Why don't Delphi modal forms get WM_SYSCOMMAND when the user clicks the taskbar button?

Question

In a Delphi (2007) program running on Windows 8.1, I would like to receive a notification when a user clicks a taskbar button that belongs to my program. Thus, I capture WM_SYSCOMMAND, which usually receives the message in this case.

This works great for the main program window.

If the modal window is active (opened using Form2.ShowModal), the same code cannot capture WM_SYSCOMMAND, either in the main or in the modal form. Who cares? And is there a way to change that?

This is the code I added in both forms:

unit unit1; interface type TForm1 = class(TForm) // [...] procedure WMSysCommand(var Msg: TWMSysCommand); message WM_SYSCOMMAND; end; // [...] implementation // [...] procedure Tf_dzProgressTest.WMSysCommand(var Msg: TWMSysCommand); begin inherited; // place breakpoint here end; // [...] end. 

I also tried using the Application.OnMessage or TApplicationEvents component and even overriding the WndProc method. Also failed to capture WM_SYSCOMMAND while the modal form was active.

Answer 1

When you click the taskbar button, the system tries to perform a minimize action on the window associated with the taskbar button. This is usually a window for the main form. This is where WM_SYSCOMMAND arises.

Now that the modal form is displayed, the main form is disabled. It was disabled when the Win32 EnableWindow function was EnableWindow . This is an integral part of modality. A modal window is only a window with the top level turned on, because you should not interact with any other top-level window.

When a window is disabled, its system menu is also disabled. This is why the system cannot perform the minimize action and why you are not getting WM_SYSCOMMAND .

Not much that you can do about this. When you show the modal form, the main window should be turned off. And at that moment he will not receive WM_SYSCOMMAND and will not know that the user has pressed the taskbar button.

Answer 2

David explained the problem very well, so I will not repeat what he said.

What I'm going to give you is working with non-blocking code.

You will need to declare an event that will tell us when the form has closed.

  TModalResultEvent = procedure(aSender: TObject; var aModal: TModalResult) of object; 

This allows us to listen to messages passing through the application.

 const WM_SYSCOMMAND1 = WM_USER + 1; type TApplicationHelper = class(TWinControl) private FListener: TWinControl; public constructor Create(AOwner: TComponent); override; procedure WMSysCommand1(var Msg: TWMSysCommand); message WM_SYSCOMMAND1; procedure FirstChance(var Msg: TMsg; var Handled: Boolean); virtual; property Listener: TWinControl read FListener write FListener; end; constructor TApplicationHelper.Create(AOwner: TComponent); begin inherited; Application.OnMessage := FirstChance; if aOwner is TWinControl then FListener := TWinControl(aOwner) else FListener := Self; end; procedure TApplicationHelper.FirstChance(var Msg: TMsg; var Handled: Boolean); begin {get in and out...this gets called alot...I would recommend only using PostMessage since it is non blocking} if Assigned(FListener) then begin if Msg.Message = WM_SYSCOMMAND then begin PostMessage(FListener.Handle, WM_SYSCOMMAND1, Msg.wParam, Msg.lParam); end; end; end; procedure TApplicationHelper.WMSysCommand1(var Msg: TWMSysCommand); begin ShowMessage('WMSYSCOMMAND1 AppHelper'); end; end. 

An example of how to invoke a Non Blocking form.

 unit IForms; interface uses Forms, Controls; type TModalResultEvent = procedure(aSender: TObject; var aModal: TModalResult) of object; IForm = interface function getEnableForm: boolean; procedure setEnableForm(const Value: boolean); Property EnableForm: boolean read getEnableForm write setEnableForm; end; implementation end. TForm1 = class(TForm, IForm) Button1: TButton; Button2: TButton; procedure Button1Click(Sender: TObject); procedure Button2Click(Sender: TObject); procedure FormCreate(Sender: TObject); procedure CheckBox1Click(Sender: TObject); procedure FormDestroy(Sender: TObject); private { Private declarations } FEnable: boolean; FAppHelper: TApplicationHelper; procedure FormModal(aSender: TObject; var aModal: TModalResult); function getEnableForm: boolean; procedure setEnableForm(const Value: boolean); //don't need it //procedure EnableChildren(aParent: TWinControl; aEnable: boolean); procedure WMSysCommand1(var Msg: TWMSysCommand); message WM_SYSCOMMAND1; public { Public declarations } Property EnableForm: boolean read getEnableForm write setEnableForm; end; var Form1: TForm1; implementation {$R *.dfm} uses Unit2, Unit3; procedure TForm1.Button1Click(Sender: TObject); var a_Form: TForm2; begin //Normal blocking code a_Form := TForm2.Create(nil); try a_Form.ShowModal; finally a_Form.Free; end; end; procedure TForm1.Button2Click(Sender: TObject); var a_Form: TForm3; begin //Non blocking code a_Form := TForm3.Create(nil); a_Form.ShowModal(Self, FormModal); end; { mrNone = 0; mrOk = idOk; mrCancel = idCancel; mrAbort = idAbort; mrRetry = idRetry; mrIgnore = idIgnore; mrYes = idYes; mrNo = idNo; mrAll = mrNo + 1; mrNoToAll = mrAll + 1; mrYesToAll = mrNoToAll + 1; } procedure TForm1.FormModal(aSender: TObject; var aModal: TModalResult); var a_Message: string; begin if aSender is TForm then a_Message := 'Form: ' + TForm(aSender).Name; Case aModal of mrNone: a_Message := a_Message + ' None'; mrOk: a_Message := a_Message + ' Ok'; mrCancel: a_Message := a_Message + ' Cancel'; mrAbort: a_Message := a_Message + ' Abort'; mrRetry: a_Message := a_Message + ' Retry'; mrYes: a_Message := a_Message + ' Yes'; mrNo: a_Message := a_Message + ' No'; mrAll: a_Message := a_Message + ' All'; mrNoToAll: a_Message := a_Message + ' No To All'; mrYesToAll: a_Message := a_Message + ' Yes To All'; else a_Message := a_Message + ' Unknown'; end; ShowMessage(a_Message); end; { procedure TForm1.EnableChildren(aParent: TWinControl; aEnable: boolean); var a_Index: integer; begin for a_Index := 0 to aParent.ControlCount - 1 do begin if aParent.Controls[a_Index] is TWinControl then EnableChildren(TWinControl(aParent.Controls[a_Index]), aEnable); aParent.Controls[a_Index].Enabled := aEnable; end; end;} function TForm1.GetEnableForm: boolean; begin //Result := FEnable; Result := Enabled; end; procedure TForm1.SetEnableForm(const Value: boolean); begin //FEnable := Value; Enabled := Value; //EnableChildren(Self, FEnable); end. procedure TForm1.FormCreate(Sender: TObject); begin FAppHelper:= TApplicationHelper.Create(Self); FAppHelper.Parent := Self; end; procedure TForm1.CheckBox1Click(Sender: TObject); begin if CheckBox1.Checked then FAppHelper.Listener := Self else FAppHelper.Listener := FAppHelper; end; procedure TForm1.FormDestroy(Sender: TObject); begin FAppHelper.Free; end; procedure TForm1.WMSysCommand1(var Msg: TWMSysCommand); begin ShowMessage('WMSYSCOMMAND1 Form1'); end; { object Form1: TForm1 Left = 84 Top = 126 Width = 514 Height = 259 Caption = 'Form1' Color = clBtnFace Font.Charset = DEFAULT_CHARSET Font.Color = clWindowText Font.Height = -11 Font.Name = 'MS Sans Serif' Font.Style = [] OldCreateOrder = False OnCreate = FormCreate OnDestroy = FormDestroy PixelsPerInch = 96 TextHeight = 13 object Button1: TButton Left = 56 Top = 56 Width = 75 Height = 25 Caption = 'Button1' TabOrder = 0 OnClick = Button1Click end object Button2: TButton Left = 256 Top = 56 Width = 75 Height = 25 Caption = 'Button2' TabOrder = 1 OnClick = Button2Click end object CheckBox1: TCheckBox Left = 256 Top = 112 Width = 97 Height = 17 Caption = 'Send to Form' Checked = True State = cbChecked TabOrder = 2 OnClick = CheckBox1Click end end } 

This is a non-blocking form.

 unit Unit3; interface uses Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms, Dialogs, Unit1, StdCtrls; type TForm3 = class(TForm) Button1: TButton; Button2: TButton; Button3: TButton; procedure FormClose(Sender: TObject; var Action: TCloseAction); procedure Button1Click(Sender: TObject); private { Private declarations } FForm: IForm; FModalResultEvent: TModalResultEvent; protected procedure DoClose; virtual; public { Public declarations } procedure ShowModal(aForm: IForm; aModalResultEvent: TModalResultEvent) overload; end; var Form3: TForm3; implementation {$R *.dfm} { object Button1: TButton Left = 32 Top = 128 Width = 73 Height = 25 Caption = 'Yes' ModalResult = 6 TabOrder = 0 OnClick = Button1Click end object Button2: TButton Left = 128 Top = 128 Width = 57 Height = 25 Caption = 'No' ModalResult = 7 TabOrder = 1 OnClick = Button1Click end object Button3: TButton Left = 216 Top = 128 Width = 57 Height = 25 Caption = 'Cancel' ModalResult = 2 TabOrder = 2 OnClick = Button1Click end } procedure TForm3.FormClose(Sender: TObject; var Action: TCloseAction); begin try DoClose; finally Action := caFree; end; end; procedure TForm3.ShowModal(aForm: TForm; aModalResultEvent: TModalResultEvent); begin FForm := aForm; FModalResultEvent := aModalResultEvent; if Assigned(FForm) then FForm.EnableForm:= False; Self.Show; end; procedure TForm3.Button1Click(Sender: TObject); begin if Sender is TButton then begin Self.ModalResult := TButton(Sender).ModalResult; Close; end; end; procedure TForm3.DoClose; var a_MR: TModalResult; begin a_MR := Self.ModalResult; if Assigned(FForm) then FForm.EnableForm := True; if Assigned(FModalResultEvent) then FModalResultEvent(Self, a_MR); end;