Synchronize (Delphi)

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Description

This example demonstrates the use and the need for the TThread Synchronize method by executing three sorting routines in three separate threads, with and without synchronization. This example is a snippet from the Threads demo in the Demos directory of your RAD Studio installation. Please refer to that demo for the remaining support code and all the design requirements. The UseSynchronize feature in not implemented in the demo, but all the changes you need are here.

Also see the procedure for running the THRDDEMO in the RAD Studio help topic "Freezing and Thawing Threads in the Debugger".

Code

type

{ TSortThread }

  PThreadSortArray = ^TThreadSortArray;
  TThreadSortArray = array[0..MaxInt div SizeOf(Integer) - 1] of Integer;

  TSortThread = class(TThread)
  private
    FBox: TPaintBox;
    FSortArray: PThreadSortArray;
    FSize: Integer;
    FA, FB, FI, FJ: Integer;
    procedure DoVisualSwap;
  protected
    procedure Execute; override;
    procedure VisualSwap(A, B, I, J: Integer);
    procedure Sort(var A: array of Integer); virtual; abstract;
  public
    constructor Create(Box: TPaintBox; var SortArray: array of Integer);
  end;

{ TBubbleSort }

  TBubbleSort = class(TSortThread)
  protected
    procedure Sort(var A: array of Integer); override;
  end;

{ TSelectionSort }

  TSelectionSort = class(TSortThread)
  protected
    procedure Sort(var A: array of Integer); override;
  end;

{ TQuickSort }

  TQuickSort = class(TSortThread)
  protected
    procedure Sort(var A: array of Integer); override;
  end;

procedure PaintLine(Canvas: TCanvas; I, Len: Integer);

var UseSynchronize, UseQueue: Boolean;

implementation

procedure PaintLine(Canvas: TCanvas; I, Len: Integer);
begin
  Canvas.PolyLine([Point(0, I * 2 + 1), Point(Len, I * 2 + 1)]);
end;

{ TSortThread }

constructor TSortThread.Create(Box: TPaintBox; var SortArray: array of Integer);
begin
  FBox := Box;
  FSortArray := @SortArray;
  FSize := High(SortArray) - Low(SortArray) + 1;
  FreeOnTerminate := True;
  inherited Create(False);
end;

{ Since DoVisualSwap uses a VCL component (i.e., the TPaintBox), it should never
  be called directly by this thread. DoVisualSwap should be called by passing
  it to the Synchronize method, which causes DoVisualSwap to be executed by the
  main VCL thread, avoiding multi-thread conflicts. See VisualSwap for an
  example of calling Synchronize. }

procedure TSortThread.DoVisualSwap;
begin
  with FBox do
  begin
    Canvas.Pen.Color := clBtnFace;
    PaintLine(Canvas, FI, FA);
    PaintLine(Canvas, FJ, FB);
    Canvas.Pen.Color := clRed;
    PaintLine(Canvas, FI, FB);
    PaintLine(Canvas, FJ, FA);
  end;
end;

{ VisusalSwap is a wrapper on DoVisualSwap, making it easier to use. The
  parameters are copied to instance variables so that they are accessible
  to the main VCL thread when it executes DoVisualSwap. }

procedure TSortThread.VisualSwap(A, B, I, J: Integer);
begin
  FA := A;
  FB := B;
  FI := I;
  FJ := J;
  if (UseSynchronize) then Synchronize(DoVisualSwap)
    else if (UseQueue) then Queue(DoVisualSwap)
      else DoVisualSwap;
end;

{ The Execute method is called when the thread starts. }

procedure TSortThread.Execute;
begin
  Sort(Slice(FSortArray^, FSize));
end;

{ TBubbleSort }

procedure TBubbleSort.Sort(var A: array of Integer);
var
  I, J, T: Integer;
begin
  for I := High(A) downto Low(A) do
    for J := Low(A) to High(A) - 1 do
      if A[J] > A[J + 1] then
      begin
        VisualSwap(A[J], A[J + 1], J, J + 1);
        T := A[J];
        A[J] := A[J + 1];
        A[J + 1] := T;
        if Terminated then Exit;
      end;
end;

{ TSelectionSort }

procedure TSelectionSort.Sort(var A: array of Integer);
var
  I, J, T: Integer;
begin
  for I := Low(A) to High(A) - 1 do
    for J := High(A) downto I + 1 do
      if A[I] > A[J] then
      begin
        VisualSwap(A[I], A[J], I, J);
        T := A[I];
        A[I] := A[J];
        A[J] := T;
        if Terminated then Exit;
      end;
end;

{ TQuickSort }

procedure TQuickSort.Sort(var A: array of Integer);

  procedure QuickSort(var A: array of Integer; iLo, iHi: Integer);
  var
    Lo, Hi, Mid, T: Integer;
  begin
    Lo := iLo;
    Hi := iHi;
    Mid := A[(Lo + Hi) div 2];
    repeat
      while A[Lo] < Mid do Inc(Lo);
      while A[Hi] > Mid do Dec(Hi);
      if Lo <= Hi then
      begin
        VisualSwap(A[Lo], A[Hi], Lo, Hi);
        T := A[Lo];
        A[Lo] := A[Hi];
        A[Hi] := T;
        Inc(Lo);
        Dec(Hi);
      end;
    until Lo > Hi;
    if Hi > iLo then QuickSort(A, iLo, Hi);
    if Lo < iHi then QuickSort(A, Lo, iHi);
    if Terminated then Exit;
  end;

begin
  QuickSort(A, Low(A), High(A));
end;

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