Author: Jonas Bilinkevicius

How can I resize a form in steps, not smooth, like the Object Inspector in Delphi?

Answer:

First, declare a message handler for the WM_SIZING message in your form:

1   TForm1 = class(TForm)
2     { ... }
3   protected
4     procedure WmSizing(var Msg: TMessage); message WM_SIZING;
5     { ... }
6   end;
7   
8   //Then define the WmSizing message handler like so:
9   
10  procedure TForm1.WmSizing(var Msg: TMessage);
11  const
12    STEP = 20;
13  var
14    Side: LongInt;
15    Rect: PRect;
16  begin
17    inherited;
18    Side := Msg.WParam;
19    Rect := PRect(Msg.LParam);
20    if (WMSZ_BOTTOM and Side = WMSZ_BOTTOM) then
21      Rect^.Bottom := Rect^.Bottom - (Rect^.Bottom mod STEP);
22    if (WMSZ_BOTTOMLEFT and Side = WMSZ_BOTTOMLEFT) then
23    begin
24      Rect^.Bottom := Rect^.Bottom - (Rect^.Bottom mod STEP);
25      Rect^.Left := Rect^.Left - (Rect^.Left mod STEP);
26    end;
27    if (WMSZ_BOTTOMRIGHT and Side = WMSZ_BOTTOMRIGHT) then
28    begin
29      Rect^.Bottom := Rect^.Bottom - (Rect^.Bottom mod STEP);
30      Rect^.Right := Rect^.Right - (Rect^.Right mod STEP);
31    end;
32    if (WMSZ_LEFT and Side = WMSZ_LEFT) then
33      Rect^.Left := Rect^.Left - (Rect^.Left mod STEP);
34    if (WMSZ_RIGHT and Side = WMSZ_RIGHT) then
35      Rect^.Right := Rect^.Right - (Rect^.Right mod STEP);
36    if (WMSZ_TOP and Side = WMSZ_TOP) then
37      Rect^.Top := Rect^.Top - (Rect^.Top mod STEP);
38    if (WMSZ_TOPLEFT and Side = WMSZ_TOPLEFT) then
39    begin
40      Rect^.Top := Rect^.Top - (Rect^.Top mod STEP);
41      Rect^.Left := Rect^.Left - (Rect^.Left mod STEP);
42    end;
43    if (WMSZ_TOPRIGHT and Side = WMSZ_TOPRIGHT) then
44    begin
45      Rect^.Top := Rect^.Top - (Rect^.Top mod STEP);
46      Rect^.Right := Rect^.Right - (Rect^.Right mod STEP);
47    end;
48    Msg.Result := 1;
49  end;


You can adjust the STEP constant to meet your needs. Likewise, you can make any edge a constant size simply by assigning a valueto it.