Question

I have developed a component to implement pan and zoom functionality for Graphics32 based ImgView32s. One can drop the component next to an TImgView32, set the Image view property of my component and all is good, and working as expected.
However, once I try to close the Form hosting my component and the ImgView32 the Delphi IDE freezes. My first thought was that the ImgView32 while still linked to my component gets destroyed before my component, so I implemented the Delphi standard notification mechanisms. Still the problem remains. Here is the source code of my component. The component is included in a runtime package and another design time package is using the runtime package and registers the component.

Update, as a result of Rob’s useful debugging tips: As it turns out, the component hangs in an endless call to the Notification method. Maybe thats a hint to someone.

unit MJImgView32PanZoom;

interface

uses Classes, Controls, Gr32, GR32_Image, GR32_Layers;

type
  TImgView32ScaleChangeEvent = procedure( OldScale, NewScale: Double ) of object;

  TimgView32PanZoom = class(TComponent)
  private
    FEnabled: Boolean;
    FMaxZoom: Double;
    FMinZoom: Double;
    FImgView32: TImgView32;
    FZoomStep: Double;
    FOrigImgMouseMove: TImgMouseMoveEvent;
    FOrigImgMouseDown: TImgMouseEvent;
    FOrigImgMouseUp: TImgMouseEvent;
    FOrigImgMouseWheel: TMouseWheelEvent;
    FOrigImgCursor: TCursor;
    FPanMouseButton: TMouseButton;
    FLastMouseDownPos : TFloatPoint;
    FPanCursor: TCursor;
    FOnScaleChanged: TImgView32ScaleChangeEvent;
    procedure imgMouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer; Layer: TCustomLayer);
    procedure imgMouseDown(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer; Layer: TCustomLayer);
    procedure SetImgView32(const Value: TImgView32);
    procedure imgMouseWheel(Sender: TObject; Shift: TShiftState; WheelDelta: Integer; MousePos: TPoint; var Handled: Boolean);
    procedure imgMouseUp(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer; Layer: TCustomLayer);
  protected
    procedure Notification(AComponent: TComponent; Operation: TOperation); override;
  public
    destructor Destroy; override;
    constructor Create(AOwner: TComponent); override;
  published
    property Enabled: Boolean read FEnabled write FEnabled;
    property MaxZoom: Double read FMaxZoom write FMaxZoom;
    property MinZoom: Double read FMinZoom write FMinZoom;
    property PanMouseButton: TMouseButton read FPanMouseButton write FPanMouseButton;
    property PanCursor: TCursor read FPanCursor write FPanCursor;
    property ZoomStep: Double read FZoomStep write FZoomStep;
    property ImgView32: TImgView32 read FImgView32 write SetImgView32;
    property OnScaleChanged: TImgView32ScaleChangeEvent read FOnScaleChanged write FOnScaleChanged;
  end;



implementation

{ TimgView32PanZoom }

constructor TimgView32PanZoom.Create(AOwner: TComponent);
begin
  inherited;
  FimgView32 := nil;
  FEnabled := True;
  FZoomStep := 0.1;
  FMaxZoom := 5;
  FMinZoom := 0.1;
  FPanMouseButton := mbLeft;
  FEnabled := True;
  FPanCursor := crDefault;
end;

destructor TimgView32PanZoom.Destroy;
begin
  ImgView32 := nil;
  inherited;
end;

procedure TimgView32PanZoom.imgMouseDown( Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer; Layer: TCustomLayer );
begin
  if not Enabled then
    Exit;
  if ( FPanMouseButton = mbLeft ) and not( ssLeft in Shift ) then
    Exit;
  if ( FPanMouseButton = mbRight ) and not( ssRight in Shift ) then
    Exit;
  FImgView32.Cursor := FPanCursor;
  Mouse.CursorPos := Point(Mouse.CursorPos.X+1, Mouse.CursorPos.Y);   // need to move mouse in order to make
  Mouse.CursorPos := Point(Mouse.CursorPos.X-1, Mouse.CursorPos.Y);   // cursor change visible
  with FImgView32, GetBitmapRect do
        FLastMouseDownPos := FloatPoint((X - Left) / Scale,(Y - Top) / Scale);
  if Assigned(FOrigImgMouseDown) then
    FOrigImgMouseDown(Sender, Button, Shift, X, Y, Layer);
end;

procedure TimgView32PanZoom.imgMouseUp(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer; Layer: TCustomLayer);
begin
  FImgView32.Cursor := FOrigImgCursor;
  if Assigned(FOrigImgMouseUp) then
    FOrigImgMouseUp(Sender, Button, Shift, X, Y, Layer);
end;

procedure TimgView32PanZoom.imgMouseMove( Sender: TObject; Shift: TShiftState; X, Y: Integer; Layer: TCustomLayer );
begin
  if not Enabled then
    Exit;
  if ( FPanMouseButton = mbLeft ) and not( ssLeft in Shift ) then
    Exit;
  if ( FPanMouseButton = mbRight ) and not( ssRight in Shift ) then
    Exit;
  with FImgView32 do
    with ControlToBitmap( Point( X, Y ) ) do
    begin
      OffsetHorz := OffsetHorz + Scale * ( X - FLastMouseDownPos.X );
      OffsetVert := OffsetVert + Scale * ( Y - FLastMouseDownPos.Y );
    end;
  if Assigned( FOrigImgMouseMove ) then
    FOrigImgMouseMove( Sender, Shift, X, Y, Layer );
end;

procedure TimgView32PanZoom.imgMouseWheel( Sender: TObject; Shift: TShiftState; WheelDelta: Integer; MousePos: TPoint; var Handled: Boolean );
var
  tmpScale: Single;
  NewHoriz, NewVert: Single;
  NewScale: Single;
begin
  if not Enabled then
    Exit;
  with FImgView32 do
  begin
    BeginUpdate;
    tmpScale := Scale;
    if WheelDelta > 0 then
      NewScale := Scale * 1.1
    else
      NewScale := Scale / 1.1;
    if NewScale > FMaxZoom then
      NewScale := FMaxZoom;
    if NewScale < FMinZoom then
      NewScale := FMinZoom;
    NewHoriz := OffsetHorz + ( tmpScale - NewScale ) * FImgView32.ControlToBitmap( FImgView32.ScreenToClient( Mouse.CursorPos ) ).X;
    NewVert := OffsetVert + ( tmpScale - NewScale ) * FImgView32.ControlToBitmap( FImgView32.ScreenToClient( Mouse.CursorPos ) ).Y;
    Scale := NewScale;
    OffsetHorz := NewHoriz;
    OffsetVert := NewVert;
    EndUpdate;
    Invalidate;
  end;
  if Assigned( FOnScaleChanged ) then
    FOnScaleChanged( tmpScale, NewScale );
  if Assigned( FOrigImgMouseWheel ) then
    FOrigImgMouseWheel( Sender, Shift, WheelDelta, MousePos, Handled );
end;

procedure TimgView32PanZoom.Notification(AComponent: TComponent; Operation: TOperation);
begin
  if (Operation = opRemove) and (AComponent = FImgView32) then
  begin
    FImgView32 := nil;
  end;
end;

procedure TimgView32PanZoom.SetImgView32(const Value: TImgView32);
begin
   if Assigned(FImgView32) then
   begin
     FImgView32.RemoveFreeNotification(Self);
     FImgView32.OnMouseMove := FOrigImgMouseMove;
     FImgView32.OnMouseDown := FOrigImgMouseDown;
     FImgView32.OnMouseWheel := FOrigImgMouseWheel;
     FImgView32.OnMouseUp := FOrigImgMouseUp;
     FImgView32.Cursor := FOrigImgCursor;
   end;

   FImgView32 := Value;
   if Assigned(FImgView32) then
   begin
     FOrigImgMouseMove := FImgView32.OnMouseMove;
     FOrigImgMouseDown := FImgView32.OnMouseDown;
     FOrigImgMouseWheel := FImgView32.OnMouseWheel;
     FOrigImgMouseUp := FImgView32.OnMouseUp;
     FOrigImgCursor := FImgView32.Cursor;
     FImgView32.OnMouseDown := imgMouseDown;
     FImgView32.OnMouseMove := imgMouseMove;
     FImgView32.OnMouseWheel := imgMouseWheel;
     FImgView32.OnMouseUp := imgMouseUp;
     FImgView32.FreeNotification(Self);
   end;
end;


end.

Answer 1

You need to call inherited in your Notification method to let the control process all notifications that occurs in the control ascendants chain. So, to fix your infinite loop (which is as you’ve described the source of the freeze) modify your Notification method this way:

procedure TimgView32PanZoom.Notification(AComponent: TComponent; 
  Operation: TOperation);
begin
  inherited;
  if (Operation = opRemove) and (AComponent = FImgView32) then
    FImgView32 := nil;
end;