(Ebook - Pdf) Kick Ass Delphi Programming

TMyGrid = class(TDrawGrid) protected

procedure SizeChanged(OldColCount, OldRowCount: Longint); override;

end;

{ In unit's implementation... }

procedure TMyGrid.SizeChanged(OldColCount, OldRowCount: Longint); begin

{ Take whatever actions necessary } end;

Overriding SizeChanged provides all the notification we need, but if we want control over the number of columns—for example, if we’re developing a grid that should have no more than three columns—SizeChanged is not a satisfactory place to shoehorn in our veto. By the time SizeChanged is called (note the past tense in the name), the change has already been made. The best we can do then, if ColCount was set to 4, is change it to 3, requiring the whole private process of change to be repeated.

What we want is front end control, and we can get this by redeclaring the ColCount property with our own access methods (see the TMyGrid declaration in Listing 7.10). Our redeclaration of the property will hide our ancestor’s ColCount, so that when a programmer writes

ColCount := AValue;

our own nonvirtual SetColCount access method will be called. As you can see from the SetColCount method in Listing 7.10, we first check to see whether the suggested value for the number of columns is less than or equal to 3. If it is, we make the change.

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could—because the other adjustments needed in the grid would not get made. We can’t call our ancestor’s SetColCount method—that’s private. And if we write

ColCount := Value;

within our own SetColCount method, we will create endless recursion and crash the stack.

The answer is to use the INHERITED keyword with the property name:

inherited ColCount := Value;

The ability to use INHERITED with an ancestor’s property name is not as well documented as its use with an inherited public or protected method. It’s a pleasant surprise, but it is entirely consistent with Object Pascal’s other pleasant surprises.

Taking Snapshots of the Screen with Delphi

In Delphi, if we want to capture the image of a form’s client area, we can call GetFormImage. But sometimes we want a snapshot of the entire form—title bar, frame, and all. Or we want a snapshot of the entire screen. If we were desperate, we might pop up a message box that says, “Press Print Screen button NOW!” and then figure out a way to get the screen’s mug off the clipboard.

But we’re not that desperate. Combining Delphi canvases with a few GDI functions makes capturing the screen in code a snap. CaptureScreenRect, in Listing 7.11, demonstrates this. It gets the screen’s device context with GetDC(0), and then it copies a rectangular area from that DC to a bitmap’s canvas. To do the copying, it uses BitBlt. The key to using BitBlt—or any GDI function—with Delphi is remembering that a canvas’ Handle is the DC that Windows needs.

Listing 7.11 SCRNCAP.PAS

{ Screen capture functions for Delphi } unit ScrnCap;

interface

uses WinTypes, WinProcs, Forms, Classes, Graphics;

function CaptureScreenRect( ARect: TRect ): TBitmap; function CaptureScreen: TBitmap;

function CaptureClientImage( Control: TControl ): TBitmap; function CaptureControlImage( Control: TControl ): TBitmap;

implementation

{ Use this to capture a rectangle on the screen... } function CaptureScreenRect( ARect: TRect ): TBitmap; var

ScreenDC: HDC; begin

Result := TBitmap.Create; with Result, ARect do

begin

Width := Right - Left; Height := Bottom - Top;

ScreenDC := GetDC( 0 ); try

BitBlt( Canvas.Handle, 0, 0, Width, Height, ScreenDC, Left, Top, SRCCOPY );

finally

ReleaseDC( 0, ScreenDC ); end;

end;

end;

{Use this to capture the entire screen... } function CaptureScreen: TBitmap;

begin

with Screen do

Result := CaptureScreenRect( Rect( 0, 0, Width, Height ));

end;

{Use this to capture just the client area of a form

or control... }

function CaptureClientImage( Control: TControl ): TBitmap; begin

with Control, Control.ClientOrigin do

Result := CaptureScreenRect( Bounds( X, Y, ClientWidth, ClientHeight ));

end;

{ Use this to capture an entire form or control... } function CaptureControlImage( Control: TControl ): TBitmap; begin

with Control do

if Parent = nil then

Result := CaptureScreenRect( Bounds( Left, Top, Width, Height ))

else

with Parent.ClientToScreen( Point( Left, Top )) do Result := CaptureScreenRect( Bounds( X, Y, Width,

Height ));

end;

end.

The remaining screen capture functions in Listing 7.11 cobble up rectangles and farm out the real work to CaptureScreenRect. CaptureScreen throws together a rectangle for the whole screen. CaptureClientImage and CaptureControlImage throw together rectangles for the client area and for the entire area of a control, respectively.

These four functions can be used to capture any arbitrary screen area, as well as the screen images of forms, buttons, memos, combo boxes, and so on. We just tell the little buggers to

say cheese—and free the bitmaps when we’re done.

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property ItemEnabled[ Index: Integer ]: Boolean read GetItemEnabled write SetItemEnabled;

end;

implementation

function TRadioBtnGroup.CheckAnyBut; var

Index: Integer; begin

Result := True;

for Index := NotThisIndex + 1 to Items.Count - 1 do if Buttons[ Index ].Enabled then

begin

Buttons[ Index ].Checked := True; Exit;

end;

for Index := 0 to NotThisIndex - 1 do if Buttons[ Index ].Enabled then begin

Buttons[ Index ].Checked := True; Exit;

end;

Result := False; end;

function TRadioBtnGroup.GetItemEnabled; begin

Result := Buttons[ Index ].Enabled; end;

procedure TRadioBtnGroup.SetItemEnabled; begin

if ( not Value ) and ( Index = ItemIndex ) and Buttons[ Index ].Checked and

( not CheckAnyBut( Index )) then ItemIndex := -1;

Buttons[ Index ].Enabled := Value; end;

function TRadioBtnGroup.GetButtons; begin

Result := Components[ Index ] as TRadioButton; end;

end.

Internally, TRadioBtnGroup uses the GetButtons method to gain access to its radio buttons. GetButtons relies on the fact that, since a radio group owns its child buttons, it holds them in its Components array. All GetButtons does is index into the Components array and safely typecast the result.

The new control tries its best to be an intelligent helper. If a checked button is disabled, the

control tries to check another button; if all the buttons are disabled, it unchecks all the buttons. Depending on your needs, you might want to change this latter behavior.

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represents the main window on which the laser beam will draw the logo.

Listing 8.1 LASERFRM.PAS

unit LaserFrm;

interface

uses

Messages, WinTypes, WinProcs, Classes, Graphics, Controls, Forms, DLPoints, SvrUtils;

private

FMousePt : TPoint; FDone : Boolean; FOriginPoint : Longint;

FOrigins : array[ TLaserOrigin ] of Longint; FCurrPoint : Integer;

FFirstPass : Boolean; FGettingPassword : Boolean; FOrigX : Integer;

FOrigY : Integer;

procedure IdleProc( Sender : TObject; var Done : Boolean ); procedure Animate;

procedure CursorOff; procedure CursorOn; procedure PopulateOrigins; procedure GetPassword;

procedure SetTopMost( TopMost : Boolean ); end;

var

FrmLaser: TFrmLaser;

implementation

{$R *.DFM}

uses

GetPWFrm, MMSystem;

const

Offset = 25; { Distance from Screen Edge of Laser Source }