Windows 8 系列（三）：挂起管理（Suspension Management ）

      从我发Windows 8系列第一篇文章：Windows 8 系列（一）：win 8 简介 到现在有一个月了，原本计划等Windows 8 beta（Windows 8 Consumer Preview）出来以后看看有什么变化，然后再来基于Windows 8 Beta 来写相关的技术博文，而不是基于Windows 8 Developer Preview，毕竟 DP(Developer Preview) 版本还有很多功能和api会在beta版本中有所修改，而且我不知道到底有多少改动。

      从现在看来，确实有部分改动，包括应用程序的生命周期都有了变化（详见Windows 8 系列（二）：Metro Style 应用程序生命周期（Metro Style Application Life Cycle））。我在此想介绍的是挂起管理，顾名思义是应用在触发挂起事件时我们需要做的一件事：保存数据。其实这个跟windows phone 中的墓碑机制有点像，只不过墓碑是15秒限制，而win 8的挂起限制是5秒。

　　在DP版本中，用vs 创建系统自带的Metro style app模板程序后，你会发现工程中有个名为SuspensionManager.cs的文件，而在Beta版本中却没有了这个文件，我觉得可能微软不想把开发者的思维限制住（比如用户是不是真的需要一个字典来存储数据），但是，我觉得对于新手来说，这个类可以方便的进行临时数据保存的管理。代码如下：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Windows.Storage;
using Windows.Storage.Streams;
using System.Runtime.Serialization;
using System.IO;
using Windows.ApplicationModel;


namespace WeiboForWindows8Beta
{
    static class SuspensionManager
    {
        static private Dictionary<string, object> sessionState_ = new Dictionary<string,object>();
        private const string filename = "_sessionState.xml";
        static private List<Type> knownTypes_ = new List<Type>();

        static public Dictionary<string, object> SessionState
        {
            get { return sessionState_; }
        }

        static public List<Type> KnownTypes
        {
            get { return knownTypes_; }
        }

        // @todo:  Worker to workaround issues with Developer Preview.
        static async public Task SaveAsync()
        {
            await Windows.System.Threading.ThreadPool.RunAsync((wiArgs) =>
            {
                SuspensionManager.SaveImplAsync().Wait();
            }, Windows.System.Threading.WorkItemPriority.Normal);
        }

        static async private Task SaveImplAsync()
        {
            // Get the output stream for the SessionState file
            StorageFile file = await ApplicationData.Current.LocalFolder.CreateFileAsync(filename, CreationCollisionOption.ReplaceExisting);
            IRandomAccessStream raStream = await file.OpenAsync(FileAccessMode.ReadWrite);
            IOutputStream outStream = raStream.GetOutputStreamAt(0);

            // Serialize the Session State
            DataContractSerializer serializer = new DataContractSerializer(typeof(Dictionary<string, object>), knownTypes_);
            serializer.WriteObject(outStream.AsStreamForWrite(), sessionState_);
            await outStream.FlushAsync();
        }

        // @todo:  Worker to workaround issues with Developer Preview.
        static async public Task RestoreAsync()
        {
            await Windows.System.Threading.ThreadPool.RunAsync((wiArgs) =>
            {
                SuspensionManager.RestoreImplAsync().Wait();
            }, Windows.System.Threading.WorkItemPriority.Normal);
        }

        static async private Task RestoreImplAsync()
        {
            // Get the input stream for the SessionState file
            StorageFile file = await ApplicationData.Current.LocalFolder.CreateFileAsync(filename, CreationCollisionOption.OpenIfExists);
            if (file == null) return;
            IInputStream inStream = await file.OpenReadAsync();

            // Deserialize the Session State
            DataContractSerializer serializer = new DataContractSerializer(typeof(Dictionary<string, object>), knownTypes_);
            sessionState_ = (Dictionary<string, object>)serializer.ReadObject(inStream.AsStreamForRead());
        }

        //获取Key对应的值
        static public object GetValueByKey(string key)
        {
            if (sessionState_.ContainsKey(key))
                return sessionState_[key];
            else
                return null;
        }
        //添加或者设置相应的值
        static public void SetValueByKey(string key, object value)
        {
            if (sessionState_.ContainsKey(key))
                sessionState_[key] = value;
            else
                sessionState_.Add(key, value);
        }
        //清除某个Key和对应的值
        static public void RemoveKey(string key)
        {
            if (sessionState_.ContainsKey(key))
                sessionState_.Remove(key);
        }
    }
}

以上代码中最后三个函数是我自己加上的，这简化了用户对字典数据的操作。

在App.xaml.cs文件中，有相关代码的调用：

using System;
using System.Text;
using WeiboForWindows8Beta.Utils;
using WeiboService;
using Windows.ApplicationModel;
using Windows.ApplicationModel.Activation;
using Windows.ApplicationModel.DataTransfer;
using Windows.Storage.Streams;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Media.Imaging;

// The Blank Application template is documented at http://go.microsoft.com/fwlink/?LinkId=234227

namespace WeiboForWindows8Beta
{
    /// <summary>
    /// Provides application-specific behavior to supplement the default Application class.
    /// </summary>
    sealed partial class App : Application
    {
        public Frame CurrentFrame { get; set; }
        /// <summary>
        /// Initializes the singleton application object.  This is the first line of authored code
        /// executed, and as such is the logical equivalent of main() or WinMain().
        /// </summary>
       public App()
        {
            this.InitializeComponent();
            this.Suspending += OnSuspending;
        }

        /// <summary>
        /// Invoked when the application is launched normally by the end user.  Other entry points
        /// will be used when the application is launched to open a specific file, to display
        /// search results, and so forth.
        /// </summary>
        /// <param name="args">Details about the launch request and process.</param>
        protected override void OnLaunched(LaunchActivatedEventArgs args)
        {
            if (args.PreviousExecutionState == ApplicationExecutionState.Terminated)
            {
                //TODO: Load state from previously suspended application
                SuspensionManager.RestoreAsync
            }


            // Create a Frame to act navigation context and navigate to the first page
            if (CurrentFrame==null)
                CurrentFrame = new Frame();
            CurrentFrame.Navigate(typeof(WeiboForWindows8Beta.View.Login),);

        }

        

        /// <summary>
        /// Invoked when application execution is being suspended.  Application state is saved
        /// without knowing whether the application will be terminated or resumed with the contents
        /// of memory still intact.
        /// </summary>
        /// <param name="sender">The source of the suspend request.</param>
        /// <param name="e">Details about the suspend request.</param>
        void OnSuspending(object sender, SuspendingEventArgs e)
        {
            //TODO: Save application state and stop any background activity
            SuspensionManager.SaveAsync()
        }    
    }
}

在构造函数中给Suspending事件添加了OnSuspending函数，应用程序会在挂起事件发生时，触发OnSuspending。

SuspensionManager.SaveAsync()会把我们之前保存到SessionState中的数据保存至_sessionState.xml文件中，而SaveAsync则是从_sessionState.xml文件中读取出来。

获取之前用C#的童鞋感觉对async 这个关键词和用法比较模糊，我会在专门的一篇文章来介绍await 和 async 这两个关键词。