Chapter 3. Coding in Objective-C

published book

This chapter covers

Creating classes to match your designs
Declaring and implementing messages and properties
Connecting code to views in Interface Builder

In the last chapter, you learned how to go from ideas to object-oriented designs. In this chapter, you’ll learn how to code those designs. By iteratively applying the concepts of sketching, designing, and coding, you’ll eventually have an app.

Figure 3.1. Sketch, design, code

To finish an app, you’ll go through this loop many times, refining it on each pass. You already know how to create designs by sending messages to classes that are organized into models, views, and controllers, so you’re halfway there.

Representing those designs as code is the most exacting of the three skills you need to create an app. Your notebook won’t complain if it doesn’t like your sketch or design, but Xcode will put up error after error if you don’t get the syntax exactly right. In reading this chapter, pay attention to every detail about creating classes, messages, and properties.

Creating classes to match your designs

In your designs, you spent a lot of time thinking about what the classes should be, what properties they should have, and what messages they need to respond to and send. You’ve also learned that classes can be in different kinds of relationships with each other. In Objective-C, you’ll see each piece of the class in two places, the header and the module, which together define the parts of a class.

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#import <UIKit/UIKit.h>
@interface HWViewController : UIViewController {
}
@end

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Declaring a message

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Table 3.1. Common classes and built-in types

Type	Description
NSString*	Text values like @"Hello" and @"Good-bye"
NSDate*	Dates like July 4, 1776
int	Positive and negative integer numbers, like -1, 0, and 5
bool	YES or NO
double	Positive and negative decimal numbers like 3.14159, 0.0, and -123.45
void	Message return values that indicate there isn’t a return value
NSMutableArray*	A collection of any number of other objects, such as a list of strings or dates
UIButton*	A button on a view
UILabel*	A label, like the one you used in the Hello World! app

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Figure 3.2. Message with no parameters or return

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Figure 3.3. Message with no parameters and an `int` return

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Figure 3.4. Message with a string parameter

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Figure 3.5. Message with multiple parameters

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Table 3.2. Declare these messages

Message description	Declaration
1. A message that returns the first name of a person object as a string
2. A message that returns how old someone is, given a birth date
3. A message that takes the name of a parent and returns a list of their children
4. A message that takes a string and returns the number of letters in it
5. A message that takes a number and returns whether it’s positive or not
6. A message that takes two yes/no values and returns yes only if the values match each other, and no otherwise
7. A message that takes a number and returns the date that is that number of days in the future
8. A message that takes two strings a state, and a capital, and returns yes if the capital is the capital of that state
9. A message that takes a first name and a last name and returns a string with them separated by a space
10. A message that takes a string and shows an alert containing the string

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-(NSString*) getFirstName;
-(double) getAge: (NSDate*)birthdate;
-(NSMutableArray*) getChildrenOfParent: (NSString*)parent;
-(int) getNumLetters: (NSString*)string;
-(bool) isPositive: (int)number;
-(bool) doesBool: (bool)a matchBool:(bool)b;
-(NSDate*) getFutureDate: (int)days;
-(bool) isCapital:(NSString*)capital ofState:(NSString*)state;
-(NSString*) getFullNameFromFirstName: (NSString*)firstName lastName: (NSString*)lastName;
-(void) showAlert:(NSString*) text;

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Figure 3.6. What errors you’ll get for each change

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Declaring a view-controller message for your views to send

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-(IBAction) actionMessage: (id)sender;

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Using properties to save data in objects

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Figure 3.7. Syntax for a field (instance variable)

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Figure 3.8. Syntax for a property

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Figure 3.9. Syntax for an `IBOutlet`

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Connecting code to views in Interface Builder

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Figure 3.10. Sketch of new Hello app

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Figure 3.11. Parts of the view

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Figure 3.12. The parts of the view that change

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Using the Connections Inspector in Interface Builder

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Figure 3.13. Say Hello view

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Figure 3.14. Check box to hide views at start

Creating outlets and actions using the assistant

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Figure 3.15. Xcode’s assistant

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Figure 3.16. Control-click and drag to create outlet

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Figure 3.17. Name the outlet

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@property (weak, nonatomic) IBOutlet UITextField *userNameTextField;
@property (weak, nonatomic) IBOutlet UILabel *userNameLabel;
@property (weak, nonatomic) IBOutlet UILabel *helloLabel;

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Figure 3.18. Control-click and drag to create action

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Figure 3.19. Set the connection type and name

Xcode will insert the following message declaration in your header:

-(IBAction) sayHello: (id)sender;

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-(IBAction) sayHello: (id)sender
{
}

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Defining the action message

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You want the sayHello message to do three things:

Figure 3.20. What the `sayHello` message does

To do that, you need to

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Figure 3.21. Property syntax

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Figure 3.22. Syntax for accessing a property of a property

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-(IBAction) sayHello: (id)sender
{
    self.userNameLabel.text = self.userNameTextField.text;
    self.helloLabel.hidden = NO;
    self.userNameLabel.hidden = NO;
}

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