Chapter 6 Sub Procedures

Transcription

1 Sub Procedures A set of statements that perform a specific task. Divide a program into smaller, more manageable blocks of code. An event procedure is written for a specific object event, while a sub procedure can perform tasks unrelated to any specific event. Procedure names should begin with an uppercase letter and then an uppercase letter should begin each word within the name. Executed by using a Call statement. Slide 1

2 The PictureBox Control (Name) should begin with pic. Image is used to add images to the Resource folder with the selected image displayed in the picture box. SizeMode is typically set to AutoSize so that the picture box is automatically sized to the image size. Visible is set to False to hide an image at run time. Size changes automatically when SizeMode is AutoSize. Slide 2

3 Changing an Image at Run Time Images must be stored in the Resources folder in the project folder. The My.Resources object is used to access an image: My.pictureBox.Image = My.Resources.imageName Slide 3

4 The LinkLabel Control The LinkLabel Control is used to add a link to a website Slide 4

5 Value Parameters A procedure can have parameters for accepting values from a calling statement. Parameters are used inside the procedure to perform the procedure's task. The data passed to a procedure are called the arguments. GiveHint() has two parameters: Sub GiveHint(ByVal firstnum As Integer, _ ByVal secondnum As Integer) Slide 5

6 Value Parameters (cont.) The order of the arguments passed corresponds to the order of the parameters in the procedure heading. The number of arguments in a procedure call must match the number of parameters in the procedure declaration. Arguments passed by value can be in the form of constants, variables, values, or expressions. Variable arguments passed by value are not changed by the procedure. Slide 6

7 Procedure Documentation Procedure documentation should include a brief description of what the procedure does. Documentation should also include the assumptions, or initial requirements, of a procedure called preconditions (pre:). Documentation should include a postcondition (post:), which is what must be true at the end of the execution of a procedure if the procedure has worked properly. Slide 7

8 Reference Parameters Slide 8

9 Reference Parameters (cont.) The order of the arguments passed corresponds to the order of the parameters in the procedure heading. ByRef parameters accept only variable arguments. Variable arguments passed by reference may be changed by the procedure. Slide 9

10 Control Object Parameters Control objects are a data type called a control class. A control object can be passed as an argument to a procedure. Control argument parameters should be declared ByRef in a procedure using the appropriate class name. Slide 10

11 Event Handler Procedures Can be written to handle more than one event. Procedure name can be changed because the events after the Handles keyword is what indicates when the procedure will execute. Slide 11

12 The HotDog Application Slide 12

13 The Tag Property Every control has a Tag property. Can be set to a string that is useful for identifying the object. When an event procedure is handling more than one object, the string in the Tag property can be used to determine which object raised the event. Slide 13

14 Function Procedures A set of statements that perform a specific task and then return a value. Built-in functions include Int() and Rnd(). A function often has at least one parameter for data that is required to perform its task. Parameters are ByVal because a function returns a single value without changing argument values. Called from within an assignment statement or another type of statement that will make use of the returned value. Slide 14