ArchiSteamFarm/ArchiSteamFarm/Helpers/ArchiCacheable.cs

//     _                _      _  ____   _                           _____
//    / \    _ __  ___ | |__  (_)/ ___| | |_  ___   __ _  _ __ ___  |  ___|__ _  _ __  _ __ ___
//   / _ \  | '__|/ __|| '_ \ | |\___ \ | __|/ _ \ / _` || '_ ` _ \ | |_  / _` || '__|| '_ ` _ \
//  / ___ \ | |  | (__ | | | || | ___) || |_|  __/| (_| || | | | | ||  _|| (_| || |   | | | | | |
// /_/   \_\|_|   \___||_| |_||_||____/  \__|\___| \__,_||_| |_| |_||_|   \__,_||_|   |_| |_| |_|
// 
// Copyright 2015-2018 Łukasz "JustArchi" Domeradzki
// Contact: JustArchi@JustArchi.net
// 
// 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.

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ArchiSteamFarm.Helpers {
	internal sealed class ArchiCacheable<T> : IDisposable {
		private readonly TimeSpan CacheLifetime;
		private readonly SemaphoreSlim InitSemaphore = new SemaphoreSlim(1, 1);
		private readonly Func<Task<(bool Success, T Result)>> ResolveFunction;

		private bool IsInitialized => InitializedAt > DateTime.MinValue;
		private bool IsPermanentCache => CacheLifetime == Timeout.InfiniteTimeSpan;
		private bool IsRecent => IsPermanentCache || (DateTime.UtcNow.Subtract(InitializedAt) < CacheLifetime);

		// Purge should happen slightly after lifetime, to allow eventual refresh if the property is still used
		private TimeSpan PurgeLifetime => CacheLifetime + TimeSpan.FromMinutes(5);

		private DateTime InitializedAt;
		private T InitializedValue;
		private Timer MaintenanceTimer;

		internal ArchiCacheable(Func<Task<(bool Success, T Result)>> resolveFunction, TimeSpan? cacheLifetime = null) {
			ResolveFunction = resolveFunction ?? throw new ArgumentNullException(nameof(resolveFunction));
			CacheLifetime = cacheLifetime ?? Timeout.InfiniteTimeSpan;
		}

		public void Dispose() {
			// Those are objects that are always being created if constructor doesn't throw exception
			InitSemaphore.Dispose();

			// Those are objects that might be null and the check should be in-place
			MaintenanceTimer?.Dispose();
		}

		internal async Task<(bool Success, T Result)> GetValue() {
			if (IsInitialized && IsRecent) {
				return (true, InitializedValue);
			}

			await InitSemaphore.WaitAsync().ConfigureAwait(false);

			try {
				if (IsInitialized && IsRecent) {
					return (true, InitializedValue);
				}

				(bool success, T result) = await ResolveFunction().ConfigureAwait(false);

				if (!success) {
					return (false, InitializedValue);
				}

				InitializedValue = result;
				InitializedAt = DateTime.UtcNow;

				if (!IsPermanentCache) {
					if (MaintenanceTimer == null) {
						MaintenanceTimer = new Timer(
							async e => await SoftReset().ConfigureAwait(false),
							null,
							PurgeLifetime, // Delay
							Timeout.InfiniteTimeSpan // Period
						);
					} else {
						MaintenanceTimer.Change(PurgeLifetime, Timeout.InfiniteTimeSpan);
					}
				}

				return (true, result);
			} finally {
				InitSemaphore.Release();
			}
		}

		internal async Task Reset() {
			if (!IsInitialized) {
				return;
			}

			await InitSemaphore.WaitAsync().ConfigureAwait(false);

			try {
				if (!IsInitialized) {
					return;
				}

				HardReset();
			} finally {
				InitSemaphore.Release();
			}
		}

		private void HardReset() {
			InitializedAt = DateTime.MinValue;
			InitializedValue = default;

			if (MaintenanceTimer != null) {
				MaintenanceTimer.Dispose();
				MaintenanceTimer = null;
			}
		}

		private async Task SoftReset() {
			if (!IsInitialized || IsRecent) {
				return;
			}

			await InitSemaphore.WaitAsync().ConfigureAwait(false);

			try {
				if (!IsInitialized || IsRecent) {
					return;
				}

				HardReset();
			} finally {
				InitSemaphore.Release();
			}
		}
	}
}