模式窗体与非模式窗体

非模式窗体从创建到销毁都需要代码来维护，防止内存未安全释放。

模式窗体就比较省心一些，只需要在代码段的try…finally中创建并维护即可。

下面用例子来说明一下两类窗体如何调用：

主窗体程序：

unit Main;

interface

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

type
  TForm8 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    procedure Button2Click(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form8: TForm8;

implementation

{$R *.dfm}

procedure TForm8.Button1Click(Sender: TObject);
begin
  try
    Form9:=TForm9.Create(Application);
    Form9.ShowModal;
  finally
    Form9.Free; //窗体的清理放在同一代码段的try...finaally中，而不是在ModalForm中的Destroy方法里。
    Form9:=nil;
  end;

end;

procedure TForm8.Button2Click(Sender: TObject);
begin
  if not Assigned(Form10) then  //防止出现多个窗体实例
  Form10:=TForm10.Create(Application); //窗体的创建方式

  Form10.Show;
end;

end.

模式窗体：

unit ModalForm;

interface

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

type
  TForm9 = class(TForm)
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form9: TForm9;

implementation

{$R *.dfm}


end.

非模式窗体：

unit ModalessForm;

interface

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

type
  TForm10 = class(TForm)
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
    procedure FormDestroy(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form10: TForm10;

implementation

{$R *.dfm}

procedure TForm10.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  Action:=caFree;
end;

procedure TForm10.FormDestroy(Sender: TObject);
begin
  Form10:=nil; //这里是Form10设为nil而不是TForm10，否则将无法再次创建窗体。
end;

end.