内容预告：
- awaitable的对象如何工作
- 并行处理的新方法
- 用async替代BackgroundWorkder
- 了解为什么不再使用线程和线程池
异步函数与基于任务的编程模型（TPM==Task Programming Model）：
- WP7.1用.NET4.0的模式支持异步编程：BeginXYZ，EndXYZ函数，如HttpWebRequest.BeginGetResponse, EndGetResponse。
- WP7.1异步事件模型：设置一个Completed事件处理器，然后用XYZAsync调用操作，如WebClient.DownloadAsync函数和DownloadCompleted enent。
- WP8中任何操作时间可能超过50毫秒的任务都以TPM的方式暴露为一个异步函数。
- 在WinRT中异步编程很重要，文件IO，网络传输都在用它。
- TPM逐渐成为编写异步代码的方式。
- WP8中很多新的API都会提供一个基于任务的API。
保证UI的流畅性：比较WP7.1中阻塞式文件IO和WP8中TPM式的文件IO：
WP7.1：
var isf = IsolatedStorageFile.GetUserStoreForApplication();
using (var fs = new IsolatedStorageFileStream("CaptainsLog.store", FileMode.Open, isf)) { StreamReader reader = new StreamReader(fs); theData = reader.ReadToEnd(); reader.Close(); };
WP8：
StorageFile storageFile = await Windows.Storage.StorageFile.GetFileFromApplicationUriAsync(new Uri("ms-appdata:///local/CaptainsLog.store ")); Stream readStream = await StorageFile.OpenStreamForReadAsync(); using (StreamReader reader = new StreamReader(readStream)) { theData = await reader.ReadToEndAsync(); }
上异步代码看起来同步：
编译器做的转换：在一个函数的签名中加上async会使编译器用状态机重新实现这个函数。使用状态机编译器可以在函数扶起和恢复的地方插入一个标记点，而不是阻止线程执行。这些点只会在你显示地用await关键字的时候插入。当你await一个异步操作的时候，以下事情都还没有做：
- 编译器打包所有当前的调用函数的状态把它们保存在堆上。
- 函数返回给调用者，允许线程干其他活。
- 当await标记的操作完成时，函数用保存的状态继续执行。
释放调用线程
：
错误处理：继续向同步代码一样用try..catch就行，当代码异步执行时，在调用的线程上抛出异常：
private async void SomeMethod() { try { string theData = await LoadFromLocalFolder();
TextBox1.Text = theData; } catch (Exception ex) { // An exception occurred from the async operation } }
调用async标记的函数时必须带await吗？当然不，不带await时，调用的函数仍然在后台线程执行，调用线程也不等待结果，async函数返回void或task，而不是Task<TResult>。当你这么做时，智能提示会警告你：
替代BackgroundWorker：如果有长时间执行的代码可以放在后台线程里运行，BackgroundWorkder在WP8仍然支持，并可以取消和报告进度。但这些用Task也能解决。
BackroundWorker:
private void LaunchTaskButton_Click(object sender, RoutedEventArgs e) { BackgroundWorker bgw = new BackgroundWorker();
bgw.RunWorkerCompleted += ((s, a) =>MessageBox.Show("BackgroundWorker has completed, result: " + (int)a.Result)); bgw.DoWork += ((s, a) => { // Simulate some long running work Thread.Sleep(5000); // Return the result a.Result = 1234; }); // Now start execution bgw.RunWorkerAsync(); }
用Task实现的等效的代码：
private async void LaunchTaskButton_Click(object sender, RoutedEventArgs e) { int result = await Task.Factory.StartNew<int>(() => { // Simulate some long running work
Thread.Sleep(5000); // Return the result return 4321; }); MessageBox.Show("BackgroundWorker has completed, result is: " + result); }
BackgroundWorkder的进度汇报：
BackgroundWorker bgw = new BackgroundWorker(); bgw.WorkerReportsProgress = true;
bgw.RunWorkerCompleted += ((s, a) => MessageBox.Show("BackgroundWorker has completed, result: " + (int)a.Result));
bgw.ProgressChanged += ((s, a) => { // Progress Indicator value must be between 0 and 1
SystemTray.GetProgressIndicator(this).Value = (double)a.ProgressPercentage/100.0; }); bgw.DoWork += ((s, a) => { // Simulate some long running work for (int i = 0; i < 10; i++) { Thread.Sleep(500); // Report progress as percentage completed
bgw.ReportProgress(i * 10); } a.Result = 1234;
// Return the result }); // Now start execution bgw.RunWorkerAsync();
基于Task的进度汇报：
IProgress<int> progressReporter = new Progress<int>((percentComplete) => // Progress Indicator value must be between 0 and 1
SystemTray.GetProgressIndicator(this).Value = (double)percentComplete / 100.0 ); int result = await Task.Factory.StartNew<int>(() => { // Simulate some long running work for (int i = 0; i < 10; i++) { Thread.Sleep(500); // Report progress as percentage completed
progressReporter.Report(i * 10); } // Return the result return 4321; } ); MessageBox.Show("BackgroundWorker has completed, result is: " + result);
BackgroundWorker的取消：
BackgroundWorker bgw = new BackgroundWorker();
bgw.WorkerReportsProgress = true;
bgw.WorkerSupportsCancellation = true;
bgw.RunWorkerCompleted += ((s, a) => {
if (a.Cancelled)
MessageBox.Show("BackgroundWorker was cancelled");
else
MessageBox.Show("BackgroundWorker has completed, result: " + (int)a.Result); } );
bgw.ProgressChanged += ((s, a) => { ... });
bgw.DoWork += ((s, a) =>{
// Simulate some long running work
for (int i = 0; i < 10; i++)
{
Thread.Sleep(500);
// Report progress as percentage completed
bgw.ReportProgress(i * 10);
// Have we been cancelled?
bgw.DoWork += ((s, a) =>
{
// Simulate some long running work
for (int i = 0; i < 10; i++)
{
Thread.Sleep(500);
// Report progress as percentage completed
bgw.ReportProgress(i * 10);
// Have we been cancelled?
if (bgw.CancellationPending)
{
a.Cancel = true;
return;
}
} a.Result = 1234; // Return the result
});
// Now start execution
bgw.RunWorkerAsync();
基于Task的取消：
var cancellationTokenSource = new CancellationTokenSource();
var cancellationToken = cancellationTokenSource.Token;
try
{
int result = await Task.Factory.StartNew<int>(() =>
{
// Simulate some long running work
for (int i = 0; i < 10; i++)
{
Thread.Sleep(500);
// Have we been cancelled?
cancellationToken.ThrowIfCancellationRequested();
}
return 4321;
// Return the result
}, cancellationToken);
MessageBox.Show("BackgroundWorker has completed, result is: " + result);
}
catch (OperationCanceledException ex)
{
MessageBox.Show("Task BackgroundWorker was cancelled");
}
