Question

I would like to know why are there 2 different ways of clearing out a listview. One is by calling listview.clear and other is listview.items.clear. Actually, this extends to many other VCL components too. which method must be used and why?

Answer 1

ListView.Clear is just a wrapper around ListView.Items.Clear with ListItems.BeginUpdate/ListItems.EndUpdate. look at the source:

procedure TCustomListView.Clear;
begin
  FListItems.BeginUpdate;
  try
    FListItems.Clear;
  finally
    FListItems.EndUpdate;
  end;
end;

From the docs:

The BeginUpdate method suspends screen repainting until the EndUpdate
method is called. Use BeginUpdate to speed processing and avoid
flicker while items are added to or deleted from a collection.

A better practice is to use BeginUpdate/EndUpdate for speed and avoiding flicker.
But the main reason to use ListView.Clear is because using a “high-level VCL methods” (As well commented by @Arnaud) is always a good idea, and the implementation might change (BTW, the method was introduced in D7).

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EDIT: I have tested the TListView with 10k Items (D7/WinXP):

• ListView.Items.Clear: ~5500 ms
• ListView.Clear: ~330 ms

Conclusion: ListView.Clear is about 16 times faster than ListView.Items.Clear when BeginUpdate/EndUpdate is not used!