Managing MDI forms

Problem/Question/Abstract:

Manage MDI forms. Each form will be visble only once. Form will be created if needed.

Answer:

{

This article describes a base class for your mainform.
(You can inherite from this form or use it as is)

Three new routines are presented. They will enable you to easily
manage MDI forms:
- Only one instance per MDI Class will be activated
- Creation of MDI class will be handled by routines if needed
- Activated MDI class will be focused if needed

}

type
  TMDIClass = class of TForm;

type
  TBaseMainForm = class(TForm)
    {... }
  public
    { Public declarations }
    function ActivateMDIClass(MDIClass: TMDIClass): TForm;
    function GetMDIClassIndex(MDIClass: TMDIClass): Integer;
    function MDIClassIsActive(MDIClass: TMDIClass): Boolean;
    {  ... }
  end;

implementation

{
Use ActivateMDIClass() to activate a mdi child class.
If the class is not created yet, it will be.
The mdi child will be shown on screen and focused.
}

function TBaseMainForm.ActivateMDIClass(MDIClass: TMDIClass): TForm;
var
  i: Integer;

begin
  // Try to find index of MDIClass form in MDI child list
  i := GetMDIClassIndex(MDIClass);

  // if index is not found (-1) then create the form
  if i = -1 then
    Result := MDIClass.Create(Application)
  else
    Result := MDIChildren[i];

  // bring it to front
  Result.Show;
  Result.BringToFront;
end;

{
  Get mdi child index of specified MDIClass.
  Returns -1 if the MDIClass does not exist as a created MDI form
}

function TBaseMainForm.GetMDIClassIndex(
  MDIClass: TMDIClass): Integer;
var
  i: Integer;
begin
  // Default index  -1 =  MDIClass not found
  Result := -1;

  // try to find a MDI child of correct MDIClass class
  for i := 0 to MDIChildCount - 1 do
    if MDIChildren[i].ClassType = MDIClass then
      Result := i;
end;

{
  Returns true is the MDIClass exists as a created MDI form
}

function TBaseMainForm.MDIClassIsActive(
  MDIClass: TMDIClass): Boolean;
begin
  Result := GetMDIClassIndex(MDIClass) <> -1;
end;

Usage Example

Create a mainform, inherited from TBaseMainForm.

Create two mdi forms called TfrmBrainstorm and TfrmReport.
Make sure ...FormStyle=fsMDIChild.
Make sure MDI childs can be closed:

procedure...FormClose(Sender: TObject;
  var Action: TCloseAction);
begin
  Action := caFree;
end;

Now use the following code to activate those mdi forms:

procedure TMainForm.OnClick1(Sender: TObject);
begin
  ActivateMDIClass(TfrmBrainstorm);
end;

procedure TMainForm.OnClick2(Sender: TObject);
begin
  ActivateMDIClass(TfrmReport);
end;