Before the value can be displayed, the control must have a value. That involves adding a property to the control and then writing the drawing code.

ActiveX controls have four types of properties:

To add the value to the die-roll control, use ClassWizard to add a custom property called Number. Follow these steps:

Before you can write code to display the value of the Number property, the property must have a value to display. Control properties are initialized in DoPropExchange().

FIG. 11.4 ClassWizard simplifies the process of adding a custom property to your die-rolling control.

This method actually implements persistence; that is, it enables the control to be saved as part of a document and read back in when the document is opened. Whenever a new control is created, the properties can’t be read from a file, so they are set to the default values provided in this method. Controls don’t have a Serialize() method.

AppWizard generated a skeleton DoPropExchange() method; this code is in Listing 11.7.

Notice the use of the version numbers to ensure that a file holding the values was saved by the same version of the control. Take away the TODO comment that AppWizard left for you, and add this line:

PX_Short() is one of many property-exchange functions that you can call—one for each property type that is supported. The parameters you supply are as follows:

The following are the PX functions:

• PX_Blob() (for binary large object [BLOB] types)

• PX_Bool()

• PX_Color() (OLE_COLOR)

• PX_Currency()

• PX_DATAPATH (CDataPathProperty)

• PX_Double()

• PX_Float()

• PX_Font()

• PX_IUnknown() (for LPUNKNOWN types, COM interface pointer)

• PX_Long()

• PX_Picture()

• PX_Short()

• PX_String()

• PX_ULong()

• PX_UShort()

• PX_VBXFontConvert()

Filling in the property’s default value is simple for some properties but not for others. For example, you set colors with the RGB() macro, which takes values for red, green, and blue from 0 to 255 and returns a COLORREF. Say that you had a property with the external name EdgeColor and the internal name m_edgecolor and you wanted the property to default to gray. You would code that like the following:

Controls with font properties should, by default, set the font to whatever the container is using. To get this font, call the COleControl method AmbientFont().

The code to display the number belongs in the OnDraw() method of the control class, CDierollCtrl. (Controls don’t have documents or views.) This function is called automatically whenever Windows needs to repaint the part of the screen that includes the control. AppWizard generated a skeleton of this method, too, shown in Listing 11.8.

As discussed in the "Scrolling Windows" section of Unit 5, "Drawing on the Screen," the framework passes the function a device context to draw in, a CRect describing the space occupied by your control, and another CRect describing the space that has been invalidated. The code in Listing 17.8 draws a white rectangle throughout rcBounds and then draws an ellipse inside that rectangle, using the default foreground color. You can keep the white rectangle for now, but rather than draw an ellipse on it, draw a character that corresponds to the value in Number. To do that, replace the last line in the skeletal OnDraw() with these lines:

These code lines convert the short value in m_number (which you associated with the Number property on the Add Property dialog box) to a CString variable called val, using the new CString::Format() function (which eliminates one of the last uses of sprintf() in C++ programming). The ExtTextOut() function draws a piece of text—the character in val—within the rcBounds rectangle. As the die-roll control is written now, that number will always be 3.

You can build and test the control right now if you would like to see how little effort it takes to make a control that does something. Unlike the other ActiveX applications, a control isn’t run as a standalone application in order to register it. Build the project and fix any typing mistakes. Choose Tools, ActiveX Control Test Container to bring up the control test container, shown in Figure 11.5.

FIG. 11.5 The ActiveX control test container is the ideal place to test your control.

Within the test container, choose Edit, Insert New Control and then choose Dieroll Control from the displayed list. As Figure 11.6 shows, the control appears as a white rectangle displaying a small number 3. You can move and resize this control within the container, but that little 3 stays doggedly in the upper-left corner. The next step is to make that number change when a user clicks the die.