Programming with Microsoft Visual Basic 2010 5th Edition Chapter Seven Sub and Function Procedures

Previewing Harvey Industries Application Open the Harvey.exe file The Harvey Industries application calculates payroll for an employee Figure 7-1 Interface showing the payroll calculations

Lesson A Objectives After studying Lesson A, you should be able to: Explain the difference between a Sub procedure and a Function procedure Create a procedure that receives information passed to it Create a Function procedure Explain the difference between passing data by value and passing data by reference

More About Sub Procedures A block of program code that performs specific task Two types of Sub procedures in Visual Basic Event procedure Procedure associated with specific object and event Independent Sub procedure Independent of any object and event Processed only when called (invoked)

Passing Variables Variables Passing by value Passing by reference Have both value and unique address Passing by value Passes a copy of the variable’s value Passing by reference Passes a variable’s address

Passing Variables by Value Provides only the contents of variable to receiving procedure (called procedure) How to pass by value: Include keyword ByVal before parameter Reasons to pass by value: Procedure needs to know the contents of variable Procedure does not need to change original value (此變數只會出現在assignment statement右邊) By default, Visual Basic passes by value example

Figure 7-5 ShowMsg procedure and btnDisplay Click event procedure (定義程序時的參數) (引用程序時的參數) Figure 7-5 ShowMsg procedure and btnDisplay Click event procedure

Passing Variables by Reference Provides the address of variable to receiving procedure Address : memory location Receiving procedure can thus change the value of the variable Reason to pass by reference Procedure needs to change variable’s contents (此變數會出現在assignment statement左邊，異動值傳回呼叫的程式) How to pass by reference Include keyword ByRef before parameter example

lstHours lstRate 43.0 7.75 Figure 7-8 CalcGrossPay procedure and btnCalc control’s Click event procedure

Passing Variables by Reference (cont’d.) Figure 7-9 Argument values before the Call CalcGrossPay statement is processed program

alias (Called by ByRef) (Called by ByVal) program Figure 7-10 Argument and parameter values after the Call statement and CalcGrossPay procedure header are processed program

Figure 7-11 Argument and parameter values after the first statement in the CalcGrossPay procedure is processed program

Figure 7-12 Argument and parameter values after the statement in the selection structure’s true path is processed program

Figure 7-13 Argument and parameter values after the CalcGrossPay procedure ends program

Function Procedures Function procedure A block of code that performs specific task Returns a value after completing its task Visual Basic provides built-in functions Can also create your own functions As datatype in header indicates return type of data Return expression type must agree with As datatype Syntax & example

Figure 7-14 Function procedure syntax, examples, and steps

Invoking Function Procedures Figure 7-15 Examples of invoking the GetNewPrice function

Sub procedure方式 Figure 7-16 CalcGrossPay function and btnCalc control’s Click event procedure

Lesson A Summary Two types of Sub procedures Event and independent Function: Performs task and returns a value Independent procedures and functions Called from application’s code using Call statement Pass by value Send a copy of variable’s contents to procedure or function Pass by reference Send variable’s address to procedure or function

Lesson B Objectives After studying Lesson B, you should be able to: Include a combo box in an interface Add items to a combo box Select a combo box item from code Determine the item either selected or entered in a combo box Code a combo box’s TextChanged event procedure

Including a Combo Box in an Interface Allows user to select from a number of choices Allows user to type an entry not on list Can save space on form List box does not share features two and three DropDownStyle property Values: Simple, DropDown (default), DropDownList comparison

Including a Combo Box in an Interface cboCity cboState cboName Figure 7-19 Examples of the combo box styles Program for this UI

Figure 7-20 Code associated with the combo boxes in Figure 7-19

Including a Combo Box in an Interface (cont’d.) To process code when Text property changes Use TextChanged event procedure Use Item collection’s Add method to add item Other properties of combo box Sorted: Sorts items in dictionary order SelectedIndex: Used to select item in list portion SelectedItem: Contains value of item selected Text: Contains the value that appears in text portion Use label control to provide keyboard access to the combo box example

Including a Combo Box in an Interface (cont’d.) Figure 7-22 Gross pay amount shown in the interface

Lesson B Summary Use ComboBox tool to add combo box to a form Specify style of combo box using DropDownStyle property Use Items collection’s Add method to add items to Combo box Use combo box’s Sorted property to sort items in combo’s list Enter code in combo box’s TextChanged event procedure To process code when Text property changes Usage of SelectedIndex, SelectedItem, or Text property: To select combo box item from code To determine item that was selected

Lesson C Objectives After studying Lesson C, you should be able to: Prevent a form from closing Round a number

Creating Harvey Industries Application Application objective is to calculate: Employee’s weekly gross and net pay (初步薪資與淨薪資) Federal withholding tax (FWT) Social Security and Medicare (FICA) tax Net pay

Coding FormClosing Event Procedure Occurs when form is about to be closed because: Computer processes Me.Close() statement User clicks Close button on form’s title bar Typical functions of FormClosing event procedure Verifying that user wants to close application Taking appropriate action based on user’s response To prevent closing Set Cancel property of FormClosing procedure’s e parameter to true

Coding the FormClosing Event Procedure Figure 7-27 Pseudocode for the FormClosing event procedure result

Coding the FormClosing Event Procedure (cont’d.) Figure 7-28 Message box displayed by the code in the FormClosing event procedure  實作練習在429~430頁

Coding btnCalc Control’s Click Event UI Figure 7-29 Pseudocode for the btnCalc control’s Click event procedure  實作練習在431~432頁

Creating the GetFwt Function GetFwt function emulates FWT table lookup Determining federal withholding tax (FWT) Evaluate weekly taxable wages (可稅週薪) and filing status Use data to look up FWT in special FWT tables How to calculate weekly taxable wages Multiply number of withholding allowances by $70.19 Subtract this result from weekly gross pay pseudocode

Creating the GetFwt Function (cont’d.) Figure 7-35 Pseudocode for the GetFwt function

Creating the GetFwt Function (Married) Figure 7-33 FWT calculations for a married (已婚的) taxpayer whose weekly taxable wages are $388.46

Creating the GetFwt Function (Single) Figure 7-34 FWT calculations for a single (單身的) taxpayer whose weekly taxable wages are $600

GetFwt Function Header and Footer Figure 7-36 GetFwt function header and footer  實作練習在435~439頁

Completing the btnCalc Control’s Click Event Procedure Must call GetFwt function from btnCalc’s Click event procedure Math.Round function Round value to specific number of decimal places Syntax: Math.Round (value[, digits]) value: Numeric expression to work on digits: Integer indicating number of places to right of decimal point  實作練習在436~438頁

Adding an Existing Form to a Solution Add an existing form to Chap07/Harvey Industries Solution Make sure that added form uses a different form name from any form name of Harvey Industries Solution in Visual Studio and disk file system The splash screen for Country Charming Inn in Chapter 1 is to be integrated into Chap07/Harvey Industries Solution The splash screen identifies the application’s author and copyright year, or shows welcome message 實作練習 : 需下載 Chap07/ Harvey Industries Solution Chap07/Splash Solution Chap02/Playtime Solution

Adding an Existing Form to a Solution Figure 1-31: Completed splash screen 40

Adding an Existing Form to a Solution Rename the form in Splash Solution Project->加入現有項目->Find Splash Form.vb to add Open a new form by Timer’s Tick event Add following codes in the Tick event of the timer tmrExit Me.Hide() tmrExit.Enabled = False Dim mForm As New frmMain mForm.ShowDialog() Me.Close() Syntax used to instruct the computer to create an object from a class: Dim variablename As New classname Otherwise, the form frmMain will show up many times. The name of the main form in Harvey Industries Solution 41

Include Image File for the Added Form VB doesn’t import image files automatically for the added form. You can do it by one of the following steps Make the statement Me.picCountry.Image = …… as a comment Click Splash Form’s PictureBox picCountry Use the task box of the PictureBox to set up its image file (image file is in the Resources folder of the Chap07/Splash Solution ) 42

Assigning the Startup Form When an application is started, VB.NET automatically invokes the form set as the Startup object of a project The setup steps for Startup object are as follows: Right click on the project in the Project Explorer Window to open the Properties Window Click the Application tab in Properties Window Click the Startup object ComboBox to set up the startup form → choose frmSplash 43

ShowDialog() vs. Show() Method A form object’s ShowDialog() method allows you to display the form object on the screen with focus transfer (dialog-style interaction) The syntax is: form.ShowDialog() The form object’s Show() method allows you to display a form object on the screen without focus transfer The syntax is: form.Show () 44

Show Another Form by a Button Download Chap02/Playtime Solution, Chap07/Harvey Industries Solution How to use a button to open a new form Project->加入現有項目->Find Playtime Form.vb to add Include Image File for the PictureBox PictureBox1 Add following codes in the Click event of a button Show Playtime on the Harvey Industries’ MainForm Me.Hide() ‘Show a form at a time Dim hForm As New frmPlaytime hForm.ShowDialog() Me.Show() frmPlaytime.Show() or .ShowDialog() ‘Show 2 forms frmPlaytime.Focus() (when using frmPlaytime.Show() ) The name of main form in Playtime Solution 45 45

Lesson C Summary Use form’s FormClosing event procedure to process code when form is about to be closed Set Cancel property of FormClosing event procedure’s e parameter to true to prevent form from being closed Use Math.Round function to round number to specific number of decimal places