使用CancellationTokenSource取消先前的异步任务

Question

I need to implement async task cancel. I know that CancellationTokenSource would help me in achieving that. But I'm unable to find a proper way.

I have a search textbox, whenever a user types in textbox, for each textchanged event I call GetStocks method as shown below,

public async Task GetStocks()
{
    var stockings = new List<Services.Models.Admin.SiteStockingLevelsModel>();
    IsBusy = true;
    cts?.Cancel();
    cts = new CancellationTokenSource();
    await Task.Run(() => { CreateStockingCollection(); });
    ValidateMaterials();
    IsBusy = false;
}

The CreateStockingCollection method is as shown below,

private void CreateStockingCollection()
{
    var stockings = _siteStockingLevelsService.GetSiteInventoryLevels(SiteId);
    CreateStockingLevelCompareCollection(stockings);
    StockingLevels =
        _mapper.Map<TrulyObservableCollection<SiteStockingLevelsModel>>(stockings);            

    ((INotifyPropertyChanged)StockingLevels).PropertyChanged +=
        (x, y) => CompareStockingChanges();
    CompareStockingChanges();
}

My requirement here is,
Example Suppose user wants to type "Abc". When user types "A" the GetStocks method will be called, immediately the user enters "b" the again the get stocks methods will be called, in this case i want to cancel the previous GetStocks task called with letter "A".

Answer 1

One of the best practices of the async programming in .Net is Async all the way . Looks like your method isn't async-based, neither does it accept CancellationToken . You need to add it to your method, in other way only Task.Run will try to cancel your task, which wouldn't work well.

Also, the only creation of the CancellationTokenSource isn't enough - you need to use it's .Token property in your code - this would exactly that token:

await Task.Run(() => { CreateStockingCollection(); }, cts.Token);

Answer 2

Cancellation is cooperative, so you you need to pass it to your own code and have it respond to that token:

public async Task GetStocks()
{
  var stockings = new List<Services.Models.Admin.SiteStockingLevelsModel>();
  IsBusy = true;
  cts?.Cancel();
  cts = new CancellationTokenSource();
  var token = cts.Token;
  await Task.Run(() => { CreateStockingCollection(token); });
  ValidateMaterials();
  IsBusy = false;
}

private void CreateStockingCollection(CancellationToken token)
{
  var stockings = _siteStockingLevelsService.GetSiteInventoryLevels(SiteId, token);
  CreateStockingLevelCompareCollection(stockings);
  StockingLevels =
      _mapper.Map<TrulyObservableCollection<SiteStockingLevelsModel>>(stockings);            

  ((INotifyPropertyChanged)StockingLevels).PropertyChanged +=
      (x, y) => CompareStockingChanges();
  CompareStockingChanges();
}

Here I'm passing it to GetSiteInventoryLevels , which sounds like that would be the long-running part of this work. GetSiteInventoryLevels must now take the CancellationToken and pass it along to whatever APIs it's using.