Copying files which the main thread adds to a stringlist using a thread

Question

I have a web creation program which, when building a site, creates hundreds of files.

When the internet root folder is situated on the local pc, the program runs fine. If the internet root folder is situated on a network drive, the copying of a created page takes longer than creating the page itself (the creation of the page is fairly optimized).

I was thinking of creating the files locally, adding the names of the created files to a TStringList and let another thread copy them to the network drive (removing the copied file from the TStringList).

Howerver, I have never, ever used threads before and I couldn't find an existing answer in the other Delphi questions involving threads (if only we could use an and operator in the search field), so I am now asking if anyone has got a working example which does this (or can point me to some article with working Delphi code) ?

I am using Delphi 7.

EDITED: My sample project (thx to the original code by mghie - who is hereby thanked once again).

  ...
  fct : TFileCopyThread;
  ...

  procedure TfrmMain.FormCreate(Sender: TObject);
  begin
     if not DirectoryExists(DEST_FOLDER)
     then
        MkDir(DEST_FOLDER);
     fct := TFileCopyThread.Create(Handle, DEST_FOLDER);
  end;


  procedure TfrmMain.FormClose(Sender: TObject; var Action: TCloseAction);
  begin
     FreeAndNil(fct);
  end;

  procedure TfrmMain.btnOpenClick(Sender: TObject);
  var sDir : string;
      Fldr : TedlFolderRtns;
      i : integer;
  begin
     if PickFolder(sDir,'')
     then begin
        // one of my components, returning a filelist [non threaded  :) ] 
        Fldr := TedlFolderRtns.Create();
        Fldr.FileList(sDir,'*.*',True);
        for i := 0 to Fldr.TotalFileCnt -1 do
        begin
           fct.AddFile( fldr.ResultList[i]);
        end;
     end;
  end;

  procedure TfrmMain.wmFileBeingCopied(var Msg: Tmessage);
  var s : string;
  begin
     s := fct.FileBeingCopied;
     if s <> ''
     then
        lbxFiles.Items.Add(fct.FileBeingCopied);
     lblFileCount.Caption := IntToStr( fct.FileCount );
  end;

and the unit

  unit eFileCopyThread;
  interface
  uses
     SysUtils, Classes, SyncObjs, Windows, Messages;
  const
    umFileBeingCopied = WM_USER + 1;
  type

    TFileCopyThread = class(TThread)
    private
      fCS: TCriticalSection;
      fDestDir: string;
      fSrcFiles: TStrings;
      fFilesEvent: TEvent;
      fShutdownEvent: TEvent;
      fFileBeingCopied: string;
      fMainWindowHandle: HWND;
      fFileCount: Integer;
      function GetFileBeingCopied: string;
    protected
      procedure Execute; override;
    public
      constructor Create(const MainWindowHandle:HWND; const ADestDir: string);
      destructor Destroy; override;

      procedure AddFile(const ASrcFileName: string);
      function IsCopyingFiles: boolean;
      property FileBeingCopied: string read GetFileBeingCopied;
      property FileCount: Integer read fFileCount;
    end;

  implementation
  constructor TFileCopyThread.Create(const MainWindowHandle:HWND;const ADestDir: string);
  begin
    inherited Create(True);
    fMainWindowHandle := MainWindowHandle;
    fCS := TCriticalSection.Create;
    fDestDir := IncludeTrailingBackslash(ADestDir);
    fSrcFiles := TStringList.Create; 
    fFilesEvent := TEvent.Create(nil, True, False, ''); 
    fShutdownEvent := TEvent.Create(nil, True, False, ''); 
    Resume; 
  end; 

  destructor TFileCopyThread.Destroy; 
  begin 
    if fShutdownEvent <> nil then 
      fShutdownEvent.SetEvent; 
    Terminate;
    WaitFor;
    FreeAndNil(fFilesEvent);
    FreeAndNil(fShutdownEvent);
    FreeAndNil(fSrcFiles);
    FreeAndNil(fCS);
    inherited;
  end;

  procedure TFileCopyThread.AddFile(const ASrcFileName: string);
  begin
    if ASrcFileName <> ''
    then begin
      fCS.Acquire;
      try
        fSrcFiles.Add(ASrcFileName);
        fFileCount := fSrcFiles.Count;
        fFilesEvent.SetEvent;
      finally
        fCS.Release;
      end;
    end;
  end;

  procedure TFileCopyThread.Execute;
  var
    Handles: array[0..1] of THandle;
    Res: Cardinal;
    SrcFileName, DestFileName: string;
  begin
    Handles[0] := fFilesEvent.Handle;
    Handles[1] := fShutdownEvent.Handle;
    while not Terminated do
    begin
      Res := WaitForMultipleObjects(2, @Handles[0], False, INFINITE);
      if Res = WAIT_OBJECT_0 + 1 then
        break;
      if Res = WAIT_OBJECT_0
      then begin
        while not Terminated do
        begin
          fCS.Acquire;
          try
            if fSrcFiles.Count > 0
            then begin
              SrcFileName := fSrcFiles[0];
              fSrcFiles.Delete(0);
              fFileCount := fSrcFiles.Count;
              PostMessage( fMainWindowHandle,umFileBeingCopied,0,0 );
           end else
               SrcFileName := '';
           fFileBeingCopied := SrcFileName;
            if SrcFileName = '' then
              fFilesEvent.ResetEvent;
          finally
            fCS.Release;
          end;

          if SrcFileName = '' then
            break;
          DestFileName := fDestDir + ExtractFileName(SrcFileName);
          CopyFile(PChar(SrcFileName), PChar(DestFileName), True);
        end;
      end;
    end;
  end;

  function TFileCopyThread.IsCopyingFiles: boolean;
  begin 
    fCS.Acquire; 
    try 
      Result := (fSrcFiles.Count > 0) 
        // last file is still being copied 
        or (WaitForSingleObject(fFilesEvent.Handle, 0) = WAIT_OBJECT_0); 
    finally 
      fCS.Release; 
    end; 
  end; 

  // new version - edited after receiving comments 
  function TFileCopyThread.GetFileBeingCopied: string; 
  begin 
     fCS.Acquire; 
     try 
        Result := fFileBeingCopied; 
     finally 
        fCS.Release; 
     end; 
  end; 

  // old version - deleted after receiving comments 
  //function TFileCopyThread.GetFileBeingCopied: string;
  //begin
  //  Result := '';
  //  if fFileBeingCopied <> ''
  //  then begin
  //    fCS.Acquire;
  //    try
  //      Result := fFileBeingCopied;
  //      fFilesEvent.SetEvent;
  //    finally
  //      fCS.Release;
  //    end;
  //  end;
  //end;

  end.

Any additional comments would be much appreciated.

Reading the comments and looking at the examples, you find different approaches to the solutions, with pro and con comments on all of them.

The problem when trying to implement a complicated new feature (as threads are to me), is that you almost always find something which seems to work ... at first. Only later on you find out the hard way that things should have been done differently. And threads are a very good example of this.

Sites like StackOverflow are great. What a community.

Answer 1

A quick and dirty solution:

type
  TFileCopyThread = class(TThread)
  private
    fCS: TCriticalSection;
    fDestDir: string;
    fSrcFiles: TStrings;
    fFilesEvent: TEvent;
    fShutdownEvent: TEvent;
  protected
    procedure Execute; override;
  public
    constructor Create(const ADestDir: string);
    destructor Destroy; override;

    procedure AddFile(const ASrcFileName: string);
    function IsCopyingFiles: boolean;
  end;

constructor TFileCopyThread.Create(const ADestDir: string);
begin
  inherited Create(True);
  fCS := TCriticalSection.Create;
  fDestDir := IncludeTrailingBackslash(ADestDir);
  fSrcFiles := TStringList.Create;
  fFilesEvent := TEvent.Create(nil, True, False, '');
  fShutdownEvent := TEvent.Create(nil, True, False, '');
  Resume;
end;

destructor TFileCopyThread.Destroy;
begin
  if fShutdownEvent <> nil then
    fShutdownEvent.SetEvent;
  Terminate;
  WaitFor;
  FreeAndNil(fFilesEvent);
  FreeAndNil(fShutdownEvent);
  FreeAndNil(fSrcFiles);
  FreeAndNil(fCS);
  inherited;
end;

procedure TFileCopyThread.AddFile(const ASrcFileName: string);
begin
  if ASrcFileName <> '' then begin
    fCS.Acquire;
    try
      fSrcFiles.Add(ASrcFileName);
      fFilesEvent.SetEvent;
    finally
      fCS.Release;
    end;
  end;
end;

procedure TFileCopyThread.Execute;
var
  Handles: array[0..1] of THandle;
  Res: Cardinal;
  SrcFileName, DestFileName: string;
begin
  Handles[0] := fFilesEvent.Handle;
  Handles[1] := fShutdownEvent.Handle;
  while not Terminated do begin
    Res := WaitForMultipleObjects(2, @Handles[0], False, INFINITE);
    if Res = WAIT_OBJECT_0 + 1 then
      break;
    if Res = WAIT_OBJECT_0 then begin
      while not Terminated do begin
        fCS.Acquire;
        try
          if fSrcFiles.Count > 0 then begin
            SrcFileName := fSrcFiles[0];
            fSrcFiles.Delete(0);
          end else
            SrcFileName := '';
          if SrcFileName = '' then
            fFilesEvent.ResetEvent;
        finally
          fCS.Release;
        end;

        if SrcFileName = '' then
          break;
        DestFileName := fDestDir + ExtractFileName(SrcFileName);
        CopyFile(PChar(SrcFileName), PChar(DestFileName), True);
      end;
    end;
  end;
end;

function TFileCopyThread.IsCopyingFiles: boolean;
begin
  fCS.Acquire;
  try
    Result := (fSrcFiles.Count > 0)
      // last file is still being copied
      or (WaitForSingleObject(fFilesEvent.Handle, 0) = WAIT_OBJECT_0);
  finally
    fCS.Release;
  end;
end;

To use this in production code you would need to add error handling, maybe some progress notifications, and the copying itself should probably be implemented differently, but this should get you started.

In answer to your questions:

should I create the FileCopyThread in the FormCreate of the main program (and let it running), will that slow down the program somehow ?

You can create the thread, it will block on the events and consume 0 CPU cycles until you add a file to be copied. Once all files have been copied the thread will block again, so keeping it over the whole runtime of the program has no negative effect apart from consuming some memory.

Can I add normal event notification to the FileCopyThread (so that I can send an event as in property onProgress:TProgressEvent read fOnProgressEvent write fOnProgressEvent; with f.i. the current number of files in the list, and the file currently processed. I would like to call this when adding and before and after the copy routine

You can add notifications, but for them to be really useful they need to be executed in the context of the main thread. The easiest and ugliest way to do that is to wrap them with the Synchronize() method. Look at the Delphi Threads demo for an example how to do this. Then read some of the questions and answers found by searching for "[delphi] synchronize" here on SO, to see how this technique has quite a few drawbacks.

However, I wouldn't implement notifications in this way. If you just want to display progress it's unnecessary to update this with each file. Also, you have all the necessary information in the VCL thread already, in the place where you add the files to be copied. You could simply start a timer with an Interval of say 100, and have the timer event handler check whether the thread is still busy, and how many files are left to be copied. When the thread is blocked again you can disable the timer. If you need more or different information from the thread, then you could easily add more thread-safe methods to the thread class (for example return the number of pending files). I started with a minimal interface to keep things small and easy, use it as inspiration only.

Comment on your updated question:

You have this code:

function TFileCopyThread.GetFileBeingCopied: string;
begin
  Result := '';
  if fFileBeingCopied <> '' then begin
    fCS.Acquire;
    try
      Result := fFileBeingCopied;
      fFilesEvent.SetEvent;
    finally
      fCS.Release;
    end;
  end;
end;

but there are two problems with it. First, all access to data fields needs to be protected to be safe, and then you are just reading data, not adding a new file, so there's no need to set the event. The revised method would simply be:

function TFileCopyThread.GetFileBeingCopied: string;
begin
  fCS.Acquire;
  try
    Result := fFileBeingCopied;
  finally
    fCS.Release;
  end;
end;

Also you only set the fFileBeingCopied field, but never reset it, so it will always equal the last copied file, even when the thread is blocked. You should set that string empty when the last file has been copied, and of course do that while the critical section is acquired. Simply move the assignment past the if block.

Answer 2

If you're somewhat reluctant to go down to the metal and deal with TThread directly like in mghie solution, an alternative, maybe quicker, is to use Andreas Hausladen's AsyncCalls.

skeleton code:

procedure MoveFile(AFileName: TFileName; const DestFolder: string);
//------------------------------------------------------------------------------
begin
  if DestFolder > '' then
    if CopyFile(PChar(AFileName), PChar(IncludeTrailingPathDelimiter(DestFolder) + ExtractFileName(AFileName)), False) then
      SysUtils.DeleteFile(AFileName)
    else
      RaiseLastOSError;
end;

procedure DoExport;
//------------------------------------------------------------------------------
var
  TempPath, TempFileName: TFileName;
  I: Integer;
  AsyncCallsList: array of IAsyncCall;
begin
  // find Windows temp directory
  SetLength(TempPath, MAX_PATH);
  SetLength(TempPath, GetTempPath(MAX_PATH, PChar(TempPath)));

  // we suppose you have an array of items (1 per file to be created) with some info
  SetLength(AsyncCallsList, Length(AnItemListArray));
  for I := Low(AnItemListArray) to High(AnItemListArray) do
  begin
    AnItem := AnItemListArray[I];
    LogMessage('.Processing current file for '+ AnItem.NAME);
    TempFileName := TempPath + Format(AFormatString, [AnItem.NAME, ...]);
    CreateYourFile(TempFileName);
    LogMessage('.File generated for '+ AnItem.NAME);
    // Move the file to Dest asynchronously, without waiting
    AsyncCallsList[I] := AsyncCall(@MoveFile, [TempFileName, AnItem.DestFolder])
  end;

  // final rendez-vous synchronization
  AsyncMultiSync(AsyncCallsList);
  LogMessage('Job finished... ');
end;

Answer 3

A good start for using thread is Delphi is found at the Delphi about site

In order to make your solution work, you need a job queue for the worker thread. A stringlist can be used. But in any case you need to guard the queue so that only one thread can write to it at any single moment. Even if the writing thread is suspended.

Your application writes to the queue. So there must be a guarded write method.

Your thread reads and removes from the queue. So there must be a guarded read/remove method.

You can use a critical section to make sure only one of these has access to the queue at any single moment.