dynamically create forms using a STRING - delphi

Question

I have 5 forms created at design time. I need to dynamically create an instance of each form and put on a tab.
My question: If the form names are in an array of strings and I call my procedure like this:
ShowForm(FormName[3]);// To show the 3rd form on a tab page.
How can I define and create the new instance for each form?
This is what I have for now:
procedure TForm1.ShowFormOnTab(pProcName:String);
var
NewForm: TfrmSetupItemCategories;//***HERE IS MY PROBLEM***
NewTab: TTabSheet;
FormName: String;
begin
NewTab := TTabSheet.Create(PageControl1);
NewTab.PageControl:= PageControl1;
NewTab.Caption:='hi';
PageControl1.ActivePage := NewTab;
if pProcName='ProcfrmSetupItemCategories' Then
begin
NewForm:=TfrmSetupItemCategories.Create(NewTab);
NewTab.Caption := NewForm.Caption;
end;
if pProcName='ProcfrmZones' Then
begin
NewForm:=TfrmZones.Create(NewTab);
NewTab.Caption := NewForm.Caption;
end;
.
.
.
end;
the line that reads "HERE IS MY PROBLEM" is where I need help. I can't reuse NewForm as a variable with a second form in this way...
Note: My problem is NOT the tab. Rather it's creating a new instance of the form using the same variable name.

Answer 1

Declare the NewForm variable as TForm:
var
NewForm: TForm;
begin
NewForm := TMyForm.Create(Tab1); //compiles OK
NewForm := TMyOtherForm.Create(Tab2); //also compiles OK
end;
I'm assuming TMyForm and TMyOtherForm both are derivatives of TForm.
DRY
You can also reduce your repeating code using a class reference variable, like this:
procedure TForm1.ShowFormOnTab(pProcName:String);
var
NewForm: TForm;
ClassToUse: TFormClass;
NewTab: TTabSheet;
FormName: String;
begin
NewTab := TTabSheet.Create(PageControl1);
NewTab.PageControl:= PageControl1;
NewTab.Caption:='hi';
PageControl1.ActivePage := NewTab;
if pProcName='ProcfrmSetupItemCategories' then
ClassToUse := TfrmSetupItemCategories
else if pProcName='ProcfrmZones' then
ClassToUse := TfrmZones
else
ClassToUse := nil;
if Assigned(ClassToUse) then
begin
NewForm := ClassTouse.Create(NewTab);
NewTab.Caption := NewForm.Caption;
//if you access custom properties or methods, this is the way:
if NewForm is TfrmZones then
TfrmZones(NewForm).ZoneInfo := 'MyInfo';
end;
end;
Register your classes and then create the forms from a string
As Sir Rufo points in his comment, you can even go further registering your classes (I'm not sure if this can be done in Lazarus, that exercise is up to you).
First, register the form classes you want to instantiate from the class name, previous to any call to your ShowFormOnTab method, for example:
procedure TMainForm.FormCreate(Sender: TObject);
begin
RegisterClass(TfrmSetupItemCategories);
RegisterClass(TfrmZones);
//and other classes
end;
Then, you can change the code to get the class reference from the class name string:
procedure TForm1.ShowFormOnTab(pProcName:String);
var
NewForm: TForm;
ClassToUse: TFormClass;
ClassNameToUse: string;
NewTab: TTabSheet;
FormName: String;
begin
NewTab := TTabSheet.Create(PageControl1);
NewTab.PageControl:= PageControl1;
NewTab.Caption:='hi';
PageControl1.ActivePage := NewTab;
//get rid of 'Proc' and add the T
//or even better, pass directly the class name
ClassNameToUse := 'T' + Copy(pProcName, 5, MaxInt);
ClassToUse := TFormClass(FindClass(ClassNameToUse));
if Assigned(ClassToUse) then
begin
NewForm := ClassTouse.Create(NewTab);
NewTab.Caption := NewForm.Caption;
//if you access custom properties or methods, this is the way:
if NewForm is TfrmZones then
TfrmZones(NewForm).ZoneInfo := 'MyInfo';
end;
end;
That way, the code remains the same for any number of classes.
For more info about this, take a look at Creating a Delphi form from a string in delphi.about.com.

Answer 2

Declare your variable as an ancestor type:
var
NewForm: TForm;
or
var
NewForm: TCustomForm;
Drawback: you'll need to cast the variable to the specific class if you want to call any methods of your form that you have declared yourself.
Use a 'soft' cast if you want to have the compiler check that NewForm is actually a TMyForm at runtime:
(NewForm as TMyForm).MyMethod;
When you are absolutely sure that NewForm is a TMyForm (like when you just created it), you can also use a 'hard' cast:
TMyForm(NewForm).MyMethod;

Answer 3

With registered classes, in the initialization of the used forms, you could shorten it to
Function CreateAndDock(pc:TPageControl;const FormName:String):Boolean;
begin
Result := false;
if Assigned(GetClass(FormName)) and GetClass(FormName).InheritsFrom(TCustomForm) then
With TFormClass( GetClass(FormName)).Create(pc.Owner) do
begin
ManualDock(pc);
Show;
Result := true;
end;
end;
procedure TForm4.Button1Click(Sender: TObject);
begin
ShowMessage(IntToStr(Integer(CreateAndDock(pagecontrol1,'TDockForm'))));
ShowMessage(IntToStr(Integer(CreateAndDock(pagecontrol1,'TNotExists'))));
end;