19th Chapter-delphi Custom Parts Development (IV.) (2)

1. Decide what to draw

Graphic parts usually have the ability to change their appearance, and the appearance of graphic control depends on the combination of some of its properties, such as gauge control's ability to determine its shape, direction, and graphically display its processes. Similarly, shape control should have the ability to decide which shapes to display.

Give shape control of this ability and add properties called shape. This requires the following three steps:

Declaring property types

declaring properties

Writing implementation Methods

⑴ declaring property types

When declaring a property of a user-defined type, you must first declare the property type. The most common custom type used for attributes is the enumeration type.

For shape control, you need to declare an enumeration that controls the ability to draw shapes, and here is the declaration of the enumeration type:

Type

Tsampleshapetype= (Sstrectangle, Sstsquare, Sstroundrect,

Sstroundsquare, Sstellipse, sstcircle);

Tsampleshape = Class (Tgraphiccontrol)

End

This allows you to declare a property with that type.

⑵ Declaration Properties

When declaring a property, it is often necessary to declare a private domain to hold the property value, and then describe the method of reading and writing the property value.

For Shape control, a field is declared to hold the current shape, and then a property is declared to read and write the domain value by means of a method.

Type

Tsampleshape=class (Tgrahpiccontrol)

Private

Fshape:tsampleshapetype;

Procedure Setshape (Value:tsampleshapetype);

Published

Property Shape:tsampleshapetype read Fshape write Setshape;

End

Now, there is only the Setshape implementation part left.

⑶ Writing Implementation method

Here is the implementation of the Setshape:

Procedure Tsampleshape.setshape (Value:tsampleshapetype);

Begin

If Fshape<>value Then

Begin

Fshape: = value;

Invalidate (True); {Force new shape to repaint}

End

End

2. Covering constructor and destructor

In order to change the default property values and initialize the objects owned by the part, you need to overwrite the inherited constructor and destructor methods.

The default size of the graphics control is the same, so you need to change the width and Height properties.

In this case, the size of shape control is initially set to 65 pixel points of the edge length.

⑴ add overlay in part declarations constructor

Type

Tsampleshape=class (Tgraphiccontrol)

Public

Constructor Create (aowner:tcomponent); Override

End

⑵ the attribute height and width with the new default value

Type

Tsampleshape=class (Tgrahiccontrol)

Published

Property Height default 65;

Property Width default 65;

End

⑶ a new constructor in the implementation part of the library unit

Constructor Tsampleshape.create (aowner:tcomponent);

Begin

Inherited Create (Aowner);

Width: = 65;

Height: = 65;

End