C#沉澱-非同步程式設計 三
GUI程式在設計上要求所有的顯示變化都必須在主GUI執行緒中完成,Windows程式是通過訊息來實現這一點的,訊息被放入由訊息泵管理的訊息佇列中。
訊息泵從列隊中取出一條訊息,並呼叫它的處理程式程式碼。當程式程式碼完成時,訊息泵獲取下一條訊息並迴圈這個過程。
由於這個架構,處理程式程式碼就必須矮小精悍,這樣才不至於扶起並阻礙其他GUI行為處理。如果某個訊息的處理程式程式碼耗時過長,訊息佇列中的訊息會產生積壓。程式將失去響應,因為在那個長時間執行的處理程式完成之前,無法處理任何訊息。
示例:下面建立一個WPF程式,在主窗體中新增一個Label與一個Button,具體工作在後端程式碼的註釋中
< Window x:Class="WpfForAsync.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns: local="clr-namespace:WpfForAsync"
mc:Ignorable="d"
Title="MainWindow" Height="151.4" Width="323.2">
<StackPanel>
<Label Name="lblStatus" Margin="10,5,10,0" >Not Doing Anything</Label>
<Button Name="btnDoStuff" Content="Do Stuff" HorizontalAlignment= "Left" Margin="10,5" Padding="5,2" Click="btnDoStuff_Click"/>
</StackPanel>
</Window>
後端程式碼:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
namespace WpfForAsync
{
/// <summary>
/// MainWindow.xaml 的互動邏輯
/// </summary>
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
}
private void btnDoStuff_Click(object sender, RoutedEventArgs e)
{
//首先讓按鈕禁用
btnDoStuff.IsEnabled = false;
//第二步將lable的文字改動
lblStatus.Content = "Doing Stuff";
//阻塞4秒鐘
Thread.Sleep(1000 * 4);
//將lable的內容改為原來的內容
lblStatus.Content = "Not Doing Anything";
//將按鈕設定為可用
btnDoStuff.IsEnabled = true;
}
}
}
事實上,當點選按鈕的時候,窗體沒有任何變化,Label也不會發生內容更改的情況,而且,主窗體在被阻塞的4秒內也無被凍結了!!!
來看程式執行的順序:
- btnDoStuff_Click將[禁用按鈕]訊息壓入佇列
- btnDoStuff_Click將[改變文字]訊息壓入佇列
- btnDoStuff_Click將被阻塞4秒
- btnDoStuff_Click將[改變文字]訊息壓入佇列
- btnDoStuff_Click將[啟用按鈕]訊息壓入佇列
- btnDoStuff_Click將退出
- 程式執行[禁用按鈕]
- 程式執行[改變文字]
- 程式執行[改變文字]
- 程式執行[啟用按鈕]
由於7~10發生的太快,我們根本看不到發生了什麼!
示例:使用非同步來解決上述問題
後端程式碼:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
namespace WpfForAsync
{
/// <summary>
/// MainWindow.xaml 的互動邏輯
/// </summary>
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
}
private async void btnDoStuff_Click(object sender, RoutedEventArgs e)
{
//首先讓按鈕禁用
btnDoStuff.IsEnabled = false;
//第二步將lable的文字改動
lblStatus.Content = "Doing Stuff";
await Task.Delay(1000 * 4);
//將lable的內容改為原來的內容
lblStatus.Content = "Not Doing Anything";
//將按鈕設定為可用
btnDoStuff.IsEnabled = true;
}
}
}
將btnDoStuff_Click方法改為一個非同步方法,非同步方法內部使用await Task.Delay(1000 * 4);
,即將遇到await的時候,程式會返回到呼叫方法,處理程式將從處理器上摘下,禁用按鈕、改變文字將被處理;當4秒之後,處理程式將自己再壓入佇列,再將之後的改變文字、啟用按鈕壓入列隊,當處理程式退出後,這兩個子訊息也將被執行,但在休息的4秒期間,是可以手動窗體的
Task.Yield
Task.Yield方法可以建立一個立即返回的awaitable。等等一個Yield可以讓非同步方法在執行後續部分的同時返回到呼叫方法。可以將其理解成離開當前的訊息佇列,回到佇列末尾,讓處理器有時間處理其他任務。
示例:
using System;
using System.Net;
using System.Diagnostics;
using System.Threading.Tasks;
using System.Collections.Generic;
using System.Threading;
namespace CodeForAsync
{
static class DoStuff
{
public static async Task<int> FindSeriesSum(int x)
{
int sum = 0;
for (int i = 1; i < x; i++)
{
sum += i;
Console.WriteLine("Yield"+i);
if (i % 10 == 0)
await Task.Yield();
}
return sum;
}
}
class Program
{
private static void CountBig(int p)
{
for (int i = 0; i < p; i++)
Console.WriteLine("CountBig"+i);
}
static void Main(string[] args)
{
Task<int> value = DoStuff.FindSeriesSum(1000);
CountBig(1000);
CountBig(1000);
CountBig(1000);
CountBig(1000);
Console.WriteLine("Sum:{0}",value.Result);
Console.ReadKey();
}
}
}
當程式遇到await Task.Yield();
時,會將當前的執行權轉讓出來,之後的CountBig(1000);
便可以提前得到執行
使用非同步Lambda表示式
語法示例:
btn.Click += async (sender, e) => {//處理工作};
完整程式碼示例:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
namespace WpfForAsync
{
/// <summary>
/// MainWindow.xaml 的互動邏輯
/// </summary>
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
this.btnDoStuff.Click += async (sender, e) =>
{
btnDoStuff.IsEnabled = false;
lblStatus.Content = "Hello";
await Task.Delay(4000);
btnDoStuff.IsEnabled = true;
lblStatus.Content = "World";
};
}
}
}
一個完整的GUI示例,請自行嘗試
<Window x:Class="WpfForAsync.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WpfForAsync"
mc:Ignorable="d"
Title="MainWindow" Height="151.4" Width="323.2" Loaded="Window_Loaded">
<StackPanel>
<Button Name="btnProcess" Width="100" Click="btnProcess_Click">Process</Button>
<Button Name="btnCancel" Width="100" Click="progressBar_Click" >Cancel</Button>
<ProgressBar Name="progressBar" Height="20" Width="200" Margin="10" HorizontalAlignment="Right"></ProgressBar>
</StackPanel>
</Window>
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Threading.Tasks;
namespace WpfForAsync
{
/// <summary>
/// MainWindow.xaml 的互動邏輯
/// </summary>
public partial class MainWindow : Window
{
CancellationTokenSource _cancellationTokenSource;
CancellationToken _cancellationToken;
public MainWindow()
{
InitializeComponent();
}
private void Window_Loaded(object sender, RoutedEventArgs e)
{
}
private async void btnProcess_Click(object sender, RoutedEventArgs e)
{
btnProcess.IsEnabled = false;
_cancellationTokenSource = new CancellationTokenSource();
_cancellationToken = _cancellationTokenSource.Token;
int completedPercent = 0;
for (int i = 0; i < 10; i++)
{
if (_cancellationToken.IsCancellationRequested)
break;
try
{
await Task.Delay(500,_cancellationToken);
completedPercent = (i+1) * 10;
}
catch (TaskCanceledException ex)
{
completedPercent = i * 10;
}
progressBar.Value = completedPercent;
}
string message = _cancellationToken.IsCancellationRequested ? string.Format("Process was cancelled at {0}%",completedPercent) : "Process completed normally";
MessageBox.Show(message,"Completion Status");
progressBar.Value = 0;
btnProcess.IsEnabled = true;
btnCancel.IsEnabled = true;
}
private void progressBar_Click(object sender, RoutedEventArgs e)
{
if (!btnProcess.IsEnabled)
{
btnCancel.IsEnabled = false;
_cancellationTokenSource.Cancel();
}
}
}
}