Add flag cache.

Marechai.App/Services/Caching/FlagCache.cs

@@ -0,0 +1,64 @@
+using System;
+using System.IO;
+using System.Net.Http;
+using System.Threading.Tasks;
+using Windows.Storage;
+using Microsoft.Extensions.Configuration;
+
+namespace Marechai.App.Services.Caching;
+
+public sealed class FlagCache
+{
+    readonly IConfiguration _configuration;
+    StorageFolder           _flagsFolder;
+
+    public FlagCache(IConfiguration configuration)
+    {
+        _configuration = configuration;
+        _              = EnsureFolderExistAsync();
+    }
+
+    async Task EnsureFolderExistAsync()
+    {
+        StorageFolder localFolder = ApplicationData.Current.LocalCacheFolder;
+        _flagsFolder = await localFolder.CreateFolderAsync("flags", CreationCollisionOption.OpenIfExists);
+    }
+
+    public async Task<Stream> GetFlagAsync(short countryCode)
+    {
+        var filename = $"{countryCode}.svg";
+
+        Stream retStream;
+
+        if(await _flagsFolder.TryGetItemAsync(filename) is StorageFile file)
+        {
+            retStream = await file.OpenStreamForReadAsync();
+
+            return retStream;
+        }
+
+        await CacheFlagAsync(countryCode);
+
+        file = await _flagsFolder.GetFileAsync(filename);
+
+        retStream = await file.OpenStreamForReadAsync();
+
+        return retStream;
+    }
+
+    async Task CacheFlagAsync(short countryCode)
+    {
+        var                       filename   = $"{countryCode}.svg";
+        string                    baseUrl    = _configuration.GetSection("ApiClient:Url").Value;
+        string                    flagUrl    = baseUrl + $"/assets/flags/{filename}";
+        using var                 httpClient = new HttpClient();
+        using HttpResponseMessage response   = await httpClient.GetAsync(flagUrl);
+        response.EnsureSuccessStatusCode();
+
+        using Stream stream = await response.Content.ReadAsStreamAsync();
+        StorageFile  file   = await _flagsFolder.CreateFileAsync(filename, CreationCollisionOption.ReplaceExisting);
+
+        using Stream fileStream = await file.OpenStreamForWriteAsync();
+        await stream.CopyToAsync(fileStream);
+    }
+}