[#8] Add auto update checker

Elwig/Helpers/Utils.cs

@@ -12,6 +12,10 @@ using System.Text;
 using System.Numerics;
 using Elwig.Models.Entities;
 using Elwig.Helpers.Billing;
+using System.Runtime.InteropServices;
+using System.Net.Http;
+using System.Text.Json.Nodes;
+using System.IO;
 
 namespace Elwig.Helpers {
     public static partial class Utils {
@@ -61,6 +65,8 @@ namespace Elwig.Helpers {
             return PhoneNrTypes.Where(t => t.Key == type).Select(t => t.Value).FirstOrDefault(type);
         }
 
+        private static readonly string[] TempWildcards = ["*.html", "*.pdf", "*.exe"];
+
         private static readonly ushort[] Crc16ModbusTable = [
             0x0000, 0xC0C1, 0xC181, 0x0140, 0xC301, 0x03C0, 0x0280, 0xC241,
             0xC601, 0x06C0, 0x0780, 0xC741, 0x0500, 0xC5C1, 0xC481, 0x0440,
@@ -302,7 +308,7 @@ namespace Elwig.Helpers {
         public static string GetSign<T>(T number) where T : INumber<T>
             => T.Sign(number) switch {
                 < 0 => "\u2212",  // minus
-                  0 => "\u00b1",  // plus minus
+                0 => "\u00b1",  // plus minus
                 > 0 => "+",
             };
 
@@ -378,5 +384,33 @@ namespace Elwig.Helpers {
                 .Select(s => new Varibute(s, varieties, attributes, cultivations))
                 .ToList();
         }
+
+        [LibraryImport("wininet.dll")]
+        [return: MarshalAs(UnmanagedType.Bool)]
+        private static partial bool InternetGetConnectedState(out int description, int reservedValue);
+
+        public static bool HasInternetConnectivity() {
+            return InternetGetConnectedState(out var _, 0);
+        }
+
+        public static async Task<(string Version, string Url, long Size)?> GetLatestInstallerUrl(string url) {
+            try {
+                using var client = new HttpClient() {
+                    Timeout = TimeSpan.FromSeconds(5),
+                };
+                var res = JsonNode.Parse(await client.GetStringAsync(url));
+                var data = res!["data"]![0]!;
+                return ((string)data["version"]!, (string)data["url"]!, (int)data["size"]!);
+            } catch {
+                return null;
+            }
+        }
+
+        public static void CleanupTempFiles() {
+            var dir = new DirectoryInfo(App.TempPath);
+            foreach (var file in TempWildcards.SelectMany(dir.EnumerateFiles)) {
+                file.Delete();
+            }
+        }
     }
 }