问题描述
我是Threads的新手，我有一个List包含一个字符串。我的目标是让多个线程对这个List工作，这个代码只针对一个线程，因为我现在正在学习，但是当我按下开始按钮时我得到了AV。
i am new to Threads, i have a List contains a strings. My goal is to make multiple threads do work to this List, this codes only for a single thread because i'm learning currently, however i get AV when i press start Button.
type
TDemoThread = class(TThread)
private
procedure Abort;
protected
procedure Execute; override;
public
List: TStringList;
end;
procedure TfrmMain.StartButton1Click(Sender: TObject);
var
i: integer;
List: Tstrings;
begin
for i := 0 to memo1.Lines.Count - 1 do
begin
List := TStringList.Create;
List.Add(memo1.Lines.Strings[i]);
end;
Thread := TDemoThread.Create(True);
Thread.FreeOnTerminate := True;
Thread.Start;
end;
procedure TDemoThread.Execute;
var
lHTTP: TIdHTTP;
i: integer;
X: Tstrings;
begin
inherited;
if Terminated then
Exit;
lHTTP := TIdHTTP.Create(nil);
X := TStringList.Create;
lHTTP.ReadTimeout := 30000;
lHTTP.HandleRedirects := True;
for i := 0 to List.Count - 1 do
try
X.Text := lHTTP.Get('https://instagram.com/' + List.Strings[i]);
S := ExtractDelimitedString(X.Text);
X.Clear;
TThread.Synchronize(nil,
procedure
begin
frmMain.Memo2.Lines.Add(List.Strings[i] + ' : ' + S);
end);
finally
end;
end;
推荐答案
我个人避免从线程更新表单他们自己。线程是数据生成器，而不是GUI管理器。所以让他们分开他们的顾虑。
Personally I'd avoid updating the form from the threads themselves. Threads are data generators here, not GUI managers. So let them separate their concerns.
我会让所有的线程将结果累积到同一个共享容器中，然后创建一个GUI线程来轮询该容器。人眼很慢，Windows GUI也很慢，所以你不应该经常更新你的GUI每秒2到3次。它只会浪费CPU负载并将表单模糊为不可读。
I'd make all the threads accumulate the results into the same shared container and then make a GUI thread to poll that container instead. Human eyes are slow and Windows GUI is slow too, so you should not update your GUI more often than 2 or 3 times per second. It would only waste CPU load and blur the form into being unreadable.
另一件事是避免使用慢速 TStringList 除非需要额外的功能（使其变慢）。常规 TList< string> 绰绰有余，而且速度更快。
Another thing would be to avoid using slow TStringList unless its extra functionality (which makes it slow) is required. The regular TList<string> is more than enough as a dumb container and is faster.
type
TDemoThread = class;
TfrmMain = class(TForm)
private
Fetchers: TThreadList<TDemoThread>;
Data: TThreadList<string>;
property inProcess: Boolean read ... write SetInProcess;
public
procedure AfterConstruction; override;
procedure BeforeDestruction; override;
....
end;
// this demo makes each thread per each line - that is actually a bad design
// one better use a thread pool working over the same queue and only have
// 20-40 worker threads for all the URLs
TDemoThread = class(TThread)
private
URL: string;
List: TThreadList<string>;
Tracker: TThreadList<TDemoThread>;
protected
procedure Execute; override;
end;
procedure TfrmMain.BeforeDestruction;
begin
while TThreadList.Count > 0 do
Sleep(100);
FreeAndNil( Fetchers );
Data.Free;
inherited;
end;
procedure TfrmMain.AfterConstruction;
begin
Fetchers := TThreadList<TDemoThread>.Create;
Data := TThreadList<string>.Create;
inherited;
end;
procedure TfrmMain.StartButton1Click(Sender: TObject);
var
i: integer;
List: Tstrings;
worker: TDemoThread;
URL: string;
begin
If inProcess then exit;
for URL in memo1.Lines do begin
worker := TDemoThread.Create(True);
worker.FreeOnTerminate := True;
worker.URL := URL;
worker.List := Data;
worker.Tracker := Fetchers;
Fetchers.Add( worker );
end;
InProcess := True;
for worker in Fetchers do
worker.Start;
end;
procedure TfrmMain.SetInProcess(const Value: Boolean);
begin
if Value = InProcess then exit; // form already is in this mode
FInProcess := Value;
memo1.ReadOnly := Value;
StartButton.Enabled := not Value;
if Value then begin
Memo2.Lines.Clear;
Data.Clear;
end;
Timer1.Delay := 500; // twice per second
Timer1.Enabled := Value;
If not Value then // for future optimisation - make immediate mode change
FlushData; // when last worker thread quits, no waiting for timer event
If not Value then
ShowMessage('Work complete');
end;
procedure TfrmMain.Timer1Timer(const Sender: TObject);
begin
FlushData;
if Fetchers.Count <= 0 then
InProcess := False;
end;
procedure TfrmMain.FlushData;
begin
Data.LockList; // next two operations should go as non-interruptible atom
try
Memo2.Lines.AddStrings( Data.ToArray() );
Data.Clear;
finally
Data.UnLockList;
end;
end;
procedure TDemoThread.Execute;
var
lHTTP: TIdHTTP;
begin
try
lHTTP := TIdHTTP.Create(nil);
try
lHTTP.ReadTimeout := 30000;
lHTTP.HandleRedirects := True;
S := ExtractDelimitedString( lHTTP.Get('https://instagram.com/' + URL) );
List.Add( S );
finally
lHTTP.Destroy;
end;
finally
Tracker.Remove( Self );
end;
end;
- http://docwiki.embarcadero.com/Libraries/Seattle/en/System.Classes.TThreadList
- http://docwiki.embarcadero.com/Libraries/Seattle/en/System.Classes.TStrings.AddStrings
PS。就个人而言，我也使用OmniThreads库，因为它通常使维护数据生成线程更容易。例如，仅管理您创建的线程数量就会设置一个属性，并确定所有线程何时完成其工作是另一个oneliner。你真的不应该创建数千个线程来获取所有的URL，而是你应该在一个线程池中有10-20个线程来获取<$ c中的URL $ c>输入队列并一个接一个地获取它们。我建议你在 - 它可以让这样的应用程序更简洁，更容易编写。 管道模式可能更适合此任务 - 因为您无论如何都要将URL列表准备为源集合。 StartButtonClick 中的一半脚手架将消失，整个 TDemoThread 类也将消失。
PS. Personally, I'd also use OmniThreads Library, as it generally makes maintaining data-generating threads easier. For example just managing how many threads did you created becomes setting one property and determining when all threads complete their work is another oneliner. You really should not create a thousand of threads to fetch all the URLs, instead you should have 10-20 threads in a Thread Pool that would take the URLs from a Input Queue and fetch them one after another. I suggest you reading about OTL's Parallel For and Fork-Join patterns at http://otl.17slon.com/tutorials.htm - it would allow making such an application more concise and easier to write. Pipeline pattern would probably be even better match for this task - since you anyway prepare URLs list as a source collection. Half the scaffolding in StartButtonClick would be gone, and the whole TDemoThread class too.
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