Tutorial - CREATING OBJECT ON THE FLY
7 Steps in Dymanic Component Creation (2)
1 * Declare an instance of a specified component type.(2/33)
2 * Called inherited create constructor to return a
reference to that component (2/33-2)
3 * Assigned Preliminary property and default value
for the object.(2/34)
4 * Assigned Parent to control to provide its visibility.(2/34-2)
5 * Make Control visible (require for Delphi 1 only)(2/35)
6 * Assigned Event Handler to the control by writing
code. (2/35-2)
7 * Detach Event Handler and Destruct the Object.(2/36)
InDirect Inheritance (2/36-2)
Dynamic Form in Delphi (2/37)
1 * Declare an instance of a specified component
type.(2/33)
This is another 7 steps (additional to standard 10 rules for OOP
you should already know).
You need to declare specific Instance to be use as references
of CLASS which you want to create, and call Uses at that Class too.
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Var
.......MyButton : TBitBin;
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2 * Called inherited create constructor to return
a reference to that component (2/33-2)
Call Method "Create" in order to put
Component up into memory which then will return to us "Reference"
as an "Instance" before put it to use.
Command "Create" is similar to function
which return address in memory of that component or object (think
of mobile phone number pinpoint where you are, but mobile phone
number is not the physical phone itself).
The difference between creating a normal class and your own class
is that you need to indicate owner by using command "Create"
to indicate how you want to destroy your component.
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.......MyButton : TBitBin.Create(Self);
.......MyButton : TBitBin.Create(Application);
.......MyButton : TBitBin.Create(Nil);
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Create(Self) indicate that when this component or CLASS
is destroy, everything which comes with this component will be destroy
too.
Create(Application) indicate that this application or form
is the OWNER of MyButton, if application or form is destroy or end
program then destroy this component too.
Create (Nil) indicate NO owner, if Nil then programmer will
decide how to destroy his component.
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3 * Assigned Preliminary property and default
value for the object.(2/34)
You can set default value for your component property (top,
left, width, height) or simply use command "SetBounds".
Some component can be non-visual which can not be seen when RUN
program, but can be seen when designing. You should not indicate
setting WHERE, but just it's property.
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.......MyButton.Caption := '&Confirm';
.......MyButton.Top :=5;
.......MyButton.Left :=5;
.......MyButton.Width :=80;
.......MyButton.Height :=25;
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Or write in short.
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.......MyButton.Caption := '&Confirm';
.......MyButton.SetBounds(5,5,80,25);
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4 * Assigned Parent to control to provide its
visibility.(2/34-2)
Our component might have to attach (cling) to some Parent control
and parent control have to be a type of TWinControl such as TForm,
TPanel, TScrollBox.
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.......MyButton.Parent :=Self
;
....................Or
.......MyButton.Parent :=Form1;
<provided that form1 exists>
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Note : Indicate Class we are
writing is Parent of MyButton
OR Form1 is parent control of MyButton
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Or use Method by using command "InsertControl" which is in
TWinControl, so that we can insert our component in TWinControl
at ease.
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.......Self.InsertControl (MyButton);
.......Form1.InsertControl (MyButton);
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PROBLEM : In case of difficulty on
setting TWinControl for be a parent control then adding this
ControlStyle : = ControlStyle + [CsAcceptControls]
;
to enable that component to be a parent control, then you can try
to set TWinControl to be a parent control again as state method
above.
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5 * Make Control visible (require for Delphi
1 only). (2/35)
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.......MyButton.Visible := True;
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6 * Assigned Event Handler to the control by
writing code. (2/35-2)
If we drop a component on form when designing we can just simply
press F11 to go to Object inspector, but when you are designing
component, you will need to write a code to assigned how you want
to handle events or actions such as onClick, OnMouseMove and etc.
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.......MyButton.OnClick := DoSomeOnClickMethod;
.......MyButton.OnMouseMove := MouseMoveRoutine;
.......MyButton.OnDblClick :=MyDoubleClickMethodRoutineForThisButton;
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7 * Detach Event Handler and Destruct the Object.(2/36)
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.......MyButton.OnClick := Nil;
.......MyButton.OnMouseMove := Nil;
.......MyButton.OnDblClick := Nil;
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.......MyButton.Free;
.......MyButton.Destroy;
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You need to destroy your object and event to give back space to
memory or else your program will keep on getting fatter. In case
of creating component on top of another component, it's better to
use this rule to destroy the object. The best way is to do it is
at Destructor of that component's parent.
If you indicate owner to that component, you DO NOT need to FREE
it in order to destroy References (instance), because Owner of the
component will automatically destroy your component for you.
You might have many owner for your component such as 1 Owner responsible
for destroy the component and 1 Owner responsible in showing the
component. IF Parent and OWNER is the same object, you can simply
write
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.......MyButton.Parent := TWinControl(Owner);
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* InDirect Inheritance (2/36-2)
Writing code to assigned event handler is not quite convenient and
mostly be applied to those experienced programmer with Delphi. It's
a wise idea to create a CLASS then override, or modify it's method
to your own preferred behavior.
See sample below
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Type
.......TMyButton = Class(TBitBtn)
.......Public
.......Constructor CreatedMeToo(AOwner:TComponent;X,Y:Integer;CapStr;String);Override;
.......Procedure Click;Override;
.......Procedure MouseMove;Override;
.......Procedure MouseDown;Override;
.......Procedure WM_MouseDown(Var Msg:TMessage) Message WM_MouseDown;
End;
Procedure TMyButton.CreateMeToo(AOwner:TComponent;X;Y:Integer;CapStr:String);
.....Begin
.......Left := X; Top :=Y;
.......Width:=50; Height:=25;
.......Caption := CapStr;
.......inherited Create(AOwner);
End;
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Note : from above we created
Class TMyButton from Class TBitBtn ....using method "Indirect",
so we can add, modify Class the way we like.
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Instead of creating ordinary TBitBtn, we can
do this.
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.......MyButton: =TMyButton.CreateMeToo(Self,5,5'&Confirm');
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Now event MouseMove and MouseDown and other Events
are already here.
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* Dynamic Form in Delphi (2/37)
One form is approximately 28-40 K (sound small but actually one
program is not that small). Form is upload in to memory and therefore
slow down your program. Dynamic form allow you to maximize your
program loading effectively and fast.
Create form when need. (PRE-PLAN form creation)
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Var
.......Form1 : TForm1;
.......Form1 : = TForm1.Create (Nil);
.......Form1.ShowModal;
.......Form1.Free;
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Note : from above if Form is
Create and have no owner (Nil), then show form "Showmodal"
and when FINISH get rid of this form.
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Create form when need. (NOT so PRE-PLAN form creation)
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Var
.......Form1 : TForm1 = Nil;
.......If Form1 = Nil
then
..........Form1:= TForm1.Create
(Application);
..........Form1.Show;
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Note : from above Form is Create
with no owner (Nil) and then assign owner to be Application,
so get rid of this form when EXIT this program.
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