVariable(name, mode) {}
Variable(const std::string& name, const base::TimeDelta poll_interval)
: BaseVariable(name, poll_interval) {}
// Gets the current value of the variable. The current value is copied to a
// new object and returned. The caller of this method owns the object and
// should delete it.
// In case of and error getting the current value or the |timeout| timeout is
// exceeded, a null value is returned and the |errmsg| is set.
// The caller can pass a null value for |errmsg|, in which case the error
// message won't be set.
virtual const T* GetValue(base::TimeDelta timeout, std::string* errmsg) = 0;
} // namespace chromeos_update_manager