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iOS Programming Basic: How Does the Hello World App Work?

I hope you enjoy the first iOS programming tutorial and already created your first app. Before we move onto the next tutorial and build a more complex app, let’s step back and have a closer look at the Hello World app. It’ll be good for you to understand some of the Objective-C syntax and the inner workings of the app.

So far you follow the step-by-step guide to build the Hello World app. But as you go through the tutorial, you may come across these questions:

  • What are those .xib. h and .m file?
  • What are those “ugly” code inside “showMessage”? What do they mean?!
  • What actually happens after you taps the “Hello World” button? How does the button trigger the “showMessage” action?
  • How does the “Run” button in Xcode work?

I want you to focus on exploring the Xcode environment so I didn’t explain any of the above in the previous post. Yet it’s essential for every developer to understand the inner details behind the code and grasp the basic concept of iOS programming. For some technical concepts, they may be a bit hard to understand particularly you have no programming background. Don’t worry, however. This is just a start. As you move on and write more code in later tutorials, you’ll get better understanding about iOS programming. Just try your best to learn as much as possible.

Interface Builder, Header and Implementation Files

First, what are those .xib. h and .m files? This is a very good question raised by one of the readers. Under the Project Navigator, you should find three main types of files – .xib. h and .m. (If you expand the “Supporting Files” folder, you’ll find other file

types such as plist and framework. But for now, let’s forget about them first. We’ll talk about them later.)

.xib – For files with .xib extension, they’re Interface Builder files that store the application’s user interface (UI). As you click on the .xib file, Xcode automatically switches to the Interface Builder for you to edit the UI of the app via drag-and-drop.

Interface Builder in Xcode

.h and .m – Files with .h extension refers to the header files while those with .m extension are the implementation files. Like most of the programming languages, the source code of Objective-C is divided into two parts: interface and implementation.

Well, to put in analogy that you can better understand both terms, let’s consider a TV remote. It’s convenient to control the volume of a TV set wirelessly with a remote. To increase the speaker volume, you press the “Volume +” button. To switch channel, you simply key in the channel number. Let me ask you. Do you know what happens behind the scene when pressing the “Volume” button? Probably not. I believe most of us don’t know how the remote communicates with the TV set and controls the speaker volume. What we just know is, that button is used for changing the volume. In this example, the button that interacts with you is the “interface” and the inner detail which is hidden behind the button is referred as the “implementation”.

Now you should have a better idea about interface and implementation. Let’s go back to the code. In Objective-C, interfaces of a class are organized in “.h” file. We use the syntax “@interface” to declare the interface of a class. Take a look at the HelloWorldViewController.h, which is the header file:

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