This recipe is a bit more complex than the previous recipes. So, I'll not explain all the code but only the fundamental parts. You can find the complete code in the book code repository (Chapter1\RECIPE06).

Let's say we want to create a tabbed interface for our software that is used to manage product orders, sales, and invoices. All the forms must be usable at the same time, without having to close the previous one. Before we begin, the following screenshot is what we want to create:

Figure 5.1: The main form containing seven embedded child forms

The project is composed of a bunch of forms. The main form has TTabControl, which allows us to switch between the active forms. All embedded forms inherit from EmbeddableForm. The most important is the method Show shown here:

procedure TEmbeddableForm.Show(AParent: TPanel);
begin
  Parent := AParent;
  BorderStyle := bsNone;
  BorderIcons := [];
  Align := alClient;
  Show;
end;

All the other forms descend from EmbeddableForm and are added to TTabControl on the main form with a line of code similar to the following one:

procedure TMainForm.MenuOrdersClick(Sender: TObject);
begin
  AddForm(TForm1.Create(self));
end;

The AddForm method is in charge of adding an actual instance of a form into the tabs, keeping a reference to it. The following code shows how it is done:

//Add a form to the stack
procedure TMainForm.AddForm(
AEmbeddableForm: TEmbeddableForm);
begin
  AEmbeddableForm.Show(Panel1);
  //each tab show the caption of the containing form and   
  //hold the reference to it
  TabControl1.Tabs.AddObject(AEmbeddableForm.Caption, AEmbeddableForm);
  ResizeTabsWidth;
  ShowForm(AEmbeddableForm);
end;

Other methods are in charge of bringing an already created form to the front when a user clicks on the Related tab, and then to close a form when the Related tab is removed (check out the ShowForm and WMEmbeddedFormClose methods).

There is a bit of code, but the concepts are simple:

Embedding a form into another TWinControl is not difficult and allows us to create flexible GUIs without using TPageControl and Frames. Probably, for the end user, this multi-tabbed GUI is probably more familiar because all the modern browsers use it, and probably, your user already knows how to use a browser with different pages or screens opened. From the developer's point of view, the multi-tabbed interface allows for much better programming patterns and practices. This technique can also be used for other scenarios where you have to embed one "screen" into another.

More flexible (and complex) solutions can be done involving the use of Observers, but in simple cases, this recipe's solution based on Windows Messaging is enough.

More information about the Observer design pattern can be found at http://sourcemaking.com/design_patterns/observer/delphi.

Another interesting solution (that doesn't rely on Windows Messaging and so is also cross-platform) may be based on the System.Messaging.TMessageManager class. More information about TMessageManager can be obtained at http://docwiki.embarcadero.com/Libraries/en/System.Messaging.TMessageManager.

Code in this recipe can be used with every component that uses TStringList to show items (TListBox, TComboBox, and so on) and can be adapted easily for other scenarios.

In the recipe code, you'll also find a nice way to show status messages generated by the embedded forms and a centralized way to show application hints in the status bar.