How to clear a TGridPanelLayout at runtime - delphi

Question

My application uses a TGridPanelLayout to present a number of images to the user. When the user clicks at the correct image, all images are cleared and a new serie of images is presented. The code for adding the images to the grid is shown below:
// Clear the grid of all controls, rows and columns and reinitialize
Grid.ControlCollection.Clear;
Grid.ColumnCollection.Clear;
Grid.RowCollection.Clear;
// Next follows some code to add columns and rows
// Display the names, n_images = rows * columns
for i := 0 to n_images - 1 do
begin
Image := TImage.Create (nil);
Image.HitTest := True;
if Sel_Array [i]
then Image.OnClick := test_positive
else Image.OnClick := test_negative;
c := i mod Options_Project.Cols;
r := i div Options_Project.Cols;
Image.Name := Format ('Image_%2.2d_%2.2d', [r, c]);
Image.Bitmap.LoadFromFile (Sel_Names [i]);
Image.Align := TAlignLayout.alClient;
Grid.AddObject (Image); // Grid: TGridPanelLayout
end; // for
This all works fine but the problem is in recreating the TGridPanelLayout. When for the second time Grid.ControlCollection.Clear is executed an access violation occurs when one of the images is freed.
How can I clear a TGridPanellayout at runtime without crash? And an additional question: is AddObject the correct way to add controls to the TGridPanelLayout? I tried AddControl but then no image was shown.
This application is tested in Windows 7.
Edit
Tom noticed that I should've assigned the .Parent and that did the trick, together with the remark of Dalija that I should've used AddControl. The code below works:
for i := 0 to n_images - 1 do
begin
Image := TImage.Create (Grid);
Image.HitTest := True;
if Sel_Array [i]
then Image.OnClick := test_positive
else Image.OnClick := test_negative;
c := i mod Options_Project.Cols;
r := i div Options_Project.Cols;
Image.Name := Format ('Image_%2.2d_%2.2d', [r, c]);
Image.Bitmap.LoadFromFile (Sel_Names [i]);
Image.Align := TAlignLayout.alClient;
Image.Parent := Grid;
Grid.ControlCollection.AddControl (Image);
end; // for
Thank you all for your help!

Answer 1

It is correct to call Grid.ControlCollection.Clear to delete the items in the collection. From Help:
Clear empties the Items array and destroys each TCollectionItem.
Note the "destroys", which means it takes ownership and resposnibility of managing the lifetime of the image.
You say:
an access violation occurs when one of the images is freed.
Do you mean actively freeing by your code? Then that is wrong and the reason for the AV.
Is the image the same that the user clicked on and that triggered the display of a new series of images? Then you need to review the code how you manipulate the image in test_positive and maybe test_negative also.
To add controls to the TGridPanelLayout you can use either
Grid.AddObject(Image);
or
Image.Parent := Grid;
Grid.Controls.Add(Image);
Note, in this case you need to set the parent in order for the image to show (and to be managed by the grid).
The above is tested with XE7.