创建Tform2时显示消息？

Question

我想在创建Tform2时向用户显示一条消息.我使用此代码,但效果不佳.

procedure TForm1.Button1Click(Sender: TObject);
var
   a:TForm2;
begin

if a=nil then
 begin
    a := TForm2.Create(Self);
    a.Show;
 end
 else
 begin
    showmessage('TForm2 is created');
 end;

end;

Answer 1

那是因为你声明a为局部变量.每次输入TForm1.Button1Click此变量时,即使可能仍有Form2,也将是全新且未初始化的.这意味着检查nil甚至不起作用.

你应该:

• 创建a一个全局(就像第一次创建表单时获得的Form2全局)
• 让aForm1的声明(你主要形式？),或者贯穿生活整个程序的其他类的数据模块的一部分.
• 根本不要使用变量,但检查一下Screen.Forms你是否有一个Form2.

[编辑]

像这样:

var
  i: Integer;
begin
  // Check
  for i := 0 to Screen.FormCount - 1 do
  begin
    // Could use the 'is' operator too, but this checks the exact class instead
    // of descendants as well. And opposed to ClassNameIs, it will force you
    // to change the name here too if you decide to rename TForm2 to a more
    // useful name.
    if Screen.Forms[i].ClassType = TForm2 then
    begin
      ShowMessage('Form2 already exists');
      Exit;
    end;
  end;

  // Create and show.
  TForm2.Create(Self).Show;
end;