Visual Basic Games: Week 3

Global variables, parameters, Select, KeyDown

Enable, Visible, Focus

State of Game

Read chapter 3

Global versus Local Variables• Variables (aka internal variables) hold data

independent of properties in controls.• Global variables are defined (persist) independently of

any particular procedure. Declared (using DIM) in the (General) section ‘above’ all the procedures.

• Local variables are defined & only persist within a procedure.

• Rule: local, except when a value must be global. • Rule: don’t economize on names or length of names.

Use i, j, n, for simple indexing, otherwise use meaningful names.

• Convention: use int, sng, bln, etc. as part of name.

Option Explicit

• Type in as first line in (General) section (this is first line in the code window).

• This will force an error if and when you use a variable without first declaring it in a DIM statement.

• WHY do this? This will warn you of any spelling errors.

Parameters

• Procedures (such as event procedures or procedures you will define yourself in the later projects) can have parameters. These are values set by VB or by the calling program.

Sub lblCard_Click(Index as Integer)Sub Form_KeyDown(KeyCode as Integer, Shift as Integer)• When you define your own procedures, these two facts will be significant

(and will be repeated)– Parameters are ‘call by reference’. The called procedure can change the value– The name of the parameter has local scope. The name in the calling procedure can

be different than the name in the called procedure.• In Cannonball, hittarget is called for two different reasons and different names are used.

Conditional statements

• IF condition Then1 or more lines of code

[Else 1 or more lines of code]End If

• Select Case expressionCase 1 or more values OR conditions

codeCase 1 or more values or conditions

code[Case Else]

code ]End Select

Chance game features

• Rules of the game:– Add up values on the dice – on a ‘first throw’, 7 or 11 wins, 2, 3 or 12 loses. For

anything else, the value becomes ‘the point’– On follow-up throws, throwing the point again means a

win. Throwing a 7 means a loss.

• Implications for implementation: need to maintain information on what the status is. This is called the state of the game. The state of the chance game is– Is it a first or a follow-up throw?– What is ‘the point’

Chance Design Tasks

• State of the game—done with global variables• Get (pseudo-) random values—use Randomize and Rnd.

Also use expression:Int(Rnd*6) + 1

• Selectively show images of dice—done with image controls, the Visible property, and using an extra control in the case of doubles (e.g., two 2s)

• Display rules: command button code: MsgBox “…”• End game: use command button code: End• Rules of game: done using Select statements within Select

statement (outer statement distinguishes between first throw and follow-up throw)

Preview: Timers

• The timer control has one event: timer.• and two critical properties:

– Interval– Enabled

• The timer event will happen if Enabled is set to True and Interval is non-zero. The timer event will happen every Interval milliseconds.– If Interval is set to 500, the timer event will happen

every ½ second.

• Timer is more like an alarm clock.

Top

Left

Top has been increased

Top and Left of the bounding rectangle

Moving ball

• Place a timer control on the form (it will not be visible). Set interval to 300, enabled to True. Place a shape control called shpBall. (make it a solid circle).

• Code in timer1.timer:shpBall.Top = shpBall.Top + .5*shpBall.Height

• Run program. You will need to use the stop button to stop it.

How to move ball….

• 1000 twips:– up– left– right

• How to move ball a different amount?

Project: control direction of object

• Objective: use keys in keypad to set (change) direction of moving ball.

• Need– Way to move (animate) a ball

– Event that signals key down = Form_KeyDown

– Key codes associated with the selected keys

– Select statement for specifying action for each case

– Way to encode right, left, up, down, and later diagonal directions

Move a ball

• Do animation using timer control and a start button that sets the interval and enables the time.

• Code in tmr.Timer (happens every interval)shpBall.Top = shpBall.Top + vspeed * shpBall.HeightshpBall.Left = shpBall.Left + hspeed * shpBall.Width

• vspeed and hspeed will be changed in the event procedure for KeyDown

Setting the two speeds

Sub Form_KeyDown(KeyCode as Integer, Shift as Integer)Select Case KeyCode

case leftkeyhspeed = -unitvspeed = 0

case upkeyhspeed = 0vspeed = -unit

case sekeyhspeed = unitvspeed = unit

….

Case Else

• Player should be given feedback for a bad keystroke

…Case else

MsgBox “Click one of the direction keys”or (for your own debugging)

MsgBox “You clicked key” & Str(KeyCode) End Select

KeyDown

• Need to make sure that the Form_KeyDown event is triggered. This requires you do disable (set the enabled property to false) other controls on the board and also setfocus for the form.

How do you know to do this: I learned from experience….

cmdStart_Click

• tmrBall.Interval = 100

• tmrBall.Enabled = True

• cmdStart.Enabled = False

• frmControl.SetFocus

Form_Load

unit = .3upkey = 104rightkey = 102downkey = 98leftkey = 100nekey = 105sekey = 99swkey = 97nwkey = 103

How to stop?

• Use a designated key (add to Form_Load)stopkey = 101

• In the Select Case statement in KeyDown add,Case stopkey

tmrBall.Enabled = False

cmdStart.Enabled = True

Global variables

• Normally, variables are active / accessible / alive just in the local subroutines and functions

• … but, you need all these variables to persist between events– vspeed, hspeed are set in KeyDown and used in

tmrBall.Timer– leftkey, rightkey, etc. are set in form_Load and used in

KeyDown

So, you need to have a DIM statement (called a declaration) in the (General) section

Review: general

• Incremental development• Important: you don’t think of things in the

complete, proper, final order. Divide and conquer = take the task and divide it into subtasks.

• Incremental testing: do first with just two directions (left and right), then up and down, then diagonals, then stop.

Review: specific

• tmrBall.Timer eventhandler moves the ball using the vspeed and hspeed, which were set elsewhere

• form_keydown eventhandler uses cases within a select case statement to set vspeed and hspeed– one of the cases will turn off the timer and reset the start

button

• cmdStart_Click eventhandler starts off the animation by setting tmrBall.Interval and tmrBall.Enabled. It also turns itself off (sets cmdStart.Enabled to false) and frmControl.Focus to True.

Assignments

• In lab, – implement the control exercise or – work on chance (dice) game

• Work on controlling ball exercise

• Complete chance to show. Work on bouncing ball, if you finish chance early.