Search for a Label by its Caption

Question

I was trying to figure out how to search for a Label by its Caption:

for I := ComponentCount - 1 downto 0 do
begin
  if Components[i] is TLabel then
    if Components[i].Caption = mnNumber then
    begin
      Components[i].Left := Left;
      Components[i].Top := Top + 8;
    end;
end;

I get an error: Undeclared identifier: 'Caption'.
How can I resolve this issue?

Answer 1

Iterating over Components[] is the wrong approach. That just yields the components that are owned by the form. You will miss any components that are added dynamically, and not owned by the form, or components that are owned by frames.

Instead you should use Controls[]. However, that only yields first generation children. If there is deeper parent/child nesting then you need to recurse. That's more work. I use some helpers to make it easy. I've wrapped them up in this unit:

unit ControlEnumerator;

interface

uses
  System.SysUtils, System.Generics.Collections, Vcl.Controls;

type
  TControls = class
  private
    type
      TEnumerator<T: TControl> = record
        FControls: TArray<T>;
        FIndex: Integer;
        procedure Initialise(WinControl: TWinControl; Predicate: TFunc<T, Boolean>);
        class function Count(WinControl: TWinControl; Predicate: TFunc<T, Boolean>): Integer; static;
        function GetCurrent: T;
        function MoveNext: Boolean;
        property Current: T read GetCurrent;
      end;
      TEnumeratorFactory<T: TControl> = record
        FWinControl: TWinControl;
        FPredicate: TFunc<T, Boolean>;
        function Count: Integer;
        function Controls: TArray<T>;
        function GetEnumerator: TEnumerator<T>;
      end;
  public
    class procedure WalkControls<T: TControl>(WinControl: TWinControl; Predicate: TFunc<T, Boolean>; Method: TProc<T>); static;
    class function Enumerator<T: TControl>(WinControl: TWinControl; Predicate: TFunc<T, Boolean>=nil): TEnumeratorFactory<T>; static;
    class function ChildCount<T: TControl>(WinControl: TWinControl; Predicate: TFunc<T, Boolean>=nil): Integer; static;
  end;

implementation

{ TControls.TEnumerator<T> }

procedure TControls.TEnumerator<T>.Initialise(WinControl: TWinControl; Predicate: TFunc<T, Boolean>);
var
  List: TList<T>;
  Method: TProc<T>;
begin
  List := TObjectList<T>.Create(False);
  Try
    Method :=
      procedure(Control: T)
      begin
        List.Add(Control);
      end;
    WalkControls<T>(WinControl, Predicate, Method);
    FControls := List.ToArray;
  Finally
    List.Free;
  End;
  FIndex := -1;
end;

class function TControls.TEnumerator<T>.Count(WinControl: TWinControl; Predicate: TFunc<T, Boolean>): Integer;
var
  Count: Integer;
  Method: TProc<T>;
begin
  Method :=
    procedure(Control: T)
    begin
      inc(Count);
    end;
  Count := 0;
  WalkControls<T>(WinControl, Predicate, Method);
  Result := Count;
end;

function TControls.TEnumerator<T>.GetCurrent: T;
begin
  Result := FControls[FIndex];
end;

function TControls.TEnumerator<T>.MoveNext: Boolean;
begin
  inc(FIndex);
  Result := FIndex<Length(FControls);
end;

{ TControls.TEnumeratorFactory<T> }

function TControls.TEnumeratorFactory<T>.Count: Integer;
begin
  Result := TEnumerator<T>.Count(FWinControl, FPredicate);
end;

function TControls.TEnumeratorFactory<T>.Controls: TArray<T>;
var
  Enumerator: TEnumerator<T>;
begin
  Enumerator.Initialise(FWinControl, FPredicate);
  Result := Enumerator.FControls;
end;

function TControls.TEnumeratorFactory<T>.GetEnumerator: TEnumerator<T>;
begin
  Result.Initialise(FWinControl, FPredicate);
end;

class procedure TControls.WalkControls<T>(WinControl: TWinControl; Predicate: TFunc<T, Boolean>; Method: TProc<T>);
var
  i: Integer;
  Control: TControl;
  Include: Boolean;
begin
  if not Assigned(WinControl) then begin
    exit;
  end;
  for i := 0 to WinControl.ControlCount-1 do begin
    Control := WinControl.Controls[i];
    if not (Control is T) then begin
      Include := False;
    end else if Assigned(Predicate) and not Predicate(Control) then begin
      Include := False;
    end else begin
      Include := True;
    end;
    if Include then begin
      Method(Control);
    end;
    if Control is TWinControl then begin
      WalkControls(TWinControl(Control), Predicate, Method);
    end;
  end;
end;

class function TControls.Enumerator<T>(WinControl: TWinControl; Predicate: TFunc<T, Boolean>): TEnumeratorFactory<T>;
begin
  Result.FWinControl := WinControl;
  Result.FPredicate := Predicate;
end;

class function TControls.ChildCount<T>(WinControl: TWinControl; Predicate: TFunc<T, Boolean>): Integer;
begin
  Result := Enumerator<T>(WinControl, Predicate).Count;
end;

end.

Now you can solve your problem like this:

var
  lbl: TLabel;
....
for lbl in TControls.Enumerator<TLabel>(Form) do
  if lbl.caption=mnNumber then
  begin
    lbl.Left := Left;
    lbl.Top := Top + 8;
  end;

Or you could make use of a predicate to put the caption test inside the iterator:

var
  Predicate: TControlPredicate;
  lbl: TLabel;
....
Predicate := function(lbl: TLabel): Boolean
  begin
    Result := lbl.Caption='hello';
  end;
for lbl in TControls.Enumerator<TLabel>(Form, Predicate) do
begin
  lbl.Left := Left;
  lbl.Top := Top + 8;
end;