Call Button1Click in Form1/Unit1 from Unit2 - delphi

Question

I've downloaded Delphi XE7 and having some problems with accessing another Units...
I need to call procedures from another units, so I'll give a very basic illustration, simple program...
This is code from main Unit1 with form and button1:
unit Unit1;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics,
Controls, Forms, Dialogs, StdCtrls, Unit2;
type
TForm1 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.Button1Click(Sender: TObject);
begin
ShowMessage('Hello');
end;
end.
And this is the code from Unit2:
unit Unit2;
interface
implementation
uses Unit1;
end.
Now, how is it possible to make procedure Button1Click like in Unit2 to showmessage let's say HelloFromUnit2 when button1 on form1 is clicked? Unit2 is codeUnit without anything..

Answer 1

Use the build in procedure for calling the Click handler
Leave form 1 the way it is:
unit Unit2;
interface
implementation
uses
Unit1;
procedure Click;
begin
if Assigned(Form1) then
Form1.Button1.Click;
end;
end.

Answer 2

Add a procedure declaration to the public section of TForm1, like this
type
TForm1 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
public
Procedure SayHello;
end;
...
procedure TForm1.SayHello;
begin
ShowMessage('Hello');
end;
end.
Then in Unit2 you would call this procedure. You would have to ensure that Form2 has already been instantiated - or create a new instance for your call.
Do not use this mechanism for event handlers!

Answer 3

The header of your post doesn't match the question in the text
"Call Button1Click in Form1/Unit1 from Unit2" vs.
"Now, how is it possible to make procedure Button1Click like in Unit2 to showmessage let's say HelloFromUnit2 when button1 on form1 is clicked?"
I answer the question in the text (as I understand it). If this is not what you intended, you might want to rephrase the question in the text.
Add, to Form1.Button1Click, a call to a new procedure in unit2
procedure TForm1.Button1Click(Sender: TObject);
begin
ShowMessage('Hello');
SayHelloFromUnit2; // <---- add this
end;
In unit2 add the following to the interface section:
procedure SayHelloFromUnit2;
and to the implementation section
uses Vcl.Dialogs;
procedure SayHelloFromUnit2;
begin
ShowMessage('Hello from unit2');
end;