There is a small new features in the VCL library in the recently released Delphi 11.3, the option to enable multiple selection in the ControlList component of the VCL library.
What is a ControlList?
The ControlList component was introduced to the VCL a few years back to allow managing extremely large lists in a totally virtual way. A control list, in fact, has a panel with a structure that is created in a single instance in memory, but is painted for each of the list items, so that is looks like you have thousands, or even millions of such elements.
This is done by asking the program to update the controls of this panel each time of the items needs to be painted. Given that the paint requests are received only for the (few) visible items, the list does only the minimum processing required to give the perception of having a large amount of elements a user can scroll and navigate.
In practical terms, in a very minimal scenario (a list only one label) the component at design time will look like this:
The code responsible to making each item unique and distinct can be like the following:
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procedure TForm19.ControlList1BeforeDrawItem(AIndex: Integer; ACanvas: TCanvas; ARect: TRect; AState: TOwnerDrawState); begin Label1.Caption := 'Item ' + (AIndex + 1).ToString; end; |
Notice that the control list items don’t exist and don’t have the concept of status. Accessing the Label1 Caption, outside of the specific ControlList event handlers use to process an element, makes no sense. After setting the list ItemCount property to 100 items, this results will be the following UI:
Now you can check the selected item by using:
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Memo1.Lines.Text := 'Single selection: ' + (ControlList1.ItemIndex + 1).ToString; |
The ControlList Multiple Selection
What is new in Delphi 11.3 is the ability to enable multiple selection. This is done with the new MultiSelect property.
With this configuration, the user of your application can click the mouse button along with the Ctrl and Shift keys to select multiple individual items (Ctrl) or all the items from the last click position (Shift), on the line of the classic behavior of a Windows ListBox with multiple selection. The same combinations can be use to unselect items, so that the Ctrl+click oepration is actually a “toggle selection” operations. Here is an example of this bare bone UI:
To check programmatically which items are selected, you can iterate over the Selected array property, with code similar to:
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for var I := 0 to ControlList1.ItemCount - 1 do if ControlList1.Selected[I] then Memo1.Lines.Add((I + 1).ToString); |
That’s all you need to do to use this new feature of the powerful ControlList VCL component.
Design. Code. Compile. Deploy.
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Nice idea but it looks like multi-selection support was added using a helper class (TCustomControlListHelper). This means it doesn’t work with C++Builder applications. Also, even though MultiSelect was added as a published property in the helper class, it doesn’t appear in the designer for Delphi applications.
We keep all point releases (i.e. 11.0, 11.1, 11.2, 11.3 in the 11.x series) ABI compatible. That means we are sometimes limited in how to make changes. Class helpers don’t break ABI compatibility, so are used to introduce new functionality and in each next major release, in this case version 12.0, are rolled back into the main class. When that’s done it will be available for C++ and in the form designer.
That said, thanks for your comment. It did provoke some discussion among the product development team about the effect of this particular change on C++ and how this can be better taken into account for future releases. We do always consider the effects of all changes on both Delphi and C++, of course, but if a change has to be made which would have an impact like this albeit with good reasons, we think we can do better at highlighting that in the release documentation and making it clear how and when that impact will be mitigated afterwards.