Speed up the display of Delphi list components

If you use a Delphi component like a ListBox, Memo, TreeList, ListView,... and you add or modify a lot of items (lines, nodes,...), the component's performance becomes very slow. This is due to the fact that after each change, it is redrawn on the screen.

Thus, modifying 10000 items of a ListBox causes 10000 redraws, and that takes from several seconds to several minutes, depending on the computer's speed and the complexity of the process. Imagine how slow it can become if the process involves inserting, deleting, modifying and swapping many thousands of items.

But you can speed up things enormously with the following tip:

Call BeginUpdate before making the changes to the items. When all changes are complete, call EndUpdate to show the changes on screen. BeginUpdate and EndUpdate prevent excessive redraws and speed up processing time when items are added, deleted, or inserted.

Here's a simple source code example for using this technique with a ListBox:

  ListBox1.Items.BeginUpdate;
for i := 1 to 10000 do
ListBox1.Items.Add('abcd');
ListBox1.Items.EndUpdate;

To give you an idea of the improvement, we timed the code above (your PC may be faster or slower):
»  without the BeginUpdate/EndUpdate lines: 4.3 seconds...
»  with BeginUpdate/EndUpdate: 0.1 seconds, that's 43 times faster!

Here's a source code example for using this technique with a Memo:

  Memo1.Lines.BeginUpdate;
for i := 1 to 5000 do
Memo1.Lines.Add('abcd');
Memo1.Lines.EndUpdate;

»  without BeginUpdate/EndUpdate: 25 seconds...
»  with BeginUpdate/EndUpdate: 0.6 seconds!