Delphi 添加项目到 ComboBox 速度

Question

I have a fairly complex and large application that hands loads and loads of data. Is there a speedy way to add items to ComboBox that doesn't take so long? On my P3 3.2ghz, the following snippet takes just under a second to add around 32,000 items. (MasterCIList is a StringList with strings typically 20 - 30 bytes long).

with LookupComboBox do
 begin
  Items.BeginUpdate;
  Items.Clear;
  for i := 0 to MasterCIList.Count - 1 do
   Items.Add(MasterCIList[i]);
  Items.EndUpdate;
 end;

Drilling down into the VCL, it appears that in TComboBoxStrings.Add , there is a call to

Result := SendMessage(ComboBox.Handle, CB_ADDSTRING, 0, Longint(PChar(S)));

I'm guessing this is really taking up time (okay, I know it is). Is there another way to populate the Items that is speedier? Any high-speed combo boxes available? I have the TMS components but they seem to be extensions of TComboBox.

For instance, I have the PlusMemo which seems to be a total rewrite of a TMemo. I can easily add a million line in a second to a PlusMemo. A TMemo, I don't think so.

Thank you so much for your time!

Answer 1

Sorry if I'm a nuisance, but I doubt a TComboBox with 32,000 items is even remotely ''usable'' --- I'd say there's a reason why it's slow: it was never meant to do this:)

Would there be a possibility to filter the data, and only load a subset? To be more concrete, in one particular database application I've been working on, the user can search for a person. We let the user type at least 3 or 4 characters of the name, and only then begin to return results in a listbox. This has greatly increased usability of the search form, also greatly speeding up the whole process.

Would you be able to follow a similar approach?

Or, on a completely different take, perhaps you could take a look at the VirtualTreeView component --- either for direct use, or for inspiration.

Answer 2

I agree that 32K items is a ridiculous amount to have in a combobox... That being said, you might try adding the items to a TStringList first and then use a combination of Begin/EndUpdate and AddStrings:

SL := TStringList.Create;
try
  // Add your items to the stringlist
  ComboBox.Items.BeginUpdate;
  try
    ComboBox.Items.AddStrings(YourStringList);
  finally
    ComboBox.Items.EndUpdate;
  end;
finally
  SL.Free;
end;

The code compiles, but I didn't test it further than that; I've never felt the need to add more than a few dozen items to a combobox or listbox. If any more items might be needed than that, I find a way to filter before populating the list so there are fewer items.

Just out of curiosity, how do you expect a user to sort through that many items to make a decision?

Answer 3

var
  Buffer: TStringList;
begin
  Buffer := TStringList.Create;

  try
    // --> Add items to Buffer here <--

    ComboBox.Items := Buffer;
  finally
    FreeAndNil(Buffer);
  end;
end;

This is the fastest way we've found to update a visual control.

The VCL does BeginUpdate, Clear, and EndUpdate internally.

If you don't believe me, profile it.

Answer 4

use backgroundworker for adding MasterCIList items.after complete adding items use only AddStrings.

procedure TForm2.BackgroundWorker1Work(Worker: TBackgroundWorker);
var
  I: Integer;
begin
  MasterCIList.BeginUpdate;
  try
    MasterCIList.Capacity := 32 * 1024; // if derminate  count of items
    for I := 1 to 32 * 1024 do
    begin
      MasterCIList.Add(IntToStr(I));
      { if need progess }
      if I mod 300 = 0 then
        Worker.ReportProgress((I * 100) div MasterCIList.Capacity);
      { if need cancelable }
      if (I mod 100 = 0) and Worker.CancellationPending then
        Break;

    end;

  finally
    MasterCIList.EndUpdate;
  end;

end;

procedure TForm2.BackgroundWorker1WorkComplete(Worker: TBackgroundWorker;
  Cancelled: Boolean);
begin

  LookupComboBox.Items.AddStrings(MasterCIList );


// AddStrings use beginupdate..endupdate in itself

 end;

Answer 5

Maybe you can try this?

"Speeding up adding items to a combobox or listbox" http://blogs.msdn.com/b/oldnewthing/archive/2004/06/10/152612.aspx

Answer 6

perhaps cmb.Items.Assign(myStringList) will help.

here's a wild idea: i haven't tried it but you might check to see if there's a way to virtually load the combobox by setting the number of items and then owner drawing. please pardon this crazy idea but i think i've heard of this being available somehow. irrelevant: this is how it's done in Palm OS...where the faster way to load the combobox is to not load it all... ;-)

Not an answer, but why on earth would you want 32,000 items in a combo box? That is a terrible way to store that much data.

i agree; it's a bad practice...

Answer 7

It's me again. I'm adding 32,000 items cause I need to. That's one of many controls in my application that has "lots" of items. I need to have that many items. It works just fine looking things up. Perfectly in fact. I'm just trying to optimize things. The users find things just fine since they are in a certain logical order.

Everything I've seem so far with Assign and AddStrings is that they eventually end up in Add with the SendMessage call. So I'll keep looking.

Thanks for the feedback.

Answer 8

I implement this in a different manner. First i removed the combobox control and take textbox control and assign it autocomplete to custom source where the custom source string collection is 32k items.I get the selected value from a new query on controls validation.

So it can replace combobox functionality. Mostly about 32k items people dont scroll but they keep entering key strokes and is catched by our custom auto complete source..

Answer 9

Perhaps you can use a database engine in the back-end and use a data aware component. Then the things will be much more quicker and usable. Perhaps if you'll try to describe what do you try to accomplish we'll help you further. In any case, your UI design is, let's say, odd . And for this perhaps the Embarcadero forums will help you better.