1.12.2015 Everything about using 2 forms... or more!

Multiple forms in Lazarus IDE
We love forms, right? So why have only one? Why not two?! Let's learn how to create multiple forms, manage them and use them as dialog boxes.

When we create a new Application Project, we get one form with our project. We love forms because every GUI project that we start, it starts from a form, a blank form. Then it gradually grows to a full application. We could consider it a building block for our applications.

For some projects it is okey to have one form. But for some others, we need extra forms. For example, a form might be needed for an About dialog, or a Settings dialog.

There is a way for using multiple forms. And in this little must-read tutorial, we're gonna learn how to use and manage multiple forms in our project.

Creating the project (as usual)

Start Lazaurs.

First we need a project. If you have one open it. But it is better to create a new project just for this practice. To create a new project start Lazarus and then click Project -> New Project, select "Application" from the list, then press OK.

This will create a project with one Form in it. It is of no surprise that by default the form is named Form1. Okey, so let's continue...

Creating a new Form

We already have a single form. Now we want to add another form. So, click File -> New Form. You will see a new form titled "Form2" in the screen.

Lazarus New form on menu

Now that you have created another form it is better to save the project to understand how they work. So, go ahead and click File -> Save All... and save the project. You can use the default file names, don't worry!

So you have saved project1.lpi, unit1.pas and unit2.pas.

How to see all your forms in current project and switch between them

If you have followed the instructions correctly, now you have 2 forms in your project. If you don't trust me, press Shift + F12 to bring up the "View Project Forms" dialog. In this dialog you will see all the forms the current project has.

View Project Forms window in Lazarus IDE, triggered by Shift + F12

You can double click on the forms from the list of that dialog to bring it up in the form design view. This will switch to that form to let you design it. You can also try the Window menu to switch between forms.

Switching forms in Lazarus is also possible with menus
Additionally, I hope you remember that we press F12 to switch between form and code view.

Showing one Form from another Form

Suppose we want to show Form2 from Form1. Form1 will have a button. When the user clicks it, Form2 will show.

So, switch to Form1, draw a TButton on the form. Set its Caption as "Show Form2" from the Object Inspector. Double click it and enter...
Well wait. We want to use Form2 in our code. But that won't work. Try writing "form2" and pressing Ctrl+Space. It won't autocorrect. That means Form2 is not found by Form1!

Although you have saved both unit1.pas and unit2.pas in the same project directory, Pascal won't find it right away. You will have to manually add it to the uses clause of Form1's code (Unit1). We will have to point a finger and tell Pascal that we want to use the TForm present in Unit2 in our code. If we don't, Pascal won't find it itself. It is similar to the situation when you add path to a library directory, still you need to add the unit name(s) in the uses clause. Or else, pascal won't add the library unit itself. That's the way Pascal works.

So in the Form1 code's uses clause, we'll have to add Unit2 in order to use Form2 in our code.

unit Unit1;


  ..., ...,

(Remember, uses clause only knows the Unit name, not the form name. So we've used Unit2, not Form2.)

To show Form2, the code would be: Form2.Show; Now we can write it since we have added Form2 in the uses clause. You will have to add the unit name to uses clause whenever you want to use one form from another.

Now let's get back to the button. Double click it then add the code:

procedure TForm1.Button1Click(Sender: TObject);

Now Run the Project (F9 or Run -> Run). Then click the button. Form2 will show. That means our code is working!

The code is working. But when you would want to use Form1 from Form2 it will give you a circular reference error. To avoid this, we should add an implementation clause then dump our reference under it. We'll be looking into this problem a bit further. But for now, delete the Unit2 from uses and add an implementation clause and keep it there:

unit Unit1;


  ..., ... {,


Accessing Form1 from Form2

We tried accessing Form2 from Form1. Now its time to to do the opposite: access Form1 from Form2.

Now here's an important note, if you add unit1 in the uses clause of unit2, it wouldn't work. It will raise unit2.pas(xxx,yyy) Fatal: Circular unit reference between Unit2 and Unit1.

unit Unit2;

  ..., ..., unit1;
  // This code is wrong! It will raise a Circular unit reference error.

So a solution to this problem is adding ANOTHER uses clause under implementation clause.

unit Unit2;

  ..., ...;


And also in Unit1 if you haven't done it, go ahead and make the change:

unit Unit1;





Now, with this cool tip you can use both forms back and forth.

Testing the link

You can add some buttons to test whether the link between the forms work.

Draw 2 buttons on Form1. Code Button1click like:

Code Button2click like:

Draw 1 button on Form2 and code on its click event:

You get the idea.

Making Form2 appear first

When we run the project, Form1 usually comes first on the screen. How can we set some other form as the first one? Let's see...

Go to Project -> Project Options. Then under Project Options click Forms. You would see 2 lists. The list on the right doesn't matter to us right now.

How to set the default form to form2 in Lazarus

Select Form2 from the list on the left and click the Up arrow icon. Make sure that Form2 is the topmost item. Then click OK.

The topmost item on that list appears first.

Showing Forms as Dialog Boxes

If you want to show a form as a dialog box, its easy! Here's the code:


It does the following:
(1) It shows the form
(2) Locks the parent form from which it was triggered
(3) Halts running code after the line Form2.ShowModal.

When the user closes the form (or hits the OK button or maybe something else to close it) the code after the line resumes to execute.

We can use this behavior to capture the settings that the user put in the dialog.

Reading settings from a Modal form

Suppose, the modal form is our dialog box. So we want to update something according to the settings that the user changed. How could we do it?

There are many ways to do that. One of the ways is described in the Free Pascal/Lazarus Wiki. Well, you can follow that instructions. But I think I should write in my own way which I find simpler.

We would use something called a ModalResult property of the modal form. The default value for ModalResult is mrNone. If the value of it changes, the modal form automatically closes and the .ShowModal returns the value of the ModalResult. We can set its value to mrOK, mrCancel, mrIgnore etc. In that case the value will be returned to the parent form. This is a very logical workflow for handling modal forms and great for handling all the cases possible.

Let's suppose we have 2 forms - Form1 is our parent form and Form2 is our modal form. On form1 I drop a button. And on form2 I create TEdit and two buttons btnOK and btnCancel. So:

1. On the OK button of the Form2 I set ModalResult to mrOK.

procedure TForm2.btnOKClick(Sender: TObject);
  // the parent form will get the mrOK value

...And the Cancel button would have:

procedure TForm2.btnCancelClick(Sender: TObject);

2. This will let the parent form's code to continue where it left off. So, we can now access every component in modal form.

procedure TForm1.Button1Click(Sender: TObject);
  // we detect that ok button has been pressed
  if Form2.ShowModal = mrOK then begin   

All the possible values for ModalResult (copy-pasted from the Controls unit):
  mrNone = 0;
  mrOK = mrNone + 1;
  mrCancel = mrNone + 2;
  mrAbort = mrNone + 3;
  mrRetry = mrNone + 4;
  mrIgnore = mrNone + 5;
  mrYes = mrNone + 6;
  mrNo = mrNone + 7;
  mrAll = mrNone + 8;
  mrNoToAll = mrNone + 9;
  mrYesToAll = mrNone + 10;
  mrClose = mrNone + 11;
  mrLast = mrClose;

So, now you are ready to make multiple forms appear in your project.
So go on and make some forms!


Tida Tida said...

Accessing Form1 from Form2:
You implemented Unit 1 in Unit 1. Shouldn't Unit 2 be implemented in Unit 1..

Adnan Shameem said...

@Tida Tida
Yes, you are right. (Oops!) Thanks for pointing out the mistake. Great to have a reader like you.
The mistake was due to writing it in my busy schedule.
The code has been fixed.


gorhin stroebel said...

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I Would Love you to check out my Project Man

Adnan Shameem said...

Happy to see you got new ideas.
Yeah sure. Post a link and I'll check it out.


gorhin stroebel said...

I would like to also help with the tutorials
I'm 14 Years Old And Work With Delphi 7
I Know:

I'm Working On A Book Called:
Graphical Programming With Delphi
Delphi Physics
I Make Videos on Delphi and Java but I never put them on youtube yet

So I want to know if I can help here?

gorhin stroebel said...

Dude Please send your email so I can email you the zip quickly. And so I have way of contacting you faster!

Adnan Shameem said...

(Sorry for late reply. My internet connection is not so steady lately.)
That's great! Good luck on your book!

And yes, you are welcome to write on LazPlanet. If you wish to write interesting and helpful articles or tutorials, you can contact me with the "Contact" page (link can be found at the top navigation).


Eddie Bole said...

First of all, thanks for the nice tute Adrian.
I'm still a bit confused about "Uses". When would you ONLY USE a "Uses" clause? Can you give another example?

I understand that to access each form from the other you need to add a "Uses Implementation" clause to prevent a circular reference error.

Adnan Shameem said...

@Eddie Bole
Well, to explain this it would take a whole new article. :)

I would explain it as much as I can--
Every project you make with Lazarus has a general uses clause at the very beginning of the unit code. You can switch to Code view of any project (F12) and you would see the general uses clause. This clause acts as an "include" function for your project. There are many codes written for you already in Lazarus/Free Pascal. But in order to use them, you'll have to "include" those code units into your project. For that you'll have write their name in the USES clause.

If you are familiar with C++, you should know that every code in C++ has some include command, such as, #include . iostream.h has all the input and output functions written into it so that you don't have to. You would then only need to include that file to use those functions in the file. Uses clause in Pascal is just like that. (The only difference being that you don't have to type in any filename, you just need to write their unit name.)

If you create a new Application project in Lazarus (Project->New Project->Application->OK) you would see that Lazarus already "included" some units in its code in the code view (F12). These units are basic Form and Component units. Even the simple ShowMessage() comes from the "Dialogs" unit. You can add different kinds of components (such as a TImage) and Lazarus will add the necessary units in the uses clause.

So a pascal code can have a general Uses clause and another Uses clause under the implementation clause. This one is special case but it works the same way. The difference is that, this uses clause cannot be seen by other units. It is just for the current unit.

General Uses clause helps you to include a code from another unit. Same happens for the Implementation Uses. We "include" Unit2 to use the components that are declared in Form2. If you go to the code of Unit2 you will see under the Form2 class declaration that there are all the components that you created on the form. Lazarus creates these code lines under the hood. By "including" Unit2, we can use those components in our Unit1 code.

For further reading you can see this wiki article.

Hope I made it clear to you.

Unknown said...

hello.. i want to ask some help.. :-)
i have 2 forms. i created Tedit in form1 and Tlabel in form2.. my design is, when i'll put something in Tedit in form1, i want it to be shown in Tlabel in form2.. how will it happen? pls. give or share some tip or advice.. thanks :-)

Adnan Shameem said...

Well, it is one of the very confusing topic of Lazarus. I understand. Let me explain the solution to your problem in detailed words...
Start Lazarus.
Create a new Application Project (Project->New Project->Application->OK)

Now you want to deal with 2 forms. So create one using File->New Form.
Save the project using File->Save All.
Draw a TEdit on Form1 and a TLabel on Form2.
Now on Unit1, under its implementation clause add a uses clause:


We would have to show the Form2 because it will not show up by default... Add this code on Form1's OnShow event... (Object Inspector -> Events -> OnShow -> [...])

procedure TForm1.FormShow(Sender: TObject);

Switch to form view (F12) and double click the TEdit. Then enter the code below:

procedure TForm1.Edit1Change(Sender: TObject);
  Form2.Label1.Caption := Edit1.Text;

Now Run it (F9) and you should have two forms on screen. (You may have to move one form to see the other.) Now write something on form1's Tedit and you should have what you want. :-)


Unknown said...

Thank u your advice helps alot

Dwi Ahmad said...

hi, good morning.. it's morning around here.
I used delphi 7, but unfortunately when I use "implementation uses" my program won't start and it says Tform1 was missing or incorrect, after I remove the "implementation uses" again, it works..
please help me.
thanks in advance.

Adnan Shameem said...

@Dwi Ahmad

I am not sure about Delphi. I have never used it. But it seems "implementation uses" can be used on Delphi:

You don't actually need to include it always. You will only need to include the unit under uses when you need to access any component from that form. Otherwise, you can just skip it.

You can also try including the unit in the normal uses clause. This should be ok in normal case, but that causes problems when that other form also "uses" this form.

Hope this helps.

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