1. EEC-492/693/793 iPhone Application Development
Lecture 6
Wenbing Zhao & Nigamanth Sridhar
12/1/2018
EEC492/693/793 - iPhone Application Development

2. Outline More in-depth Objective-C Control Fun App (part III)
Objective lifecycle
autorelease
Control Fun App (part III)
Objective-C part is shamelessly taken from Stanford CS193P winter 2010 lecture 3
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EEC492/693/793 - iPhone Application Development
EEC492/693/793 - iPhone Application Development 2

3. EEC492/693/793 - iPhone Application Development
Object Lifecycle
Creating objects
Memory management
Destroying objects
12/1/2018
EEC492/693/793 - iPhone Application Development

4. EEC492/693/793 - iPhone Application Development
Object Creation
Two step process
allocate memory to store the object
initialize object state
+ alloc
Class method that knows how much memory is needed
- init
Instance method to set initial values, perform other setup
Create = Allocate + Initialize
Person *person = nil;
person = [[Person alloc] init];
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EEC492/693/793 - iPhone Application Development

5. Implementing Your own -init Method
#import “Person.h”
@implementation Person
-(id)init {
// allow superclass to initialize its state first
if (self = [super init]) {
age = 0;
name
// do other initialization... }
return self;
@end
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EEC492/693/793 - iPhone Application Development

6. EEC492/693/793 - iPhone Application Development
Multiple init Methods
Classes may define multiple init methods
- (id)init;
- (id)initWithName:(NSString *)name;
- (id)initWithName:(NSString *)name age:(int)age;
Less specific ones typically call more specific with default values
- (id)init {
return [self Name”]; }
- (id)initWithName:(NSString *)name {
return [self initWithName:name age:0];
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EEC492/693/793 - iPhone Application Development

7. Finishing Up With an Object
Person *person = nil;
person = [[Person alloc] init];
[person Jones”];
[person setAge:32];
[person castBallot];
[person doSomethingElse];
// What do we do with person when we’re done?
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EEC492/693/793 - iPhone Application Development

8. EEC492/693/793 - iPhone Application Development
Memory Management
Allocation
Destruction C
Malloc
Free
Objective-C
Alloc
dealloc
Calls must be balanced, otherwise, your program may leak or crash
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EEC492/693/793 - iPhone Application Development

9. EEC492/693/793 - iPhone Application Development
Reference Counting
Every object has a retain count
Defined on NSObject
As long as retain count is > 0, object is alive and valid
+alloc and -copy create objects with retain count of 1
-retain increments retain count
-release decrements retain count
When retain count reaches 0, object is destroyed
-dealloc method invoked automatically
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EEC492/693/793 - iPhone Application Development

10. EEC492/693/793 - iPhone Application Development
Balanced Calls
Person *person = nil;
person = [[Person alloc] init];
[person Jones”];
[person setAge:32];
[person castBallot];
[person doSomethingElse];
// When we’re done with person, release it
[person release]; // person will be destroyed here
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EEC492/693/793 - iPhone Application Development

11. Reference Counting in Action
Person *person = [[Person alloc] init];
Retain count begins at 1 with +alloc
[person retain];
Retain count increases to 2 with -retain
[person release];
Retain count decreases to 1 with -release
Retain count decreases to 0, -dealloc automatically called
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EEC492/693/793 - iPhone Application Development

12. Messaging deallocated objects
Person *person = [[Person alloc] init];
// ...
[person release]; // Object is deallocated
person = nil;
[person doSomething]; // No effect
Tuesday,
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EEC492/693/793 - iPhone Application Development

13. Implementing a -dealloc method
#import Person
(void)dealloc { // Do any cleanup that’s necessary // ...
// when we’re done, call super to clean us up [super dealloc]; }
@end
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EEC492/693/793 - iPhone Application Development

14. Object Lifecycle Recap
Objects begin with a retain count of 1
Increase and decrease with -retain and -release
When retain count reaches 0, object deallocated automatically
You never call dealloc explicitly in your code
Exception is calling -[super dealloc]
You only deal with alloc, copy, retain, release
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EEC492/693/793 - iPhone Application Development

15. EEC492/693/793 - iPhone Application Development
Object Ownership
#import <Foundation/Foundation.h>
@interface Person: NSObject {
// instance variables
NSString *name; // Person class “owns” the name
int age; }
// method declarations
- (NSString *)name;
- (void)setName:(NSString *)value;
- (int)age;
- (void)setAge:(int)age;
- (BOOL)canLegallyVote;
- (void)castBallot;
@end
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EEC492/693/793 - iPhone Application Development

16. EEC492/693/793 - iPhone Application Development
Object Ownership
#import "Person.h“
@implementation Person
- (NSString *)name {
return name; }
(void)setName:(NSString *)newName {
if (name != newName) {
[name release]; // if we don’t do this, we would lose the reference
// to the object pointed to by name after the assignment, which would
// cause a memory leak
name = [newName retain]; // Person is taking ownership of newName
// name’s retain count has been bumped up by 1
@end
There are many alternatives to the implementation provided above
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EEC492/693/793 - iPhone Application Development

17. Releasing Instance Variables
#import “Person.h”
@implementation Person
- (void)dealloc {
// Do any cleanup that’s necessary
[name release];
// when we’re done, call super to clean us up
[super dealloc]; }
@end
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EEC492/693/793 - iPhone Application Development

18. Returning a Newly Created Object
Attempt #1
Not ideal: caller must remember to call release on the NSString after using it
- (NSString *)fullName {
NSString *result;
result = [[NSString alloc]
firstName, lastName];
return result; }
12/1/2018
EEC492/693/793 - iPhone Application Development

19. Returning a Newly Created Object
Attempt #2
Wrong: result is release too early! Method would return bogus value
- (NSString *)fullName {
NSString *result;
result = [[NSString alloc]
firstName, lastName];
[result release];
return result; }
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EEC492/693/793 - iPhone Application Development

20. Returning a Newly Created Object
Attempt #3
Just right: result is released, but not right away
Caller gets valid object and could retain if needed
- (NSString *)fullName {
NSString *result;
result = [[NSString alloc]
firstName, lastName];
[result autorelease];
return result; }
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EEC492/693/793 - iPhone Application Development

21. Autoreleasing Objects
Calling -autorelease flags an object to be sent release at some point in the future
Let you fulfill your retain/release obligations while allowing an object some additional time to live
Makes it much more convenient to manage memory
Very useful in methods which return a newly created object
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EEC492/693/793 - iPhone Application Development

22. Method Names & Autorelease
Methods whose names includes alloc, copy, or new return a retained object that the caller needs to release
NSMutableString *string = [[NSMutableString alloc] init];
// We are responsible for calling -release or -autorelease
[string autorelease];
All other methods return autoreleased objects
NSMutableString *string = [NSMutableString string];
// The method name doesn’t indicate that we need to release it
// So don’t- we’re cool!
This is a convention- follow it in methods you define!
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EEC492/693/793 - iPhone Application Development

23. How does -autorelease work?
Object is added to current autorelease pool
Autorelease pools track objects scheduled to be released
When the pool itself is released, it sends -release to all its objects
UIKit automatically wraps a pool around every event dispatch
Autorelease is not garbage collection
Objective-C on iPhone OS does not have garbage collection
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EEC492/693/793 - iPhone Application Development

24. Autorelease Pools (in pictures)
[object release];
Pool
Pool
Pool created
Pool
Pool
Pool
Objectives autoreleased
Here go into pool
[object release];
Pool created
Pool created
Pool created
Pool created
Pool created
Pool created
Pool created
Pool created
Pool created
[object
autorelease];
[object release];
Pool released
Pool created
Pool created
Pool created
Pool created
Launch app
App initialized
Load main nib
Handle event
Exit app
Wait for event
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EEC492/693/793 - iPhone Application Development

25. Hanging Onto an Autoreleased Object
Many methods return autoreleased objects
Remember the naming conventions...
They’re hanging out in the pool and will get released later
If you need to hold onto those objects you need to retain them
Bumps up the retain count before the release happens
name = [NSMutableString string];
// We want the name to remain valid!
[name retain];
// ...
// Eventually, we’ll release it (maybe in our -dealloc?)
[name release];
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EEC492/693/793 - iPhone Application Development

26. Control Fun App (Part III)
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EEC492/693/793 - iPhone Application Development

27. Implementing the Action Sheet and Alert
Action sheets are used to force the user to make a choice between two or more items
The user is unable to continue using the app until they have tapped one of the buttons
Action sheets are often used to confirm a potentially dangerous or irreversible action
A view that forces the user to make a choice before they are allowed to continue using their app is known as a modal view
Alerts also force users to respond before they are allowed to continue using their app
Alerts are used more to inform the user that something important or unusual has occurred
Often present a single button: an alert is not used to solicit a choice
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EEC492/693/793 - iPhone Application Development

28. Showing the Action Sheet
In Control_FunViewController.m, the button’s action method:
- (IBAction)buttonPressed {
UIActionSheet *actionSheet = [[UIActionSheet alloc]
you sure?"
delegate:self // our view controller itself is the delegate
Way!"
I'm Sure!"
otherButtonTitles:nil]; // only two buttons shown
[actionSheet showInView:self.view]; // the view is the parent of actionSheet
[actionSheet release]; }
Why self.view: because view is private instance variable, and it must be accessed via the accessor
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EEC492/693/793 - iPhone Application Development

29. Conforming to the Action Sheet Delegate Method
Action sheets and alerts both use delegates so that they know what object to notify when the user has responded
In the Control Fun app, we want the UIViewController object to be notified when the action sheet is dismissed
Our controller, therefore, would act as the delegate
It must conform to a protocol called UIActionSheetDelegate
Should declare action method in header file:
- (void)actionSheet:(UIActionSheet *)actionSheet
didDismissWithButtonIndex:(NSInteger)buttonIndex
@interface Control_FunViewController : UIViewController <UIActionSheetDelegate> {
UITextField *nameField;
UITextField *numberField;
UILabel *sliderLabel;
...
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EEC492/693/793 - iPhone Application Development

30. Implementing the Protocol method
- (void)actionSheet:(UIActionSheet *)actionSheet
didDismissWithButtonIndex:(NSInteger)buttonIndex {
if (buttonIndex != [actionSheet cancelButtonIndex]) {
NSString *msg = nil;
if (nameField.text.length > 0)
msg = [[NSString alloc] initWithFormat:
@"You can breathe easy, everything went OK.", nameField.text];
else
msg can breathe easy, everything went OK.";
UIAlertView *alert = [[UIAlertView alloc] was done"
message:msg
delegate:self
otherButtonTitles:nil];
[alert show]; [alert release]; [msg release]; }
Alerts, unlike action sheets, are not tied to a particular view, so we just tell the alert to show
itself without specifying a parent view.
Add this method to Control_FunViewController.m
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EEC492/693/793 - iPhone Application Development

31. EEC492/693/793 - iPhone Application Development
Spiffing up the Button
Most buttons you see on your iPhone are drawn using images
You can specify a kind of template image that the iPhone will use when drawing your buttons
Offered in the Apple UICatalog sample code:
Go to UICatalog app’s Images subfolder, and add blueButton.png and whiteButton.png to the Control Fun project
Go to Interface Builder, single-click the Do Something button, and open the attributes inspector
Use 1st pop-up menu to change the type to Custom
More work to be done in Xcode
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EEC492/693/793 - iPhone Application Development

32. EEC492/693/793 - iPhone Application Development
Spiffing up the Button
Build a custom button using the viewDidLoad method
The code sets the background image for the button, and while being touched, it change from white image to blue image
Go to Xcode, add viewDidLoad: method to Control_FunViewController.m
- (void)viewDidLoad {
UIImage *buttonImageNormal = [UIImage
UIImage *stretchableButtonImageNormal = [buttonImageNormal
stretchableImageWithLeftCapWidth:12 topCapHeight:0];
[doSomethingButton setBackgroundImage:stretchableButtonImageNormal
forState:UIControlStateNormal];
UIImage *buttonImagePressed = [UIImage
UIImage *stretchableButtonImagePressed = [buttonImagePressed
[doSomethingButton setBackgroundImage:stretchableButtonImagePressed
forState:UIControlStateHighlighted]; }
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EEC492/693/793 - iPhone Application Development

33. Control States and Stretchable Images
Every UI control has four control states:
Normal: default state
Highlighted: when it is currently being used
For a button, this would be while the user has a finger on the button
Disabled: it has been turned off (by unchecking the Enabled checkbox in the inspector
Selected: this control is turned on or selected
It can continue to be selected when the user is no longer directly using the control
Not all controls support this state
Stretchable images
Resizable image that knows how to resize itself intelligently (maintains correct appearance)
End caps should not be resized If
we had specified the button image right in Interface Builder, it would resize the entire image
evenly, and our button would look weird at most sizes.
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EEC492/693/793 - iPhone Application Development

34. EEC492/693/793 - iPhone Application Development
Finishing Up
Add viewDidUnload: method to Control_FunViewController.m to free up outlets when the view is unloaded
Save, build and run!
- (void)viewDidUnload {
self.nameField = nil; // release the old object and retain nil
self.numberField = nil;
self.sliderLabel = nil;
self.leftSwitch = nil;
self.rightSwitch = nil;
self.doSomethingButton = nil;
[super viewDidUnload]; }
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EEC492/693/793 - iPhone Application Development