The target for this post is to show how you can use interfaces to decouple units in your application.
Good developers know that loose coupling between units is a good thing. This is usually done with interfaces.
Consider this small application:
MainForm.pas
unit MainForm;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;
type
TMain = class(TForm)
lblResult: TLabel;
btnResult: TButton;
txtResult: TEdit;
procedure btnResultClick(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Main: TMain;
implementation
{$R *.dfm}
uses
Dialog; // Dialog must be here in uses
procedure TMain.btnResultClick(Sender: TObject);
var
sName: string;
begin
sName := TDialogForm.Execute(txtResult.Text);
if sName <> '' then
begin
txtResult.Text := sName;
end;
end;
end.
MainForm.dfm
object Main: TMain
Left = 0
Top = 0
Caption = 'Main'
ClientHeight = 87
ClientWidth = 187
Color = clBtnFace
Font.Charset = DEFAULT_CHARSET
Font.Color = clWindowText
Font.Height = -12
Font.Name = 'Segoe UI'
Font.Style = []
TextHeight = 15
object lblResult: TLabel
Left = 8
Top = 54
Width = 32
Height = 15
Caption = 'Result'
end
object btnResult: TButton
Left = 8
Top = 20
Width = 159
Height = 25
Caption = 'Get result'
TabOrder = 0
OnClick = btnResultClick
end
object txtResult: TEdit
Left = 46
Top = 51
Width = 121
Height = 23
TabOrder = 1
end
end
Dialog.pas
unit Dialog;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;
type
TDialogForm = class(TForm)
btnOk: TButton;
btnCancel: TButton;
lblName: TLabel;
txtName: TEdit;
public
class function Execute(const AName: string): string;
end;
implementation
{$R *.dfm}
{ TDialogForm }
class function TDialogForm.Execute(const AName: string): string;
var
oDialog: TDialogForm;
begin
oDialog := TDialogForm.Create(nil);
oDialog.txtName.Text := AName;
if oDialog.ShowModal = mrOk then
begin
Result := oDialog.txtName.text;
end;
end;
end.
Dialog.dfm
object DialogForm: TDialogForm
Left = 0
Top = 0
BorderIcons = [biMinimize, biMaximize]
BorderStyle = bsDialog
Caption = 'Dialog'
ClientHeight = 114
ClientWidth = 216
Color = clBtnFace
Font.Charset = DEFAULT_CHARSET
Font.Color = clWindowText
Font.Height = -12
Font.Name = 'Segoe UI'
Font.Style = []
DesignSize = (
216
114)
TextHeight = 15
object lblName: TLabel
Left = 24
Top = 24
Width = 32
Height = 15
Caption = 'Name'
end
object btnOk: TButton
Left = 50
Top = 68
Width = 65
Height = 31
Anchors = [akRight, akBottom]
Caption = '&Ok'
Default = True
ModalResult = 1
TabOrder = 0
end
object btnCancel: TButton
Left = 137
Top = 68
Width = 71
Height = 31
Anchors = [akRight, akBottom]
Cancel = True
Caption = '&Cancel'
ModalResult = 2
TabOrder = 1
end
object txtName: TEdit
Left = 62
Top = 21
Width = 150
Height = 23
TabOrder = 2
TextHint = 'Enter any name here'
end
end
InterfaceDemo.dpr
program InterfaceDemo;
uses
Vcl.Forms,
MainForm in 'MainForm.pas' {Main},
Dialog in 'Dialog.pas' {DialogForm};
{$R *.res}
begin
Application.Initialize;
Application.MainFormOnTaskbar := True;
Application.CreateForm(TMain, Main);
Application.Run;
end.
Now I want to be able to create the dialog without have the dialog unit in uses. To do this I can use interfaces. It doesn't matter in this small demo but can be nice in an application with hundreds of forms.
First a new unit GUIAsInterface.pas
It has 3 interfaces
type
IInterfaceRegistry = interface
['{1F44813F-A8EA-4C5C-B7E9-34626108C171}']
procedure RegisterClass(const IID: TGUID; aClass: TComponentClass);
end;
IGuiDialog = interface
['{22E22636-9312-4606-90CB-497D1E07FF2F}']
function ShowModalForm(ACustomForm: TCustomForm): integer;
end;
IGUI = interface
['{49C1E5D8-92CE-486C-9082-FC257C1F2A6A}']
function GuiDialog: IGuiDialog;
procedure RegisterForm(const IID: TGUID; aClass: TComponentClass); overload;
end;
Some records and classes that implements the interfaces
type
TInterfaceClassRegistryEntry = record
IID: TGUID;
ComponentClass: TComponentClass;
end;
TInterfaceInstanceRegistryEntry = record
IID: TGUID;
InstanceAsComponent: TComponent;
end;
TClassRegistryArray = array of TInterfaceClassRegistryEntry;
TInterfaceRegistryArray = array of TInterfaceInstanceRegistryEntry;
TInterfaceRegistry = class(TComponent, IInterfaceRegistry)
private
fClasses: TClassRegistryArray;
fInstances: TInterfaceRegistryArray;
class var _Instance: IInterfaceRegistry;
procedure InternalRegisterInstance(const IID: TGUID; aComponent: TComponent; out Obj);
function FindClass(const IID: TGUID): TComponentClass;
procedure RegisterClass(const IID: TGUID; aClass: TComponentClass);
protected
function QueryInterface(const IID: TGUID; out Obj): HResult; override;
public
procedure BeforeDestruction; override;
end;
TGUI = class(TInterfaceRegistry, IGUI)
private
function GuiDialog: IGuiDialog;
procedure RegisterForm(const IID: TGUID; aClass: TComponentClass); overload;
end;
And a global method
function GUI: IGUI;
begin
if not Assigned(TInterfaceRegistry._Instance) then
begin
if _UnitFinalized then
Raise Exception.Create('Attempt to access Gui after finalization.')
else
TInterfaceRegistry._Instance := TGui.Create(Application);
end;
result := TInterfaceRegistry._Instance as IGUI;
end;
uInterfaces.pas containing the interfaces. In this case only one.
unit uInterfaces;
interface
type
IForm = interface
end;
ITestDlg = interface
['{E80A465B-6908-47B7-AC12-0835D4DE79AD}']
function Execute(const AName: string): string;
end;
implementation
end.
The idea is to have array of classes and interfaces in TInterfaceRegistry.
At startup all interfaces are registred to a form in initialization section of the form.
To add more forms and interfaces just add them to uInterfaces.pas and modify the dialog to implement the new interface like this
type
TDialogForm = class(TForm, ITestDlg)
The registration
initialization
Gui.RegisterForm(ITestDlg, TDialogForm);
As GUIAsInterface now handle the create and free of dialog it cane be removed from Execute in Dialog.pas
So when the form is needed all you need to do is (GUI as someInterface).someMethod where someInterface and SomeMethod can be anything.
In this case the dialog is created with
In this case the dialog is created with
sName := (GUI as ITestDlg).Execute(txtResult.Text);
Method Execute can have any signature. It is decided in uInterfaces.pas.
The complete project that use interfaces is committed to
https://github.com/bero/InterFaceWithGUI
You can compare the code without interfaces here https://github.com/bero/InterFaceWithGUI/tree/ccfda52d8269d8634e201ccefe0fdb10be4e6cdf
You can compare the code without interfaces here https://github.com/bero/InterFaceWithGUI/tree/ccfda52d8269d8634e201ccefe0fdb10be4e6cdf
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