一、Task.Yield
Task.Yield簡單來說就是創建時就已經完成的Task，或者說執行時間為0的Task，或者說是空任務，也就是在創建時就將Task的IsCompeted值設定為0，
我們知道await的Task完成時會釋放執行緒，然后從執行緒池中申請新的執行緒繼續執行await之后的代碼，那產生的空任務又意義何在呢？
事實上，Task.Yield產生的空任務僅僅是借await做嫁衣來達到執行緒切換的目的，即讓await之后的操作重新去執行緒池排隊申請新執行緒來繼續執行，
這樣一來，假如有一個優先級低但執行時間長的任務，可以將它拆分成多個小任務，每個小任務執行完成后就重新去執行緒池中排隊申請新執行緒來執行
下一個小任務，這樣任務就不會一直霸占著某個執行緒了（出讓執行權），讓別的優先急高或執行時間短的任務可以去執行，而不是干瞪眼著急，
class Program { static void Main(string[] args) { #region async & await入門三之Task.Yield const int num = 10000; var task = YieldPerTimes(num); for (int i = 0; i < 10; i++) { Task.Factory.StartNew(n => Loop((int)n), num / 10); } Console.WriteLine($"Sum: {task.Result}"); Console.Read(); #endregion } /// <summary> /// 回圈 /// </summary> /// <param name="num"></param> private static void Loop(int num) { for (var i = 0; i < num; i++) ; Console.WriteLine($"Loop->Current thread id is:{Thread.CurrentThread.ManagedThreadId}"); Thread.Sleep(10); } /// <summary> /// 分批出讓執行權 /// </summary> /// <param name="times"></param> /// <returns></returns> private static async Task<int> YieldPerTimes(int num) { var sum = 0; for (int i = 1; i <= num; i++) { sum += i; if (i % 1000 == 0) { Console.WriteLine($"Yield->Current thread id is:{Thread.CurrentThread.ManagedThreadId}"); Thread.Sleep(10); await Task.Yield(); } } return sum; } }View Code
運行結果如下：
二、在WinForm中使用異步Lambda運算式
public Main() { InitializeComponent(); //異步運算式：async (sender, e) btnDoIt.Click += async (sender, e) => { DoIt(false, "開始搬磚啦..."); await Task.Delay(3000); DoIt(true, "終于搬完了，"); }; } private void DoIt(bool isEnable, string text) { btnDoIt.Enabled = isEnable; lblText.Text = text; }View Code
運行結果如下：
三、滾動條應用
private CancellationTokenSource source; private CancellationToken token; public ProcessBar() { InitializeComponent(); } /// <summary> /// 初始化 /// </summary> private void InitTool() { progressBar1.Value = 0; btnDoIt.Enabled = true; btnCancel.Enabled = true; } /// <summary> /// 開始任務 /// </summary> /// <param name="sender"></param> /// <param name="e"></param> private async void btnDoIt_Click(object sender, EventArgs e) { btnDoIt.Enabled = false; source = new CancellationTokenSource(); token = source.Token; var completedPercent = 0; //完成百分比 const int loopTimes = 10; //回圈次數 const int increment = 100 / loopTimes; //進度條每次增加的進度值 for (var i = 1; i <= loopTimes; i++) { if (token.IsCancellationRequested) { break; } try { await Task.Delay(200, token); completedPercent = i * increment; } catch (Exception) { completedPercent = i * increment; } finally { progressBar1.Value = completedPercent; } } var msg = token.IsCancellationRequested ? $"任務被取消，已執行進度為：{completedPercent}%，" : $"任務執行完成，"; MessageBox.Show(msg, "提示", MessageBoxButtons.OK, MessageBoxIcon.Information); progressBar1.Value = 0; InitTool(); } /// <summary> /// 取消任務 /// </summary> /// <param name="sender"></param> /// <param name="e"></param> private void btnCancel_Click(object sender, EventArgs e) { if (btnDoIt.Enabled) return; btnCancel.Enabled = false; source.Cancel(); } }View Code
運行結果如下：
四、BackgroundWorker
與async & await不同的是，有時候可能需要一個額外的執行緒，它在后臺持續完成某個任務并不時與主執行緒通信，這時就需要用到BackgroundWorker類，
（主要用于GUI程式）
private readonly BackgroundWorker bgWorker = new BackgroundWorker(); public ProcessBar() { InitializeComponent(); //設定BackgroundWorker屬性 bgWorker.WorkerReportsProgress = true; //能否報告進度更新 bgWorker.WorkerSupportsCancellation = true; //是否支持異步取消 //連接BackgroundWorker物件的處理程式 bgWorker.DoWork += bgWorker_DoWork; bgWorker.ProgressChanged += bgWorker_ProgressChanged; bgWorker.RunWorkerCompleted += bgWorker_RunWorkerCompleted; } /// <summary> /// 開始執行后臺操作觸發，即呼叫BackgroundWorker.RunWorkerAsync時發生， /// </summary> /// <param name="sender"></param> /// <param name="e"></param> private static void bgWorker_DoWork(object sender, DoWorkEventArgs e) { if (!(sender is BackgroundWorker worker)) { return; } for (var i = 1; i <= 10; i++) { //判斷程式是否已請求取消后臺操作 if (worker.CancellationPending) { e.Cancel = true; break; } worker.ReportProgress(i * 10); //觸發BackgroundWorker.ProgressChanged事件 Thread.Sleep(200); //執行緒掛起200毫秒 } } /// <summary> /// 呼叫BackgroundWorker.ReportProgress(System.Int32)時發生 /// </summary> /// <param name="sender"></param> /// <param name="e"></param> private void bgWorker_ProgressChanged(object sender, ProgressChangedEventArgs e) { progressBar1.Value = e.ProgressPercentage; //異步任務的進度百分比 } /// <summary> /// 當后臺操作已完成或被取消或引發例外時發生 /// </summary> /// <param name="sender"></param> /// <param name="e"></param> private void bgWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e) { MessageBox.Show(e.Cancelled ? $@"任務已被取消，已執行進度為：{progressBar1.Value}%" : $@"任務執行完成，已執行進度為：{progressBar1.Value}%"); progressBar1.Value = 0; } /// <summary> /// 開始任務 /// </summary> /// <param name="sender"></param> /// <param name="e"></param> private void btnDoIt_Click(object sender, EventArgs e) { //判斷BackgroundWorker是否正在執行異步操作 if (!bgWorker.IsBusy) { bgWorker.RunWorkerAsync(); //開始執行后臺操作 } } /// <summary> /// 取消任務 /// </summary> /// <param name="sender"></param> /// <param name="e"></param> private void btnCancel_Click(object sender, EventArgs e) { bgWorker.CancelAsync(); //請求取消掛起的后臺操作 }View Code
運行結果如下：
參考自：
https://www.cnblogs.com/dudu/archive/2018/10/24/task-yield.html
https://www.cnblogs.com/liqingwen/p/5877042.html
后記：
關于更詳細的BackgroundWorker知識，可查看此篇博客：
https://www.cnblogs.com/sparkdev/p/5906272.html
轉載請註明出處，本文鏈接：https://www.uj5u.com/net/89692.html
標籤：C#