For Dummies Objective-C
Contents
1. variable Understanding How Computer Languages Work While conceptually it is pretty easy to understand computer programming all you are doing is giving it a set of instructions and some data on which to perform those instructions one of the challenges as I mentioned previ ously is that it s not that easy to tell a computer what to do Computers don t speak English although computer scientists have been working on that for years think of trying to do that as the Computer Scientist Full Employment Act A computer actually has its own language made up of ones and zeros For that matter Objective C is not something a computer can understand either but it is a language that can be turned into those ones and zeros by using a compiler A compiler is nothing more than a program that translates Objective C instructions into computer code Creating a computer program To create a computer program using a computer language follow these steps see Figure 1 3 1 Decide what you want the computer to do You can have the computer write a line of text on the monitor or create an online multiplayer game that will take two years to complete It really doesn t matter Chapter 1 Computer Programming Exposed 15 2 Break the task you want the computer to complete into a series of modules that contain the instructions the computer follows to do what you want and then provide the data it needs to do that The series of modul2. Chapter 1 Computer Programming Exposed In This Chapter Understanding the basics of computer programming Getting how computer languages work Knowing how Objective C works J vv at it from the outside computer programming can appear compli cated and a bit mysterious But once I let you in on a few of the secrets you ll realize that when you write a computer program whether it is a small program that s just a few lines or one that is tens or even hundreds of thou sands of lines you are generally doing the same thing 1 Getting input from a keyboard or touch screen or even something stored on your computer The input might be instructions to the program itself for example to display the Web page developer apple com or to print a docu ment such as Chapter 1 or to process data like enter your Apple ID and Password when you log on to the Mac Dev Center the browser is just another program or even to process a list of credit card transactions stored on a computer 2 Doing something based on or with the input Your browser may go on the Internet and access the page correspond ing to developer apple com or your word processing program may display a Print dialog and print the chapter at least that is what mine does Based on your input the program may also go out and use data it has stored or even has access to over the Internet For example when you enter your Apple ID and Password eventually a computer ac
3. Mac OS X For Dummies to really learn about the frameworks Using Xcode 3 2 You will be using the Xcode 3 2 developer tools package that was released with Mac OS X 10 6 Snow Leopard This is an improvement over Xcode 3 1 that was included in Leopard and I will assume that you are using both Xcode 3 2 and Mac OS X 10 6 in this book Using Objective C Version 2 0 You will be learning Version 2 0 of the Objective C language which was released with Mac OS X 10 5 and yes you should care Version 2 0 has some new and very useful features such as declared properties fast enumeration and garbage collection which greatly simplify memory management unfortunately garbage collection is not available on the iPhone As I explain these new features I will remind you that they are available only in Objective C Version 2 0 which works only with Mac OS X 10 5 or later and the iPhone OS If possible I ll also indicate some workarounds if you need to write applications that run under earlier versions of the OS but in general writing applications that run under earlier versions of the OS will be up to you
4. Running a computer program Just as you don t need to be a weatherman to know which way the wind blows you don t need to be an engineer who understands the intimate details of a computer to write a world class application Most people don t find it that difficult to learn to drive a car While you don t have to know all that stuff about internal combustion engines fuel injection drive trains and transmissions you do need to know a little bit about how a car works That means knowing about how to turn it on make it go forward make it go backward make it stop generally a very valuable piece of infor mation make it turn left or right and so on 6 Part l Getting to the Starting Line int main int argc char argv char theSandwich I hate peanut butter and jelly printf theSandwich return 0 Compiler Idea hate peanut butter and jelly Figure 1 3 How pro gramming works es In the same way you do need to know a little bit about how computers work to have what you do to write a computer program make sense When you run a computer program the computer does its primary work in a part of the machine you cannot see the CPU which executes the program instructions that are loaded into the computer s memory This is a fast temporary form of storage that is in one of those chips you see when you Chapter 1 Computer Programm
5. andwich example int main int argc char argv char theSandwich I hate peanut butter and jelly printf theSandwich return 0 Compiler hate peanut butter and jelly 11 2 Part l Getting to the Starting Line This is what you do to get that output 1 You write instructions for the computer to follow Unfortunately the computer can t speak English or read for that matter so you use something called a compiler to take the instructions you have written in the Objective C language and translate it into something the computer can understand 2 You provide data for the computer to use In this case you write I hate peanut butter and jelly and then the computer follows the instructions you have given it on what to do with that data Voila you see I hate peanut butter and jelly displayed on your com puter screen Fundamentally programs manipulate numbers and text and all things consid ered a computer program has only two parts variables and other structures which hold data and instructions which perform operations on that data Examining a simple computer program Is there really any difference between a chef reading a recipe and creating a peanut butter and jelly sandwich and a computer following some instructions to display something on a monitor Quite frankly no Here is the simple Objective C progra
6. cesses a database to see if your Apple ID and Password are both valid and if so allows you access to the site and displays the site for you 3 Displaying the results of your adroitness on a monitor or storing it away for future use There is no doubt that computers are engineering marvels But what will make you a good programmer is not your understanding of all that wizardry 0 Part l Getting to the Starting Line No what will make you a good programmer is taking the time to really under stand the world of the user and what you can do with a computer to make things better For example when I travel I often zone out on the fact that even though it looks like monopoly money foreign currency actually does amount to something in dollars I could use a computer to keep track of my budget and convert foreign currency into dollars for me Writing a program simply involves detailing the steps the computer needs to follow in a language the computer understands but lll get to that You know something like subtract the amount he just spent from the amount he started with or multiply the amount in foreign currency times the exchange rate Is it hard No not really It can be pedestrian but even more often it is fun Why a Computer Program Is Like a Peanut Butter and Jelly Sandwich At its heart yes it does have one computer programming is actually not that alien to most people If you don t believe me take the followin
7. es is often referred to as your application architec ture The data you provide to the computer can be some text or graph ics or where the hidden treasure is or the euro US dollar exchange rate 3 Run the instructions through the compiler A compiler is actually just another program albeit one that uses your instructions as data for its instructions on how to turn Objective C into computer code 4 Link the result to other precompiled modules As you will see the code you write is a relatively small part of what makes up your program The rest is made up of all the plumbing you need to run the program open and close windows and do all that user interface stuff Fortunately that code is provided for you in a form that is easy to attach link to your program A linker program takes your code identifies all the things it needs collects all pieces together from the disk and combines them into the executable program you see in your applications or utilities folder 5 Store that output somewhere You usually store the output on a hard disk but it can be anything the computer can access like punch cards 6 Run the program When you want to run the program say the user double clicks the pro gram icon the operating system Mac OS X for example which is also just another program gets the program from where it s stored and loads it into memory and then the CPU central processing unit executes the instructions
8. g program ming test Now don t peek ahead for the answer Okay The Never Fail Programming Test Write down the recipe for making a peanut butter and jelly sandwich Answer If what you wrote down looks anything like Recipe Peanut Butter and Jelly Sandwich Ingredients Peanut Butter Jelly 2 slices of bread Directions Place the two slices of bread close to each other Spread peanut butter on one slice of bread Spread jelly on the other slice of bread Put one slice of bread on top of the other then you re ready to go While this example may seem overly simple it generally illustrates what programming is all about When you write a program in Objective C all you Chapter 1 Computer Programming Exposed aaa Figure 1 1 The peanut butter and jelly program outputs data are doing is providing a set of instructions for the computer to follow The preceding example is not perfect but actually it is much closer to illustrat ing how Objective C programming works than you might think So consider ing the peanut butter and jelly sandwich example here is how you get your lunch made if you are lucky enough to have a chef 1 You give your chef the recipe 2 He or she gets the ingredients together and then follows the instruc tions on what to do with the ingredients Voila a peanut butter and jelly sandwich Figure 1 1 shows how a computer program works using the peanut butter and jelly s
9. ines for the time being return 0 Figure 1 2 shows the similarities between the program and the recipe for making a sandwich Recipe Peanut Butter and Jelly Sandwich Data Ingredients int main int argc char argv Peanut butter Jelly Variables gt char theSandwich 2 slices of bread I hate peanut butter and jelly Directions Instructions gt printf theSandwich 1 Place the two slices of bread close to each other Operation 2 Spread peanut butter on one slice of bread return 0 print the data 3 Spread jelly on the other slice of bread 4 Put one slice of bread on top of the other You can think of the following ingredients as variables that represent the data For example peanut butter is the name you give to pureed peanuts and whatever else is in peanut butter jelly the name you give to some fruit that s been processed and put in a jar and so on 14 Part l Getting to the Starting Line Peanut Butter Jelly 2 slices of bread Similarly Place the two slices of bread close to each other Spread peanut butter on one slice of bread Spread jelly on the other slice of bread Put one slice of bread on top of the other are simply instructions on how to take the ingredients and make a sandwich Spread peanut butter on one slice of bread is the instruction Actually spreading the peanut butter is the operation you are performing on the pureed peanuts being referenced by the peanut butter
10. ing Exposed 7 si Figure 1 4 Finding things in memory look inside a computer as opposed to the hard disk which is slower and permanent storage It requests the data it needs from memory processes it and writes new data back to memory millions of times every second But if the data is all in memory the CPU needs to be able to find a particular instruction or piece of data How does it do that The location in memory for each instruction and each piece of data is iden tified by an address like the mailboxes in the post office or an apartment house you see in Figure 1 4 and notice that the first address for a mailbox in your computer is always 0 But these are very small mailboxes that can hold only one character of information at a time not technically true but good enough referred to as a byte So for all practical purposes although again not technically true you can think of the smallest division of memory as a byte with each byte being able to be addressed on its own The good news is that if you need more mailboxes they are yours for the taking So if you get more than one letter a day the number of mailboxes assigned to you will increase to hold all the letters you need them to Address What Is Objective C Anyway Objective C is an object oriented programming language which means that it was created to support a certain style of programming Yes I k
11. m that displays I hate peanut butter and jelly onthe computer screen aae meuna ae ieee Clause euziepyll 4 char theSandwich I hate peanut butter and jelly printf theSandwich return 0 This program shows you how to display a line of text on your computer screen The best way to understand programming code is to take it apart line by line int main int argc char argv Dn Figure 1 2 A computer program can be com pared toa peanut but ter and jelly sandwich recipe D Chapter 1 Computer Programming Exposed 13 Ignore the first line it s not important now It just provides your program with some information it can use I ll explain exactly what that line means over the next few chapters char theSandwich I hate peanut butter and jelly theSandwich is what is known as a variable The best way to think of it for now is as a bucket that holds some kind of data I get more precise in Chapter 4 char tells you what kind of variable it is in this case theSandwich is a bunch of characters text known as a string while technically a string is more than that for now that description is good enough for our purposes I hate peanut butter and jelly is the data that the variable contains printf theSandwich printf is an instruction that tells the computer to display this is called an operation whatever data is in the theSandwich bucket You can also safely ignore the last two l
12. n application It provides support for win dows and other user interface items as well as many of the other things that are needed in most applications When you use Cocoa to develop your appli cation all you need to do is add the application s specific functionality the content and the controls and views that enable the user to access and use that content to the Cocoa framework Now two excellent books explain the use of frameworks on the Mac and iPhone One is Cocoa Programming for Mac OS X For Dummies by Erick Tejkowski The other is iPhone Application Development For Dummies by Neal Goldstein I know a shameless plug 20 Part l Getting to the Starting Line Framework or library What is the difference between a library anda application be designed divided into modules framework A library is a set of reusable func in a certain way application architecture to tions or data structures that are yours to use A use it like to think that while you use a library framework on the other hand has an architec a framework uses you ture or programming model which requires an Vour suite of development tools The two main development tools you use are Xcode and Interface Builder You ll be using Xcode throughout this book which I explain in Chapter 2 I talk a little about Interface Builder in Chapters 17 and 18 but again pick up copies of iPhone Application Development For Dummies and Cocoa Programming for
13. now it is hard to believe but even things like programming have different styles in fact a lot of them although the two heavyweights are object oriented and procedural Unless you re a dyed in the wool member of a particular camp it is really unnecessary to get into that discussion here or probably ever But you will I promise intimately understand what object oriented programming is by the time you re done with this book and you ll probably wonder why anyone would ever want to program in any other way 18 Part l Getting to the Starting Line But it takes more than a language to write a program it takes a village So who lives in the Objective C village Most object oriented development envi ronments consist of several parts 1 An object oriented programming language 1 A runtime environment 1 A framework or library of objects and functions 1 A suite of development tools This is where for many people things start to cloud up You mean I have to learn more than the language and what is all this stuff about runtime envi ronment and frameworks and libraries The answer is yes but not to worry Pll take you slowly through each part The following sections cover each part of the Objective C development environment Understanding programming languages When you write a program you write it as series of statements Some of these statements are about data You may allocate areas of memory to use for data in your p
14. o ignore both sides and get on with your development There are some things I really like about the language and others I don t but in essence it is what it is and it is what you ll use Running your program in a runtime environment One of features of Objective C is its runtime system This is one of those things that gets linked into your program in Step 4 in the section Creating a computer program It acts as a kind of operating system like the Mac or iPhone OS for an individual Objective C program It is this runtime system that is responsible for making some of the very powerful features of Objective C work Objective C s runtime environment also makes it possible to use tools like Interface Builder l explain Interface Builder in Chapters 17 and 18 to create user interfaces with a minimum of work I m all for that and after you learn about Interface Builder you will be too Using frameworks and libraries The framework you will use is called Cocoa It came along with Objective C when Apple acquired NeXT In 1996 when it was called NextSTEP I have worked in many development environments over my life and Objective C and Cocoa are hands down my favorite Cocoa allows you to write applications for Mac OS X and a version of it allows you to write applications for the iPhone If the operating system does the heavy lifting vis a vis the hardware the framework provides all the stuff you need to make your application a
15. rogram as well as describe how data is structured Other state ments are really instructions for the computer to do something Here is an example of an Objective C statement that adds together b and c and assigns the result to a and you thought you d never use all that algebra you learned in school a bec Ge Statements like these use operators like or or tell modules functions or objects to do something to or with the data For now think of functions or objects as simply a packaged series of statements that perform a task It might help to think of operators and modules as words you use to create sen tences the statements that tell the computer what to do Chapters 4 5 and 6 cover operators functions objects and modules in detail When most people want to learn how to program they usually focus on the language I want to program in C for example Or C is a real dog give me Java any day People really do become passionate about languages and believe me it is best to keep out of the way when an unstoppable force meets an immovable object What you really should keep in mind unless computer science is your life is that what you want to learn is how to create applications What makes that Chapter 1 Computer Programming Exposed 9 easy or difficult is not just the language but the application development tools available to you as well Objective C has its fans and its detractors My advice to you is t
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