daemon/property_monitor.h

/*
 * Copyright (C) 2020 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
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#pragma once

#include <sys/system_properties.h>

#include <atomic>
#include <condition_variable>
#include <functional>
#include <mutex>
#include <string>
#include <unordered_map>

using PropertyMonitorCallback = bool(std::string value);

struct PropertyMonitorData {
    std::function<PropertyMonitorCallback> callback;
    const prop_info* prop_info;
    uint32_t serial;
};

// This class is thread-unsafe: all operations must be guarded by mutexes if they can occur on
// different threads.
struct PropertyMonitor {
    PropertyMonitor() = default;
    ~PropertyMonitor() = default;

    // Register a callback on a specified property, and immediately invoke it on the current value.
    // If the property is not defined, the callback will be invoked with the empty string.
    //
    // Only one callback can be registered on a property at once: subsequent registrations will
    // delete the original callback.
    void Add(std::string property, std::function<PropertyMonitorCallback> callback);

    // Run the PropertyMonitor indefinitely.
    //
    // This will run until a callback returns false. If a callback returns false, this does not
    // return immediately: it will run the other callbacks for changed properties first.
    void Run();

  private:
    std::unordered_map<std::string, PropertyMonitorData> properties_;
    uint32_t last_serial_ = 0;
};