Delphi-对话框打开时键盘钩处于活动状态

Question

On an application I have a keyboard hook which close a MDI child form when the Escape button is pressed. The issue appear when a TOpenDialog class descendant is opened(with Execute ).Consider the following code (which is only for example)

unit Unit4;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm4 = class(TForm)
    OpenDialog1: TOpenDialog;
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

function KeyboardProc(code: integer; wp: WPARAM; lp: LPARAM): LResult stdcall;
function CanDoCloseOnEscape: boolean;

var
  Form4: TForm4;
  KeybHook : HHook;


implementation

{$R *.dfm}

function CanDoCloseOnEscape: boolean;
var
  Control: TWinControl;
  Form: TForm;
begin
  Control := Screen.ActiveControl;
  Form := Screen.ActiveForm;
  Result := true;
end;

function KeyboardProc(code: integer; wp: WPARAM; lp: LPARAM): LResult stdcall;
begin
  case wp of
    VK_ESCAPE:
      if CanDoCloseOnEscape then
      begin
        PostMessage(Screen.ActiveForm.Handle, WM_Close, 0, 0);
        exit;
      end;
  end;
end;

procedure TForm4.Button1Click(Sender: TObject);
begin
 if OpenDialog1.Execute then
  begin
    ShowMessage('executed');
  end;
end;

initialization
  KeybHook := SetWindowsHookEx(WH_KEYBOARD, KeyboardProc, hInstance,GetWindowTask(application.Handle));

finalization
  UnhookWindowsHookEx(KeybHook);

end.

Control := Screen.ActiveControl;

• is not taking the opendialog as the active control.

So, even the dialog is still opened the keyboard hook is executed and the form get closed.

Answer 1

The fundamental problem is that you are (ab)using a keyboard hook. This is a global event and you need to detect more local keyboard events. You should arrange instead for your Delphi forms to listen for the ESC key being pressed.

Off the top of my head, I'm not sure whether the MDI form or the MDI children will receive the event, but whichever it is you need to handle it there. Likely you'll need to set KeyPreview to be True for this to work.

On a more general note, if a global approach was to be appropriate, a keyboard hook is still the wrong tool. The tool you'd use would be the OnMessage event of the Application object. This is wired into the application's main message queue, and any Delphi modal message pumps run from ShowModal calls. Putting handling there would mean that you would detect keyboard events destined for VCL forms, but you would not pick up keyboard events for non-VCL modal windows like file dialogs, Win32 message boxes etc.