delphi - Components not being found by FindComponent()

Question

I have created a login form which holds buttons corresponding to users' names held in an Access file. The buttons are created in the OnCreate event because I don't want to have to create them each time the screen is shown.

The buttons display as expected, and I have created LogOn and LogOff procedures, which both work as I expected.

The next step was to only display the buttons for users who are currently logged on to the system. To do this, I have created the following code in the OnActivate event:

procedure TUserInForm.FormActivate(Sender: TObject);
var
   btn : TLBButton;
begin
   With UserQuery do begin;
      first;
      while (not eof) do begin
         BtnName := FieldByName('UserName').AsString;
         Btn := TLBButton(FindComponent(BtnName));
         if (Btn <> nil) then
            if (FieldByName('LoggedIn').AsBoolean = True) then Btn.Visible := True else Btn.Visible := False;
         next;
      end;
   end;
end;

However, it doesn't find any of the buttons - they are all nil. The code throws an Access Violation exception if I remove the nil check. But, at no point in the code do I destroy the buttons or the form itself. The buttons exist, because I can see them on the form. The BtnName variable is global within the unit. I've checked that the BtnName variable is populated correctly from the table.

I've used code similar to this to find components before, with no problems. In fact, I "stole" the code shown above from another procedure (with the obvious changes) in which it works fine. The logs show no errors.

Can anyone suggest some approach to fixing this problem? It's very frustrating!

question from:https://stackoverflow.com/questions/65861927/components-not-being-found-by-findcomponent

Answer 1

FindComponent() searches the owned-components-list of the component that it is called on. I’m assuming your OnCreate handler creates the buttons with the Form as their Owner. But the with block will cause FindComponent() to be called on the UserQuery component instead of the Form. That would explain why the buttons are not being found.

So, you can either:

• use Self.FindComponent() instead, since the OnActivate handler is being called on the Form, so Self will point at the Form:

procedure TUserInForm.FormActivate(Sender: TObject);
var
  Btn : TLBButton;
  BtnName : string;
begin
  with UserQuery do begin
    First;
    while (not Eof) do begin
      BtnName := FieldByName('UserName').AsString;
      Btn := TLBButton(Self.FindComponent(BtnName));
      if (Btn <> nil) then
        Btn.Visible := FieldByName('LoggedIn').AsBoolean;
      Next;
    end;
  end;
end;

• get rid of the with block altogether, as it is generally considered to be bad practice to use with anyway in non-trivial cases (this situation is a good example of why):

procedure TUserInForm.FormActivate(Sender: TObject);
var
  Btn : TLBButton;
  BtnName : string;
begin
  UserQuery.First;
  while (not UserQuery.Eof) do begin
    BtnName := UserQuery.FieldByName('UserName').AsString;
    Btn := TLBButton(FindComponent(BtnName));
    if (Btn <> nil) then
      Btn.Visible := UserQuery.FieldByName('LoggedIn').AsBoolean;
    UserQuery.Next;
  end;
end;

• if you want to keep using the with block, you can move the button search to a separate function that does not exist in the UserQuery component, so the with won’t be confused about which component to call the function on:

procedure TUserInForm.FormActivate(Sender: TObject);
var
  Btn : TLBButton;
  BtnName : string;
begin
  with UserQuery do begin
    First;
    while (not Eof) do begin
      BtnName := FieldByName('UserName').AsString;
      Btn := FindButton(BtnName);
      if (Btn <> nil) then
        Btn.Visible := FieldByName('LoggedIn').AsBoolean;
      Next;
    end;
  end;
end;

function TUserInForm.FindButton(const BtnName: string): TLBButton;
begin
  Result := TLBButton(FindComponent(BtnName));
end;

Now, that being said, it would be a better design to add the created buttons to a list that you manage for yourself, rather than a list that the VCL manages on your behalf. Then you will always know exactly where to find the buttons. For example:

type
  TUserInForm = class(TForm)
    UserQuery: TQuery;
    ...
    procedure FormActivate(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
    ...
  private
    Buttons: TList;
    function FindButton(const BtnName: string): TLBButton;
    ...
  end;

...

procedure TUserInForm.FormCreate(Sender: TObject);
var
  Btn : TLBButton;
begin
  Buttons := TList.Create;
  ...
  Btn := TLBButton.Create(Self);
  Btn.Name := ... 
  Buttons.Add(Btn);
  ...
end;

procedure TUserInForm.FormDestroy(Sender: TObject);
begin
  Buttons.Free;
end;

procedure TUserInForm.FormActivate(Sender: TObject);
var
  Btn : TLBButton;
  BtnName : string;
begin
  UserQuery.First;
  while (not UserQuery.Eof) do begin
    BtnName := UserQuery.FieldByName('UserName').AsString;
    Btn := FindButton(BtnName);
    if (Btn <> nil) then begin
      Btn.Visible := UserQuery.FieldByName('LoggedIn').AsBoolean;
    end;
    UserQuery.Next;
  end;
end;

function TUserInForm.FindButton(const BtnName: string): TLBButton;
var
 i: integer;
begin
  for i := 0 to Buttons.Count-1 do begin
    Result := TLBButton(Buttons[i]);
    if Result.Name = BtnName then Exit;
  end;
  Result := nil;
end;