TThread.Synchronize Method (TThreadProcedure)

Executes a method call within the main thread.

Pascal

procedure Synchronize(AThreadProc: TThreadProcedure); overload;

C++

__fastcall Synchronize(TThreadProcedure AThreadProc);

Description

Synchronize causes the call specified by AMethod to be executed using the main thread, thereby avoiding multi-thread conflicts. The current thread is passed in the AThread parameter.

If you are unsure whether a method call is thread-safe, call it from within the Synchronize method to ensure that it executes in the main thread.

Execution of the current thread is suspended while Method executes in the main thread.

Warning: Do not call Synchronize from within the main thread. This can cause an infinite loop.

Note: You can also protect unsafe methods using critical sections or the multi-read exclusive-write synchronizer.

Delphi Examples:

Copy Code

{
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 synchronizaiton.
This example is a snippet from the Threads demo in the the
Demos directory of your RAD Studio installation.  Please refer
to that demo for the remaining support code and all design
requirements. The UseSynchronize feature in not implemented
in the demo, but all the changes you need are here.
}
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: 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 they are accessable
by 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 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;