Title: paint a moving progress bar using a background thread?
{
Question:
I am trying query to display records in a dbgrid.however, due to size
of tables and joins takes a while for the query to Execute.is
there any way to Show a prorgess bar with a timer that increments
position but continues to work while the query is being executed.BTW,
using access so BDE is not used.
Answer:
A progress bar would not be an ideal choice since you cannot determine up
front how long the query will take, so you do not know the range the progress
bar has to cover.A simple kind of animation that tells the user basically
only that the application is not hung would be more appropriate.One could do
such a thing in a secondary thread but it would have to be done using the
plain Windows API and * no * Synchronize calls (since the main thread is
blocked in the BDE call).Here is an example: unit anithread;
}
interface
uses
Classes, Windows, Controls, Graphics;
type
TAnimationThread = class(TThread)
private
{ Private declarations }
FWnd: HWND;
FPaintRect: TRect;
FbkColor, FfgColor: TColor;
FInterval: integer;
protected
procedure Execute; override;
public
constructor Create(paintsurface : TWinControl; {Control to paint on }
paintrect : TRect; {area for animation bar }
bkColor, barcolor : TColor; {colors to use }
interval : integer); {wait in msecs between
paints}
end;
implementation
constructor TAnimationThread.Create(paintsurface : TWinControl;
paintrect : TRect; bkColor, barcolor : TColor; interval : integer);
begin
inherited Create(True);
FWnd := paintsurface.Handle;
FPaintRect := paintrect;
FbkColor := bkColor;
FfgColor := barColor;
FInterval := interval;
FreeOnterminate := True;
Resume;
end; { TAnimationThread.Create }
procedure TAnimationThread.Execute;
var
image : TBitmap;
DC : HDC;
left, right : integer;
increment : integer;
imagerect : TRect;
state : (incRight, incLeft, decLeft, decRight);
begin
Image := TBitmap.Create;
try
with Image do
begin
Width := FPaintRect.Right - FPaintRect.Left;
Height := FPaintRect.Bottom - FPaintRect.Top;
imagerect := Rect(0, 0, Width, Height);
end; { with }
left := 0;
right := 0;
increment := imagerect.right div 50;
state := Low(State);
while not Terminated do
begin
with Image.Canvas do
begin
Brush.Color := FbkColor;
FillRect(imagerect);
case state of
incRight:
begin
Inc(right, increment);
if right imagerect.right then
begin
right := imagerect.right;
Inc(state);
end; { if }
end; { Case incRight }
incLeft:
begin
Inc(left, increment);
if left = right then
begin
left := right;
Inc(state);
end; { if }
end; { Case incLeft }
decLeft:
begin
Dec(left, increment);
if left then
begin
left := 0;
Inc(state);
end; { if }
end; { Case decLeft }
decRight:
begin
Dec(right, increment);
if right then
begin
right := 0;
state := incRight;
end; { if }
end; { Case decLeft }
end; { Case }
Brush.Color := FfgColor;
FillRect(Rect(left, imagerect.top, right, imagerect.bottom));
end; { with }
DC := GetDC(FWnd);
if DC 0 then
try
BitBlt(DC,
FPaintRect.Left,
FPaintRect.Top,
imagerect.right,
imagerect.bottom,
Image.Canvas.handle,
0, 0,
SRCCOPY);
finally
ReleaseDC(FWnd, DC);
end;
Sleep(FInterval);
end; { While }
finally
Image.Free;
end;
InvalidateRect(FWnd, nil, True);
end; { TAnimationThread.Execute }
end.
{Usage:
Place a TPanel on a form, size it as appropriate.Create an instance of the
TanimationThread call like this: procedure TForm1.Button1Click(Sender : TObject);
}
var
ani : TAnimationThread;
r : TRect;
begin
r := panel1.clientrect;
InflateRect(r, - panel1.bevelwidth, - panel1.bevelwidth);
ani := TanimationThread.Create(panel1, r, panel1.color, clBlue, 25);
Button1.Enabled := False;
Application.ProcessMessages;
Sleep(30000); // replace with query.Open or such
Button1.Enabled := True;
ani.Terminate;
ShowMessage('Done');
end;