19th Chapter-delphi Custom Parts Development (IV.) (1)

19.3.2 Create a graphic part

Graphic control is a kind of simple component. Because a pure graphic part does not need keyboard focus, it does not have a window handle. Application users with graphics control can still control with the mouse, but there is no keyboard interface.

The graphical part provided in this example is Tshape. The shape part is located on the additional page of the component palette. The shape part in this example is different, so it is called Tsampleshape.

Creating a graphics part requires the following three steps:

Create and register a part

Advertisement (Publishing) inherited properties

Add Graphics Features

19.3.2.1 Create and register parts

The creation of each part starts in the same way, in this case, as follows:

Set up a part unit called shapes

Inherit from Tgraphiccontrol, call the new part Tsampleshape

Register on the Component Palette's Samples page Tsampleshape

Unit Shapes

Intertace

Use Sysutils, Wintypes, Winprocs, Messages, Classes,

graphics,controls,forms;

Type

Tsampleshape=class (Tgraphiccontrol)

End

Implementation

Procedure Register;

Begin

Registercomponents (' Samples ', [Tsampleshape]);

End

End.

19.3.2.2 To advertise inherited properties

Once you decide on a part type, you can decide which properties and events are visible to the user in the protected part of the parent class. Tgraphiccontrol has published all properties that are controlled as graphics, so just advertise the properties that respond to mouse and drag-and-drop events.

Type

Tsampleshape=class (Tgraphiccontrol)

Published

Property dragcursor;

Property dragmode;

Property ondragdrop;

Property ondragover;

Property Onenddrag;

Property OnMouseDown;

Property OnMouseMove;

Property OnMouseup;

End

In this way, the shape controls the ability to interact with the user through the mouse and drag-and-drop.

19.3.2.3. Increase graphics capability

Once you have declared the graphic part and advertised the inherited properties, you can add graphics functionality to the part. Now you need to know two points:

Decide what to draw

How to draw part graphics

In the case of shape control, you need to add some properties that enable the user to change the shape at design time.