我有一个搜索方法,它向UI返回搜索建议。每当用户在搜索框中输入一个新字符时,这个方法就会被触发。
我已经添加了一些取消代码来取消以前的搜索请求。这是有效的,但不是所有的时间。private CancellationTokenSource cancellationTokenSource;
private async Task UserSearch(string searchCriteria)
{
Debug.WriteLine("Searching for {0}....", searchCriteria);
try
{
var cts = new CancellationTokenSource();
this.Suggestions = await this.SearchAsync(searchCriteria, cts.Token);
}
catch (OperationCanceledException)
{
Debug.WriteLine("Search({0}) cancelled", searchCriteria);
}
}
private async Task<IList<string>> SearchAsync(string searchCriteria, CancellationToken cancelToken)
{
CancellationTokenSource previousCts = this.cancellationTokenSource;
CancellationTokenSource linkedCts = CancellationTokenSource.CreateLinkedTokenSource(cancelToken);
this.cancellationTokenSource = linkedCts;
// if previous task running cancel it
if (previousCts != null)
{
previousCts.Cancel();
}
linkedCts.Token.ThrowIfCancellationRequested();
List<string> results =
(await this.searchProvider.SearchAsync(searchCriteria, linkedCts.Token)).ToList();
Debug.WriteLine("Search({0}) returned {1} records", searchCriteria, results.Count);
linkedCts.Dispose();
this.cancellationTokenSource = null;
return results;
}
例如。我得到以下调试消息:
SearchTerm changing to: Di
Searching for Di....
SearchTerm changing to: Dia
Searching for Dia....
Search(Di) cancelled
SearchTerm changing to: Diap
Searching for Diap....
Search(Diap) returned 323 records
Search(Dia) returned 3230 records
正如你所看到的,第一个搜索被取消了,但第二个没有,它在最后一个搜索之后返回,给用户不正确的结果。
我如何确保以前的任务总是被取消?
我想你可能把你的解决方案复杂化了一点。您所需要做的就是查看是否有正在进行的尚未取消的现有操作,并将其取消。然后执行新的搜索。没有测试过,但是我认为这个应该可以。
private CancellationTokenSource cancellationTokenSource;
private async Task UserSearch(string searchCriteria)
{
Debug.WriteLine("Searching for {0}....", searchCriteria);
try
{
if(cancellationTokenSource != null &&
!cancellationTokenSource.IsCancellationRequested)
{
cancellationTokenSource.Cancel();
}
cancellationTokenSource = new CancellationTokenSource();
this.Suggestions = await this.searchProvider.SearchAsync(searchCriteria, linkedCts.Token);
Debug.WriteLine("Search({0}) returned {1} records", searchCriteria, results.Count);
}
catch (OperationCanceledException)
{
Debug.WriteLine("Search({0}) cancelled", searchCriteria);
}
}
您取消之前的查询太晚了,请尽快查询(直接在接收到用户输入后,而不是在下一个查询中)
我想问题是多线程访问this.cancellationTokenSource。如果说线程2取消线程1太晚了(searchProvider。SearchAsync已完成)。cancellationTokenSource可以在赋值后被清除(即线程2执行this)。cancellationTokenSource = linkedCts;之后的线程1调用this。cancellationTokenSource = null;)。这将有效地完全禁止取消线程2
因此,你最好在开始下一个搜索之前取消一个搜索,就像@Ned Stoyanov的建议