Delphi多线程学习（10）：Label(VCL)同步的问题

上文中，多线程同步主窗体的Label的Caption属性值，发现一个问题：使用Synchronize用于同步的时候，主窗体好像死掉一样；而直接用子程序为Label的引用赋值，则有时会出现“Canvas  does not allow drawing”错误。书上说VCL同步一定要用Synchronize，而不能直接访问。
    测试：
{主窗体}
unit Unit2;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;

type
TForm2 = class(TForm)
Label1: TLabel;
Button1: TButton;
Label2: TLabel;
Label3: TLabel;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;

var
Form2: TForm2;

implementation

uses TestThread;

{$R *.dfm}

procedure TForm2.Button1Click(Sender: TObject);
begin
TTestThread.Create(Label1);
//TTestThread.Create(Label2);
//TTestThread.Create(Label3);
end;

end.{多线程类}
unit TestThread;

interface

uses
Classes,StdCtrls;

type
TTestThread = class(TThread)
private
{ Private declarations }
FLabel:TLabel;
Fstr:string;
procedure UpdateLabel;
protected
procedure Execute; override;
public
constructor Create(Lab:TLabel);
end;

implementation

uses Unit2,SysUtils,windows;

{ TTestThread }

constructor TTestThread.Create(Lab: TLabel);
begin
FLabel:=Lab;
Inherited Create(False);
FreeOnTerminate:=True;
end;

procedure TTestThread.Execute;
var
i:Integer;
begin
{ Place thread code here }
for i := 0 to 20000 do
begin
if Not Terminated then
begin
Fstr:=Format('线程ID：%d,第%d个循环',[GetCurrentThreadID,i]);
//UpdateLabel;
Synchronize(UpdateLabel);
end;
end;

end;

procedure TTestThread.UpdateLabel;
begin
FLabel.Caption:=Fstr;
end;

end.
经过测试，只创建一个线程，用Synchronize同步主窗体的一个Label，那么程序无问题，可以见到同步的过程。当�