As is traditional in programming courses, your first app will be a Hello World example. Specifically, the words “Hello World” will appear at the touch of a button on screen.

The screencast below walks you through what you need to do in Xcode, from creating a new project to running your app in the iOS simulator. The code itself is explained in more detail afterwards.

THE CODE…

(ViewController) Header file

• .h file extension
• Establishes the controls used in the app’s interface, in this example the label and the button controls. It kind of serves as the link between your user interface (i.e. what you created in the xib file) and the app’s main code (i.e. in the implementation file)
• Sets the name of the controls used and their ‘event’ (to put it into Visual Basic terms), in this case: outlet and action
• Outlet is tagged with the label. Think of outlet being another way of saying output - the control that we called ‘label’ is designed to display or ‘output’ text
• Action is tagged with the button. This tells Objective-C that when a button is clicked something will occur – this is the ‘action’

Note that other than the setup of controls in this project, no other code goes into the header file. How these controls will be used is written in the implementation file.

IBOutlet UILabel *label; creates an instance of a label object in the header file.

• IBOutlet is short for Interface Builder Outlet - that is, what follows is related to an object that you clicked and dragged from the utilities panel (in Xcode) to the xib file (i.e. iPhone mockup screen).
• UILabel states that the outlet object being linked from the xib (interface) file to the header file is a label
• *label; is the name of the instance of a label object. In this example our label instance is also called label, but we could have called it anything we wanted to…

- (IBAction)button:(id)sender; creates an instance of a button object.

• - (IBAction)button: indicates that the object created will perform some kind or behaviour or ‘action’. In this example, a button object (called ‘button’ in this case) is being instantiated.
• (id)sender; tells Xcode that the button will send messages, when clicked, to the button’s method, which we’ll create next in the implementation file.

(View Controller) Implementation file

• .m file extension
• Contains the majority of the code in a project
• Indicates what will happen, exactly, when the end-user touches the button

• So this code is within a method (that is, one or more statements that outline what will happen when the method is executed).
• A method encloses it’s statements within curly braces { }
• One or more statements can be written inside the curly braces
• Each statement is like a one line instruction to the compiler
• Each statement ends with a semi-colon ;

Finally, what we’re adding to the end of the implementation file in this project is the button’s method (i.e. the code that will run (yes, even if it is only one line!) when the button is clicked.

Objective-C says - (IBAction)button:(id)sender {

Visual Basic .Net says something like Private Sub myButton_Click… etc

Objective-C says label.text = @”Hello World”;

Visual Basic .Net says something like lblOutput.Text = “Hello World”

We’ll extend the Hello World example to include capturing user input in the next tutorial, Xcode for beginners 03 – Using a text box to input data

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