Unexpected behavior of command history unit - delphi

Question

I wrote such module to store there last changes of picture in my paint application " in Delphi
unit HistoryQueue;
interface
uses
Graphics;
type
myHistory = class
constructor Create(Size:Integer);
public
procedure Push(Bmp:TBitmap);
function Pop():TBitmap;
procedure Clean();
procedure Offset();
function isEmpty():boolean;
function isFull():boolean;
function getLast():TBitmap;
protected
historyQueueArray: array of TBitmap;
historyIndex, hSize:Integer;
end;
implementation
procedure myHistory.Push(Bmp:TBitmap);
var tbmp:TBitmap;
begin
if(not isFull) then begin
Inc(historyIndex);
historyQueueArray[historyIndex]:=TBitmap.Create;
historyQueueArray[historyIndex].Assign(bmp);
end else begin
Offset();
historyQueueArray[historyIndex]:=TBitmap.Create;
historyQueueArray[historyIndex].Assign(bmp);
end;
end;
procedure myHistory.Clean;
var i:Integer;
begin
{ for i:=0 to hSize do begin
historyQueueArray[i].Free;
historyQueueArray[i].Destroy;
end; }
end;
constructor myHistory.Create(Size:Integer);
begin
hSize:=Size;
SetLength(historyQueueArray, hSize);
historyIndex:=-1;
end;
function myHistory.isEmpty: boolean;
begin
Result:=(historyIndex = -1);
end;
function myHistory.isFull: boolean;
begin
Result:=(historyIndex = hSize);
end;
procedure myHistory.Offset; {to handle overflow}
var i:integer;
begin
//historyQueueArray[0]:=nil;
for i:=0 to hSize-1 do begin
historyQueueArray[i]:=TBitmap.Create;
historyQueueArray[i].Assign(historyQueueArray[i+1]);
end;
end;
function myHistory.Pop: TBitmap;
var
popBmp:TBitmap;
begin
popBmp:= TBitmap.Create;
popBmp.Assign(historyQueueArray[historyIndex]);
Dec(historyIndex);
Result:=popBmp;
end;
function myHistory.getLast: TBitmap; {this function I use when I need refresh the cnvas when I draw ellipse or rect, to get rid of traces and safe previous changes of the picture}
var
tBmp:TBitmap;
begin
tBmp:= TBitmap.Create;
tBmp.Assign(historyQueueArray[historyIndex]);
Result:=tBmp;
end;
end.
And thats how I use it
procedure TMainForm.FormCreate(Sender: TObject);
var
cleanBmp:TBitmap;
begin
{...}
doneRedo:=false;
redomode:=false; undomode:=false;
//init arrays
picHistory:=myHistory.Create(10); //FOR UNDO
tempHistory:=myHistory.Create(10); //FOR REDO
cleanbmp:=TBitmap.Create;
cleanbmp.Assign(imgMain.Picture.Bitmap);
picHistory.Push(cleanbmp);
cleanbmp.Free;
{...}
end;
procedure TMainForm.btnUndoClick(Sender: TObject);
var redBmp:TBitmap;
begin
undoMode:=true;
//if there were some changes
if(not picHistory.isEmpty) then begin
redBmp:=TBitmap.Create;
redBmp.Assign(picHistory.getLast);
//clean canvas
imgMain.Picture.Bitmap:=nil;
//get what was there before
imgMain.Canvas.Draw(0,0, picHistory.Pop);
//and in case if we will not make any changes after UNDO(clicked one or more times)
//and call REDO then
tempHistory.Push(redBmp);//we save what were on canvas before UNDOand push it to redo history
redBmp.Free;
end;
end;
procedure TMainForm.btnRedoClick(Sender: TObject);
var undBmp:TBitmap;
begin
redoMode:=true;
if(not tempHistory.isEmpty) then begin
doneRedo:=True;
undBmp:=TBitmap.Create;
undBmp.Assign(tempHistory.getLast);
imgMain.Picture.Bitmap:=nil;
MainForm.imgMain.Canvas.Draw(0,0, tempHistory.Pop);
//same history (like with UNDO implementation) here but reverse
picHistory.Push(undBmp);
undBmp.Free;
end;
end;
{...}
procedure TMainForm.imgMainMouseUp(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
var bmp:TBitmap;
begin
//if mouse were down and then it's up this means we drew something
//and must save changes into history to be able to make UNDO
{...}
bmp:=TBitmap.Create;
try
bmp.Assign(imgMain.Picture.Bitmap);
picHistory.Push(bmp);
//if there are some changes added after redo then we clean redo history
if (doneRedo) then begin
tempHistory.Clean;
doneRedo:=false;
end;
finally
bmp.Free;
//sor of refresh
imgMain.Canvas.Draw(0,0, picHistory.getLast);
end;
{...}
But the problem is it works not way I expected. an example:
If I push undo button once - nothing happens. On twice - it does what it should at once.
And if I drew an ellipse, then click undo once and start draw new one - last drawn ellipse just dissaperas!
Here's the elipse draw method in case if it could be helpful to find out the problem
procedure TMainForm.ellipseDraw(X, Y: Integer);
begin
imgMain.Canvas.Pen.Color:=useColor;
imgMain.Canvas.Brush.Color:=scndColor;
imgMain.Canvas.Pen.Width:=size;
if(mouseIsDown) then begin
imgMain.Canvas.Draw(0,0, picHistory.getLast); //there gonna be no bizzare traces from figures
imgMain.Canvas.Ellipse(dX, dY, X,Y);
end;
end;

Answer 1

Answer
If I push undo button once - nothing happens. On twice - it does what it should at once.
That is indeed exactly what your code does:
In imgMainMouseUp you add the current picture to the undo list, and
In btnUndoClick you retrieve the last bitmap from the undo list, which is the same as currently seen on the Image.
The solution - to this specific question - is to add the previous bitmap to the undo list instead of the current one.
Bonus
And to address David's comment concerning the leaking, your implementation leaks Bitmaps because:
The routines Pop and getLast return a newly local created Bitmap. This places the responsibility for its destruction on the caller ot the routines. Your MainForm code does not destroy those Bitmaps, thus they are memory leaks. The solution is to simply return the item in the array, instead of creating a new Bitmap.
In the Offset routine, you again create new Bitmaps and leak all previous ones. Just assign Queue[I] to Queue[I + 1].
In the Push method, you forget to free the last item.
The class does not have a destructor, which again places the responsibility for the destruction of all Bitmaps on the user of the object with the need to call Clean, which it does not. The solution is to add a destructor to your object which calls Clean.
Besides these leaks, there are more problems with your code. Here some fixes and tips:
Since dynamic arrays are zero-based, your isFull routine does not return True when it should. It should be implemented as Result := historyIndex = hSize - 1;
Your array is not a queue (FIFO), but a stack (LIFO).
Your Pop routine does not check for an empty list.
Altogether, your history class could better look like:
uses
SysUtils, Graphics;
type
TBitmapHistory = class(TObject)
private
FIndex: Integer;
FStack: array of TBitmap;
procedure Offset;
public
procedure Clear;
function Count: Integer;
constructor Create(ACount: Integer);
destructor Destroy; override;
function Empty: Boolean;
function Full: Boolean;
function Last: TBitmap;
function Pop: TBitmap;
procedure Push(ABitmap: TBitmap);
end;
implementation
{ TBitmapHistory }
procedure TBitmapHistory.Clear;
var
I: Integer;
begin
for I := 0 to Count - 1 do
FreeAndNil(FStack[I]);
FIndex := -1;
end;
function TBitmapHistory.Count: Integer;
begin
Result := Length(FStack);
end;
constructor TBitmapHistory.Create(ACount: Integer);
begin
inherited Create;
SetLength(FStack, ACount);
FIndex := -1;
end;
destructor TBitmapHistory.Destroy;
begin
Clear;
inherited Destroy;
end;
function TBitmapHistory.Empty: Boolean;
begin
Result := FIndex = -1;
end;
function TBitmapHistory.Full: Boolean;
begin
Result := FIndex = Count - 1;
end;
function TBitmapHistory.Last: TBitmap;
begin
if Empty then
Result := nil
else
Result := FStack[FIndex];
end;
procedure TBitmapHistory.Offset;
begin
FStack[0].Free;
Move(FStack[1], FStack[0], (Count - 1) * SizeOf(TBitmap));
end;
function TBitmapHistory.Pop: TBitmap;
begin
if not Empty then
begin
Result := Last;
Dec(FIndex);
end;
end;
procedure TBitmapHistory.Push(ABitmap: TBitmap);
begin
if Full then
Offset
else
Inc(FIndex);
FStack[Findex].Free;
FStack[FIndex] := TBitmap.Create;
FStack[Findex].Assign(ABitmap);
end;
Remarks:
There also exists a specialized class TObjectStack for this in the Contnrs unit which you could override/exploit.
There are also concerns with your MainForm code, but I politely leave that up to you to fix.