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A common feature of many applications is a series of navigation buttons that allow users to move through a table. These typically include buttons to move the record pointer to the next or prior record in the table, as well as to the top or bottom record in the table.
Designing the Navigation Buttons
Each of the buttons will have some characteristics and functionality in common, so it is a good idea to create a navigation button class. Then the individual buttons can easily derive this common appearance and functionality. This parent class is the Navbutton class defined later in this section.
Once the parent class is defined, the following subclasses define the functionality and appearance specific to each of the four navigation buttons: navTop, navPrior, navNext, navBottom.
Finally, a container class, vcr, is created and each of the navigation buttons is added to the container class. The container can be added to a form or a toolbar to provide table navigation functionality.
NAVBUTTON Class Definition
To create Navbutton, save the following six class definitions (Navbutton, navTop, navBottom, navPrior, navNext, and vcr) to a program file such as Navclass.prg.
Definition of the Generic Navigation Commandbutton Class
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Define the parent class of the navigation buttons. Give the class some dimensions. Include a custom property, |
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If |
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Using _SCREEN.ActiveForm.Refresh instead of THISFORM.Refresh allows you to add the class to a form or a toolbar and have it function equally well. |
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End the class definition. |
The specific navigation buttons are all based on the Navbutton class. The following code defines the Top button for the set of navigation buttons. The remaining three navigation buttons are defined in the following table. The four class definitions are similar, so only the first one has extensive comments.
Definition of the Top Navigation Button Class
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Define the Top navigation button class and set the Caption property. |
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Create method code to be executed when the Click event for the control occurs. |
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Call the Click event code in the parent class, |
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End the Click procedure. |
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End the class definition. |
The other navigation buttons have similar class definitions.
Definition of the Other Navigation Button Classes
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Define the Next navigation button class and set the Caption property. |
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Include the code to set the record pointer to the next record in the table. End the class definition. |
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Define the Prior navigation button class and set the Caption property. |
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Include the code to set the record pointer to the previous record in the table. End the class definition. |
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Define the Bottom navigation button class and set the Caption property. |
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Include the code to set the record pointer to the bottom record in the table. End the class definition. |
The following class definition contains all four navigation buttons so that they can be added as a unit to a form. The class also includes a method to set the TableAlias property of the buttons.
Definition of a Table Navigation Control Class
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Begin the class definition. The Height property is set to the same height as the command buttons it will contain. |
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Add the navigation buttons. |
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This method is used to set the TableAlias property of the buttons. TableAlias is defined in the parent class |
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End class definition. |
Once you have defined the class, you can subclass it or add it to a form.
Creating a Subclass Based on the New Class
You can also create subclasses based on vcr that have additional buttons such as Search, Edit, Save, and Quit. For example, vcr2 includes a Quit button:
Definition of a Table Navigation Control Subclass
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Define a class based on |
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When the user clicks |
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End class definition. |
Vcr2 has everything that vcr does, plus the new command button, and you don't have to rewrite any of the existing code.
Changes to VCR Reflected in the Subclass
Because of inheritance, changes to the parent class are reflected in all subclasses based on the parent. For example, you could let the user know that the bottom of the table has been reached by changing the IF EOF( ) statement in navNext.Click to the following:
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IF EOF( ) GO BOTTOM SET MESSAGE TO "Bottom of the table" ELSE SET MESSAGE TO ENDIF | |
You could let the user know that the top of the table has been reached by changing the IF BOF( ) statement in navPrior.Click to the following:
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IF BOF() GO TOP SET MESSAGE TO "Top of the table" ELSE SET MESSAGE TO ENDIF | |
If these changes are made to the navNext and navPrior classes, they will also apply automatically to the appropriate buttons in vcr and vcr2.
Adding VCR to a Form Class
Once vcr is defined as a control, the control can be added in the definition of a container. For example, the following code added to Navclass.prg defines a form with added navigation buttons:
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DEFINE CLASS NavForm AS Form ADD OBJECT oVCR AS vcr ENDDEFINE | |
Running the Form Containing VCR
Once the form subclass is defined, you can display it by loading the class definition, creating an object based on the subclass and calling the Show method for the form:
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SET PROCEDURE TO navclass ADDITIVE
frmTest = CREATEOBJECT("navform")
frmTest.Show | |
If you don't call the SetTable method of oVCR (the VCR object in NavForm) when the user clicks the navigation buttons, the record pointer moves in the table in the currently selected work area. You can call the SetTable method to specify what table to move through.
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frmTest.oVCR.SetTable("customer") | |
 Note: |
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When the user closes the form, frmTest is set to a null value (.NULL.). To release the object variable from memory, use the RELEASE command. Object variables created in program files are released from memory when the program is completed.
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