How to make a combo box with fulltext search autocomplete support?

I would like a user to be able to type in the second or third word from a TComboBox item and for that item to appear in the AutoSuggest dropdown options

For example, a combo box contains the items:

Mr John Brown
Mrs Amanda Brown
Mr Brian Jones
Mrs Samantha Smith

When the user types "Br" the dropdown displays:

Mr John Brown
Mrs Amanda Brown
Mr Brian Jones

and when the user types "Jo" the dropdown displays:

Mr John Brown
Mr Brian Jones

The problem is that the AutoSuggest functionality only includes items in the dropdown list that begin with what the user has input and so in the examples above nothing will appear in the dropdown.

Is it possible to use the IAutoComplete interface and/or other related interfaces to get around this issue?

Solution

The following example uses the interposed class of the TComboBox component. The main difference from the original class is that the items are stored in the separate StoredItems property instead of
the Items as usually (used because of simplicity).

The StoredItems are being watched by the OnChange event and whenever you change them (for instance by adding or deleting from this string list), the current filter will reflect it even when the combo
list is dropped down.

The main point here is to catch the WM_COMMAND message notification CBN_EDITUPDATE which is being sent whenever the combo edit text is changed but not rendered yet. When it arrives, you just search through the StoredItems list for what you have typed in your combo edit and fill the Items property with matches.

For text searching is used the ContainsText so the search is case insensitive. Forgot to mention,
the AutoComplete feature has to be turned off because it has its own, unwelcomed, logic for this purpose.

unit Unit1;

interface

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

type
  TComboBox = class(StdCtrls.TComboBox)
  private
    FStoredItems: TStringList;
    procedure FilterItems;
    procedure StoredItemsChange(Sender: TObject);
    procedure SetStoredItems(const Value: TStringList);
    procedure CNCommand(var AMessage: TWMCommand); message CN_COMMAND;
  public
    constructor Create(AOwner: TComponent); override;
    destructor Destroy; override;
    property StoredItems: TStringList read FStoredItems write SetStoredItems;
  end;

type
  TForm1 = class(TForm)
    ComboBox1: TComboBox;
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

constructor TComboBox.Create(AOwner: TComponent);
begin
  inherited;
  AutoComplete := False;
  FStoredItems := TStringList.Create;
  FStoredItems.OnChange := StoredItemsChange;
end;

destructor TComboBox.Destroy;
begin
  FStoredItems.Free;
  inherited;
end;

procedure TComboBox.CNCommand(var AMessage: TWMCommand);
begin
  // we have to process everything from our ancestor
  inherited;
  // if we received the CBN_EDITUPDATE notification
  if AMessage.NotifyCode = CBN_EDITUPDATE then
    // fill the items with the matches
    FilterItems;
end;

procedure TComboBox.FilterItems;
var
  I: Integer;
  Selection: TSelection;
begin
  // store the current combo edit selection
  SendMessage(Handle, CB_GETEDITSEL, WPARAM(@Selection.StartPos),
    LPARAM(@Selection.EndPos));
  // begin with the items update
  Items.BeginUpdate;
  try
    // if the combo edit is not empty, then clear the items
    // and search through the FStoredItems
    if Text <> '' then
    begin
      // clear all items
      Items.Clear;
      // iterate through all of them
      for I := 0 to FStoredItems.Count - 1 do
        // check if the current one contains the text in edit
        if ContainsText(FStoredItems[I], Text) then
          // and if so, then add it to the items
          Items.Add(FStoredItems[I]);
    end
    // else the combo edit is empty
    else
      // so then we'll use all what we have in the FStoredItems
      Items.Assign(FStoredItems)
  finally
    // finish the items update
    Items.EndUpdate;
  end;
  // and restore the last combo edit selection
  SendMessage(Handle, CB_SETEDITSEL, 0, MakeLParam(Selection.StartPos,
    Selection.EndPos));
end;

procedure TComboBox.StoredItemsChange(Sender: TObject);
begin
  if Assigned(FStoredItems) then
    FilterItems;
end;

procedure TComboBox.SetStoredItems(const Value: TStringList);
begin
  if Assigned(FStoredItems) then
    FStoredItems.Assign(Value)
  else
    FStoredItems := Value;
end;

procedure TForm1.FormCreate(Sender: TObject);
var
  ComboBox: TComboBox;
begin
  // here's one combo created dynamically
  ComboBox := TComboBox.Create(Self);
  ComboBox.Parent := Self;
  ComboBox.Left := 10;
  ComboBox.Top := 10;
  ComboBox.Text := 'Br';

  // here's how to fill the StoredItems
  ComboBox.StoredItems.BeginUpdate;
  try
    ComboBox.StoredItems.Add('Mr John Brown');
    ComboBox.StoredItems.Add('Mrs Amanda Brown');
    ComboBox.StoredItems.Add('Mr Brian Jones');
    ComboBox.StoredItems.Add('Mrs Samantha Smith');
  finally
    ComboBox.StoredItems.EndUpdate;
  end;

  // and here's how to assign the Items of the combo box from the form 
  // to the StoredItems; note that if you'll use this, you have to do
  // it before you type something into the combo's edit, because typing 
  // may filter the Items, so they would get modified
  ComboBox1.StoredItems.Assign(ComboBox1.Items);
end;    

end.