04，WP8的async和await

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");
}