JSON対応版ローカルストレージアクセス(PCLStorage)

HelloPCLStorage.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Threading.Tasks;
using Newtonsoft.Json;
using PCLStorage;
using Xamarin.Forms;

namespace HelloPCLStorage
{
	public class App : Application
	{

		private Entry usernameEntry;

		public App()
		{
			var saveButton = new Button
			{
				Text = "保存"
			};

			usernameEntry = new Entry
			{
				Placeholder = "Username"
			};
			// The root page of your application
			var content = new ContentPage
			{
				Title = "HelloPCLStorage",
				Content = new StackLayout
				{
					VerticalOptions = LayoutOptions.Center,
					Children = {
						new Label {
							HorizontalTextAlignment = TextAlignment.Center,
							Text = "Welcome to Xamarin Forms!"
						},
						usernameEntry,
						saveButton
					}
				}
			};

			saveButton.Clicked += async (sender, e) =>
			{
				await SaveSettings();
			};

			MainPage = new NavigationPage(content);

		}

		// 

		private string settingsFilename = "settings.dat";


		public async Task SaveSettings()
		{
			// 保存先のフォルダインスタンスを生成
			IFolder rootFolder = FileSystem.Current.LocalStorage;
			// 保存先のファイルを開く（存在いている場合は上書き
			IFile file = 
				await rootFolder.CreateFileAsync(
					settingsFilename, 
					CreationCollisionOption.ReplaceExisting);
			// 単一項目のみなので、文字列をそのまま保存
			Dictionary<string, string> settings = new Dictionary<string, string>();
			// 辞書に追加
			settings.Add("username", usernameEntry.Text);
			// JSONに変換
			string jsonString = JsonConvert.SerializeObject(settings);
			Debug.WriteLine(jsonString);
			await file.WriteAllTextAsync(jsonString);
		}


		public async Task LoadSettings()
		{
			// 保存先のフォルダインスタンスを生成
			IFolder rootFolder = FileSystem.Current.LocalStorage;
			ExistenceCheckResult result = await rootFolder.CheckExistsAsync(settingsFilename);
			switch (result)
			{
				case ExistenceCheckResult.FileExists:
					// 存在する時だけ取得反映
					IFile file = await rootFolder.GetFileAsync(settingsFilename);
					var jsonString = await file.ReadAllTextAsync();
					try
					{
						var settings = 
							(Dictionary<string, string>)JsonConvert.DeserializeObject(
								jsonString, 
								typeof (Dictionary<string, string>));
						usernameEntry.Text = settings["username"];
					}
					catch (JsonException jex)
					{
						// 失敗した場合
						Debug.WriteLine(jex.Message);
						Debug.WriteLine(jsonString);

					}
					break;
				case ExistenceCheckResult.FolderExists:
				case ExistenceCheckResult.NotFound:
				default:
					break;
			}
		}

		protected override async void OnStart()
		{
			// Handle when your app starts
			await LoadSettings();
		}

		protected override void OnSleep()
		{
			// Handle when your app sleeps
		}

		protected override void OnResume()
		{
			// Handle when your app resumes
		}
	}
}