win8中完成进度

 public sealed partial class WorkItem : Page
    {
        private IAsyncAction _threadPoolWorkItem;
        private ManualResetEvent _sleep = new ManualResetEvent(false);

        public WorkItem()
        {
            this.InitializeComponent();
        }

        // 在线程池中找一个线程去执行指定的方法，并指定其优先级
        private void btnCreateWorkItem_Click_1(object sender, RoutedEventArgs e)
        {
            _threadPoolWorkItem = Windows.System.Threading.ThreadPool.RunAsync(
                (threadPoolWorkItem) =>
                {
                    int percent = ; // 用于模拟执行进度（0 - 100）
                    while (percent < )
                    {
                        // 当前线程 sleep 100 毫秒
                        _sleep.WaitOne();

                        // 如果 IAsyncAction 被取消了则退出此 handler 的执行
                        if (threadPoolWorkItem.Status == AsyncStatus.Canceled)
                            break;

                        percent++;

                        var ignored = Dispatcher.RunAsync(CoreDispatcherPriority.High,
                            () =>
                            {
                                lblProgress.Text = "进度：" + percent.ToString() + "%";
                            });
                    }
                },
                WorkItemPriority.High);

            // IAsyncAction 完成之后（比如任务完成了或者任务取消了）
            // 关于 IAsyncAction 的详细说明请参见 XamlDemo/Thread/Async
            _threadPoolWorkItem.Completed = new AsyncActionCompletedHandler(
                async (IAsyncAction threadPoolWorkItem, AsyncStatus status) =>
                {
                    await Dispatcher.RunAsync(CoreDispatcherPriority.High,
                        () =>
                        {
                            switch (status)
                            {
                                case AsyncStatus.Completed:
                                    lblMsg.Text = "任务完成了";
                                    break;
                                case AsyncStatus.Canceled:
                                    lblMsg.Text = "任务取消了";
                                    break;
                                case AsyncStatus.Started:
                                case AsyncStatus.Error:
                                    break;
                            }
                        });
                });

            lblProgress.Text = "进度：0%";
            lblMsg.Text = "任务开始了";
        }

        // 取消任务
        private void btnCancelWorkItem_Click_1(object sender, RoutedEventArgs e)
        {
            if (_threadPoolWorkItem != null)
            {
                _threadPoolWorkItem.Cancel();
                _threadPoolWorkItem = null;
            }
        }

        // 通过 async await 简化 ThreadPool.RunAsync() 的使用（关于 async 和 await 的详细说明请参见 XamlDemo/Thread/Async）
        private async void btnCreateWorkItemByAwait_Click_1(object sender, RoutedEventArgs e)
        {
            lblProgress.Text = "";
            lblMsg.Text = "";

            string result = "";

            await Windows.System.Threading.ThreadPool.RunAsync(
                delegate
                {
                    new ManualResetEvent(false).WaitOne();
                    result = "在线程池中找一个线程去执行指定的逻辑，然后通过 await 返回 UI 线程";
                });

            lblMsg.Text = result;
        }
    }