unit UnMain;

 interface

 uses

   Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,

   Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, Vcl.Imaging.jpeg,

   Vcl.ExtCtrls;

 type

   TFrmMain = class(TForm)

     Edit1: TEdit;

     Button1: TButton;

     Image1: TImage;

     procedure Button1Click(Sender: TObject);

   private

     { Private declarations }

     procedure Test(str:string); {定义一个用于回调的过程}

   public

     { Public declarations }

   end;

 var

   FrmMain: TFrmMain;

 implementation

 {引用子窗体单元}

 uses UnChild;

 {$R *.dfm}

 procedure TFrmMain.Button1Click(Sender: TObject);

 begin

   {调用Unit2的接口方法}

   CallUnit2(test);

 end;

 {回调过程的实现部分}

 procedure TFrmMain.test(str: string);

 begin

   {将str值副给Edit1}

   Edit1.Text:=str;

 end;

 end.

 unit UnChild;

 interface

 uses

   Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,

   Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;

 type

    TFuncCallBack = procedure(str:string) of object;

   TFrmChild = class(TForm)

     Edit1: TEdit;

     Button1: TButton;

     procedure Button1Click(Sender: TObject);

   private

   public

     {定义一个回调函数类型的变量}

     aFuncCallBack:TFuncCallBack;

   end;

    {提供给Unit1调用的接口方法,注意里面的参数的类型}

     procedure CallUnit2(FuncCallBack:TFuncCallBack);

 var

   FrmChild: TFrmChild;

 implementation

 {$R *.dfm}

 {接口方法的实现部分}

 procedure CallUnit2(FuncCallBack:TFuncCallBack);

 begin

   Application.CreateForm(TFrmChild,FrmChild);

   {将参数赋值给FuncCallBack}

   FrmChild.aFuncCallBack:=FuncCallBack;

   FrmChild.ShowModal;

 end;

 procedure TFrmChild.Button1Click(Sender: TObject);

 begin

   {当点击Form2的按钮时将Form2中的Edit的值传递给了Form1中的Edit}

   {是不是很神奇？我并没有uses Unit1,但却改变了Form1中Edit的Text属性}

   aFuncCallBack(Edit1.Text);

   ModalResult:=mrOk;

 end;

 end.