创建Tform2时显示消息？ [英] Show message when Tform2 is created?

问题描述

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。

I want to when Tform2 is created then show a message to user. I use this code, but not work well.

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;

推荐答案

这是因为您声明 a 作为局部变量。每次您输入 TForm1.Button1Click 时，即使可能仍然存在一个Form2，此变量也将是全新的且未初始化。

That's because you declare a as a local variable. Each time you enter TForm1.Button1Click this variable will be brand new and uninitialized even though there might still be a Form2. That means that the check for nil won't even work.

您应该：

• 使 a 成为全局变量（如首次创建表单时所获得的Form2全局变量）


• 使<$ c Form1声明的$ c> a （您是主表单？）的一部分，还是贯穿整个程序的其他类的数据模块。


• 根本不使用变量，而是检查 Screen.Forms 来查看是否有Form2。 li>
  

• Make a a global (like the Form2 global you get when you first create a form)
• Make a part of the declaration of Form1 (you main form?) or a datamodule of other class that lives throughout your entire program.
• Don't use a variable at all, but check Screen.Forms to see if you got a Form2 in there.

就像这样：

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;

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