Moving controls in a gridpanel with Delphi

Question

In a previous question here I asked about drag n drop within the gridpanel.

Drag N Drop controls in a GridPanel

The question I have next is that I am having weird behavior whenever I try to move controls diagonally when they are near other controls. Controls that not suppose to move are shifting cells. Up and down, sideways it is fine. But diagonal moves, when the moved cell contents are on the same row/column with other cells which hold controls will cause unexpected shifts. I have tried beginupdate/endupdate the shifts still happen. There is a LOCK function for the gridpanel but lock anything. It happens when the drop is on an empty cell, and even cells that already have contents.

here is the test project (Delphi 2010 w/o exe) http://www.mediafire.com/?xmrgm7ydhygfw2r

type
  TForm1 = class(TForm)
    GridPanel1: TGridPanel;
    btn1: TButton;
    btn3: TButton;
    btn2: TButton;
    lbl1: TLabel;
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
    procedure GridPanelDragDrop(Sender, Source: TObject; X, Y: Integer);
    procedure btnDragOver(Sender, Source: TObject; X, Y: Integer;
      State: TDragState; var Accept: Boolean);
    procedure btnDragDrop(Sender, Source: TObject; X, Y: Integer);
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure SetColumnWidths(aGridPanel: TGridPanel);
var
  i,pct: Integer;
begin
  aGridPanel.ColumnCollection.BeginUpdate;
  pct:=Round(aGridPanel.ColumnCollection.Count/100);
  for i := 0 to aGridPanel.ColumnCollection.Count - 1 do begin
    aGridPanel.ColumnCollection[i].SizeStyle := ssPercent;
    aGridPanel.ColumnCollection[i].Value     := pct;
  end;
  aGridPanel.ColumnCollection.EndUpdate;
end;

procedure SetRowWidths(aGridPanel: TGridPanel);
var
  i,pct: Integer;
begin
  aGridPanel.RowCollection.BeginUpdate;
  pct:=Round(aGridPanel.RowCollection.Count/100);
  for i := 0 to aGridPanel.RowCollection.Count - 1 do begin
    aGridPanel.RowCollection[i].SizeStyle := ssPercent;
    aGridPanel.RowCollection[i].Value     := pct;
  end;
  aGridPanel.RowCollection.EndUpdate;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  btn1.OnDragOver := btnDragOver;
  btn2.OnDragOver := btnDragOver;
  btn3.OnDragOver := btnDragOver;
  GridPanel1.OnDragOver := btnDragOver;
  GridPanel1.OnDragDrop := GridPanelDragDrop;

  btn1.OnDragDrop := btnDragDrop;
  btn2.OnDragDrop := btnDragDrop;
  btn3.OnDragDrop := btnDragDrop;

  SetColumnWidths(GridPanel1);
  SetRowWidths(GridPanel1);
end;

procedure TForm1.btnDragOver(Sender, Source: TObject; X, Y: Integer;
  State: TDragState; var Accept: Boolean);
begin
  Accept := (Source is TButton);
end;

procedure TForm1.btnDragDrop(Sender, Source: TObject; X, Y: Integer);
var
  src_x,src_y, dest_x, dest_y: Integer;
  btnNameSrc,btnNameDest: string;
  src_ctrlindex,dest_ctrlindex:integer;
begin
  if Source IS tBUTTON then
  begin
    //GridPanel1.ColumnCollection.BeginUpdate;
    btnNameSrc := (Source as TButton).Name;
    btnNameDest := (Sender as TButton).Name;
    src_ctrlindex := GridPanel1.ControlCollection.IndexOf(Source as tbutton);
    src_x := GridPanel1.ControlCollection.Items[src_ctrlindex].Column;
    src_y := GridPanel1.ControlCollection.Items[src_ctrlindex].Row;

    dest_ctrlindex := GridPanel1.ControlCollection.IndexOf(Sender as tbutton);
    dest_x := GridPanel1.ControlCollection.Items[dest_ctrlindex].Column;
    dest_y := GridPanel1.ControlCollection.Items[dest_ctrlindex].Row;

    GridPanel1.ControlCollection[src_ctrlindex].Column := dest_x;
    GridPanel1.ControlCollection[src_ctrlindex].Row := dest_y;
    //GridPanel1.ColumnCollection.EndUpdate;

    lbl1.Caption := Format('"%s" from cell %d:%d to Cell %s=%d:%d', [btnNameSrc,src_x,src_y,btnNameDest,dest_x,dest_y]);

  end;
end;

procedure TForm1.GridPanelDragDrop(Sender, Source: TObject; X, Y: Integer);
var
  DropPoint: TPoint;
  CellRect: TRect;
  i_col, i_row, src_x,src_y, dest_x, dest_y: Integer;
  btnNameSrc,btnNameDest: string;
  src_ctrlindex:integer;
begin
  if Source is tbutton then
  begin
    btnNameSrc := (Source as TButton).Name;
    btnNameDest := '';
    src_ctrlindex := GridPanel1.ControlCollection.IndexOf(Source as tbutton);
    src_x := GridPanel1.ControlCollection.Items[src_ctrlindex].Column;
    src_y := GridPanel1.ControlCollection.Items[src_ctrlindex].Row;

    DropPoint := Point(X, Y);
    for i_col := 0 to GridPanel1.ColumnCollection.Count-1 do
      for i_row := 0 to GridPanel1.RowCollection.Count-1 do
      begin
        CellRect := GridPanel1.CellRect[i_col, i_row];
        if PtInRect(CellRect, DropPoint) then
        begin
          // Button was dropped over Cell[i_col, i_row]
          dest_x := i_col;
          dest_y := i_row;
          Break;
        end;
      end;
    lbl1.Caption := Format('"%s" from cell %d:%d to Cell %s=%d:%d', [btnNameSrc,src_x,src_y,btnNameDest,dest_x,dest_y]);

    GridPanel1.ControlCollection[src_ctrlindex].Column := dest_x;
    GridPanel1.ControlCollection[src_ctrlindex].Row := dest_y;
  end;
end;

Answer 1

This is not about dragging, when an item's both column and row are changing the change occurs in two steps. With your code, first the column, then the row. If in the column change, f.i., there happens to be already an other control, this other control is pushed aside, even if its cell is not the ultimate location of the target cell of the moving control.

Begin/EndUpdate will not work, the control collection never checks the update count. What can you do is to use a protected hack to access the control item's InternalSetLocation method. This method has a 'MoveExisting' parameter which you can pass 'False'.

type
  THackControlItem = class(TControlItem);

procedure TForm1.GridPanelDragDrop(Sender, Source: TObject; X, Y: Integer);
var
  [...]
begin
  if Source is tbutton then
  begin

    [...]

    lbl1.Caption := Format('"%s" from cell %d:%d to Cell %s=%d:%d', [btnNameSrc,src_x,src_y,btnNameDest,dest_x,dest_y]);

    THackControlItem(GridPanel1.ControlCollection[src_ctrlindex]).
        InternalSetLocation(dest_x, dest_y, False, False);
//    GridPanel1.ControlCollection[src_ctrlindex].Column := dest_x;
//    GridPanel1.ControlCollection[src_ctrlindex].Row := dest_y;
  end;
end;

You might need to test if the target cell is empty or not before calling 'InternalSetLocation' depending on what you expect to be the correct control movement.