Home > Articles > Home & Office Computing > Mac OS X

  • Print
  • + Share This
This chapter is from the book

Back in Xcode

In Xcode, you will see that Foo.h and Foo.m have been added to the project. Most programmers would put these files under the Classes group (Figure 2.27). Drag the files into the Classes group if they aren't there already.

02fig27.jpgFigure 2.27 The New Class in Xcode

If this is the first time that you are seeing Objective-C code, you may be alarmed to discover that it looks quite different from C++ or Java code. The syntax may be different, but the underlying concepts are the same. For example, in Java a class would be declared like this:

import com.megacorp.Bar;
import com.megacorp.Baz;

public class Rex extends Bar implements Baz {
...methods and instance variables...
}

This says, "The class Rex inherits from the class Bar and implements the methods declared in the Baz interface."

The analogous class in Objective-C would be declared like this:

#import <megacorp/Bar.h>
#import <megacorp/Baz.h>

@interface Rex : Bar <Baz> {
...instance variables...
}
...methods...
@end

If you know Java, Objective-C really isn't so strange. Note that like Java, Objective-C allows only single inheritance; that is, a class has only one superclass.

Types and Constants in Objective-C

Objective-C programmers use a few types that are not found in the rest of the C world.

  • id is a pointer to any type of object.

  • BOOL is the same as char, but is used as a Boolean value.

    YES is 1.

    NO is 0.

  • IBOutlet is a macro that evaluates to nothing. Ignore it. (IBOutlet is a hint to Interface Builder when it reads the declaration of a class from a .h file.)

  • IBAction is the same as void. It also acts as a hint to Interface Builder.

  • nil is the same as NULL. We use nil instead of NULL for pointers to objects.

Look at the Header File

Click on Foo.h. Study it for a moment. You should see that it declares Foo to be a subclass of NSObject. Instance variables are declared inside the curly braces.

#import <Cocoa/Cocoa.h>

@interface Foo : NSObject
{
   IBOutlet NSTextField *textField;
}
- (IBAction)generate:(id)sender;
- (IBAction)seed:(id)sender;
@end

#import is similar to the C preprocessor's #include. However, #import ensures that the file is included only once.

Notice that the declaration of the class starts with @interface. The @ symbol is not used in the C programming language. To minimize conflicts between C code and Objective-C code, Objective-C keywords are prefixed by @. Here are a few other Objective-C keywords: @end, @implementation, @class, @selector, and @encode.

In general, you will find entering code easier if you turn on syntax-aware indention. In Xcode's Preferences, select the Indentation pane. Check the box labeled Syntax-aware indenting, as shown in Figure 2.28.

02fig28.jpgFigure 2.28 The New Class in Xcode

Edit the Implementation File

Now look at Foo.m. It contains the implementations of the methods. In C++ or Java, you might implement a method something like this:

public void increment(Object sender) {
    count++;
    textField.setIntValue(count);
}

In English, you would say, "increment is a public instance method that takes one argument that is an object. The method doesn't return anything. The method increments the count instance variable and then sends the message setIntValue() to the textField object with count as an argument."

In Objective-C, the analogous method would look like this:

- (void)increment:(id)sender
{
    count++;
    [textField setIntValue:count];
}

Objective-C is a very simple language. It has no visibility specifiers: All methods are public, and all instance variables are protected. (Actually, there are visibility specifiers for instance variables, but they are rarely used. The default is protected, and that works nicely.)

In Chapter 3, we will explore Objective-C in all its beauty. For now, just copy the methods:

#import "Foo.h"

@implementation Foo

- (IBAction)generate:(id)sender
{
    // Generate a number between 1 and 100 inclusive
    int generated;
    generated = (random() % 100) + 1;

    // Ask the text field to change what it is displaying
    [textField setIntValue:generated];
}

- (IBAction)seed:(id)sender
{
    // Seed the random number generator with the time
    srandom(time(NULL));
    [textField setStringValue:@"Generator seeded"];
}

@end

(Remember that IBAction is the same as void. Neither method returns anything.)

Because Objective-C is C with a few extensions, you can call functions (such as random() and srandom()) from the standard C and Unix libraries.

Build and Run

Your application is now finished. To build and run the application, click on the hammer/green circle toolbar item (Figure 2.29). If your app is already running, the toolbar item will be disabled; quit your app before trying to run it again.

02fig29.jpg Figure 2.29

If your code has an error, the compiler's message indicating a problem will appear at the view in the upper-right corner. If you click on the message, the erroneous line of code will be selected in the view on the lower right. In Figure 2.29, the programmer has forgotten a semicolon.

Launch your application. Click the buttons and see the generated random numbers. Congratulations—you have a working Cocoa application.

awakeFromNib

Notice that your application is flawed: When the application first starts, instead of anything interesting, the words System Font Text appear in the text field. Let's fix that problem. You will make the text field display the time and date that the application started.

The nib file is a collection of objects that have been archived. When the program is launched, the objects are brought back to life before the application handles any events from the user. Notice that this mechanism is a bit unusual—most GUI builders generate source code that lays out the user interface. Instead, Interface Builder allows the developer to edit the state of the objects in the interface and save that state to a file.

After being brought to life but before any events are handled, all objects are automatically sent the message awakeFromNib. You will add an awakeFromNib method that will initialize the text field's value.

Add the awakeFromNib method to Foo.m. For now, just type it in. You will understand it later on. Briefly, you are creating an instance of NSCalendarDate that represents the current time. Then you are telling the text field to set its value to the new calendar date object:

- (void)awakeFromNib
{
    NSCalendarDate *now;
    now = [NSCalendarDate calendarDate];
    [textField setObjectValue:now];
}

The order in which the methods appear in the file is not important. Just make sure that you add them after @implementation and before @end.

You will never have to call awakeFromNib; it gets called automatically. Simply build and run your application again. You should now see the date and time when the app runs (Figure 2.30).

02fig30.jpgFigure 2.30 Completed

In Cocoa, a lot of things (like awakeFromNib) get called automatically. Some of the confusion that you may experience as you read this book will come from trying to figure out which methods you have to call and which will get called for you automatically. I'll try to make the distinction clear.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020