rozeridilar/Objective-C-Basics

Objective-C-Basics

Belows are some notes from objc.

Cocoa Touch Class = .m + .h files.

• ViewController.m -> implementation file
• ViewController.h -> header file
• ViewController.swift is the mix of both above.

.h files

A class that has the definition of the methods.

• + sign before adding methods declared that method is static.
• - is the method that can be accessed via class instances.

Below is an example of NSString class's header file:

+ (instancetype)localizedStringWithFormat:(NSString *)format, ... NS_FORMAT_FUNCTION(1,2);
- (nullable instancetype)initWithCString:(const char *)nullTerminatedCString encoding:(NSStringEncoding)encoding;

Access Modifiers

• If you want to show your property in public you should add your property in .h files, like example below:

ViewController.h file:

// MARK: - My import statements
#import <UIKit/UIKit.h>

@interface ViewController : UIViewController

//Add my property in public
@property (nonatomic, strong) NSString *name;

@end

• If you want your property inside of the class and not available outside of the class you should implement it in .m file like below:

ViewController.m file:

//MARK: - Import Statements
#import "ViewController.h"

@interface ViewController ()

@property (nonatomic, strong) NSString *privateName;

@end

@implementation ViewController

//MARK: - LifeCycles

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
}

//MARK: - Actions
- (IBAction)clickSampleButtonAction:(id)sender {
}


@end

• Calling properties in many ways:

    //Call the name property in many ways:
    NSLog(@"Name: %@", self.name);
    self.name = @"Rozeri";
    NSLog(@"Name: %@", self.name);
    _name = @"Dilar";
    NSLog(@"Name: %@", self.name);
    [self setName:@"Rozeri Dilar"];
    NSLog(@"Name: %@", _name);

Basically call with _name and set with [self setName: ]

.m files

Has the implementation of the methods.

Importing other files

Unlike swift obj-C requires imports to reach other classes. To import another obj-c class in your file you should add that classes .h file into your .m file. For further explanation see how ViewController.m imported Person file.

Proporties & Instance Variables

• Properties are public and global.

  • it is also an instance variable under the hood.
  • it also creates getter&setter methods automatically.

• Instance Variables/iVars are private, not accesible by other classes.

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface Person : NSObject
{
    //Below variables are instance variables and are not publics/cannot be accessed via other classes.
    BOOL *hasTwoCats;
}

//Below variables are properties and can be accessed via other classes.
@property (nonatomic, strong) NSString *firstName;
@property (nonatomic, strong) NSString *lastName;

@end

NS_ASSUME_NONNULL_END

Getters & Setters

When creating properties,

• if you do not want to set your property in every case you should declare them explicitly in .m files by overriding the set methods like below.

Person.h:

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface Person : NSObject

@property(nonatomic) NSUInteger age;

@end

NS_ASSUME_NONNULL_END

Person.m:

@implementation Person

-(void) setAge:(NSUInteger)age{
    //Here --age-- is the parameter that is the newValue in Swift.
    // --_age-- is the instance variable that is our actual variable.
    
    //I will increase age only if the new age param is greater than the actual one. Otherwise, do not do anything.
    if(age > _age){
        _age = age;
    }
}
@end

Calling the above example:

    Person *rozeri = [[Person alloc] init];
    rozeri.age = 27;
    
    //Below I want to make the age lesser which is impossible.
    [rozeri setAge:15];
    NSLog(@"Age: %lu", rozeri.age); //Age will return 27.

• Lets say you want to return something explicitly when some user calls your property's getter,

For below example, there is this person, NSObject instance, that only returns "Rozeri" as a name. Even the developer calls set method, it cannot be changed.

Person.h:

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface Person : NSObject

@property (nonatomic, strong) NSString *name;

@end

NS_ASSUME_NONNULL_END

**Person.m: **

#import "Person.h"

@implementation Person

//Whenever other class's calls .name it will only return "Rozeri". Except if it is already "Rozeri Dilar".
-(NSString*) name{
    if([_name isEqualToString:@"Rozeri Dilar"]){
        return @"Rozeri Dilar";
    }
    return @"Rozeri";
}

@end

Calling the above example:

    Person *rozeri = [[Person alloc] init];

    [rozeri setName:@"Some Other Name"];
    NSLog(@"Name: %@", rozeri.name); //Name will return "Rozeri".
    
    [rozeri setName:@"Rozeri Dilar"];
    NSLog(@"Name: %@", rozeri.name); //Name will return "Rozeri Dilar".

Pointers

Pointer basically points a location in memory on your device.

• You will use pointers when accessing class objects.(You can also reach primitive types's pointers too, but it is better to ease this process by only using pointers in class objects. Because in Objective-C you can use native types without dealing with pointers.)

ViewController.h:

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController

@property (nonatomic, strong) NSString *name; // Use pointer when dealing with class objects.
@property (nonatomic) int *pointerAge; // I do not need this, since I can reach native primitive types without dealing with pointers.
@property (nonatomic) int age; // use like this.
@property (nonatomic) NSInteger money; //NSInteger is just like a traditional int in C. It's a typedef. There are others like NSUInteger, CGFloat, etc. that all are synonyms for primitive types.
@property (nonatomic) NSNumber *bankBalance;//NSNumber is useful when you need to stick a number into an NSArray or NSDictionary. The standard practice is to use these collections versus rolling your own; the drawback is that they can only contain Objective-C objects.

@end

ViewController.m:

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    //To call name value, since it is a pointer we must allocate an instance in the memory.
    self.name = [[NSString alloc] init];
    self.name = @"";
    
    NSArray *myArray = [[NSArray alloc] init];
    [myArray arrayByAddingObject:3];//will not compile.
    [myArray arrayByAddingObject:[NSNumber numberWithInt:3]];//ok
    [myArray arrayByAddingObject:self.bankBalance];//ok
}


@end

Importance on ARC: When using pointers, you are creating instances via allocating from memory, so once you are done with it, you must release it from the memory.

• NSInteger is just like a traditional int in C. It's a typedef. There are others like NSUInteger, CGFloat, etc. that all are synonyms for primitive types.

• NSNumber is useful when you need to stick a number into an NSArray or NSDictionary. The standard practice is to use these collections versus rolling your own; the drawback is that they can only contain Objective-C objects.

• NSNumber essentially wraps an int (or float, etc) into an Obj-C object (similar to C#/Java's concept of 'boxing' a primitive type) that can be added to an array.

• For performance reasons, if you can, use primitive types (like int, float, int[]). However, sometimes you cannot avoid NSArray/NSNumber, such as when you are reading or writing entries into a .plist file.

https://stackoverflow.com/questions/1285098/whats-the-difference-between-nsnumber-and-nsinteger

Objective-C Strings

Lets dive into some definitions for Strings:

• A literal is a value, which is immutable by definition.

• A constant is a read-only variable or pointer.

Example:

 const int age = 27;
 age = 25;//Cannot assign to variable 'age' with const-qualified type 'const int'

• A string literal is a expression like @"". The compiler will replace this with an instance of NSString.

Example:

    NSString *name = @"Rozeri";
    name = @"Dilar";
    
    NSLog(@"Name is: %@", name); // prints: Name is: Dilar

• A string constant is a read-only pointer to NSString.

    NSString *const firstName = @"Rozeri";
    firstName = "Dilar";//Cannot assign to variable 'firstName' with const-qualified type 'NSString *const __strong'

• All @"" expressions are indeed immutable. They are replaced at compile time with instances of NSConstantString, which is a specialized subclass of NSString with a fixed memory layout. This also explains why NSString is the only object that can be initialized at compile time.

https://stackoverflow.com/a/12327650/10616887

Below are some NSString usage examples such as formatting string, appending, comparisons:

    NSString *lastName = @"Rozeri";
    lastName = @"Dilar";
    
    NSLog(@"Last Name is: %@", lastName); // prints: Last Name is: Dilar
    
    //Conceting a String
    NSString *firstName = @"Rozeri";
    NSString *fullName = [NSString stringWithFormat:@"%@ Dilar", firstName];
    NSLog(@"FullName: %@%@", fullName, @"."); // prints: FullName: Rozeri Dilar.
    
    //Lets remove the . in the NSLog above and add this to another variable.
    NSString *lastSentence = [fullName stringByAppendingString:@"."];
    NSLog(@"FullName: %@", lastSentence); // prints: FullName: Rozeri Dilar.
    
    //Comparisons:
    NSString *str1 = @"Some Example";
    NSString *str2 = @"Some Other Example";
    NSString *str3 = @"some other example"; //lower case of str2

    if (![str1 isEqualToString:str2]){
        NSLog(@"str1 is equal to str2.");//This will run. --> Attention on --!-- in if statement.
    }else{
        NSLog(@"str1 is NOT equal to str2.");
    }
    
    if ([str3.lowercaseString isEqualToString: str2.lowercaseString]){
        NSLog(@"str3 is equal to str2 in LOWERCASE.");//will print
    }
    
    if ([str2 caseInsensitiveCompare:str3] == NSOrderedSame){
        NSLog(@"str3 is equal to str2 in LOWERCASE.");//will print
    }

Atomic & Non-Atomic Properties

atomic will ALWAYS guarantee

• If two different people want to read and write at the same time, your paper won't just burn! --> Your application will never crash, even in a race condition.
• If one person is trying to write and has only written 4 of the 8 letters to write, then no can read in the middle, the reading can only be done when all 8 letters is written --> No read(get) will happen on 'a thread that is still writing', i.e. if there are 8 bytes to bytes to be written, and only 4 bytes are written——up to that moment, you are not allowed to read from it. But since I said it won't crash then it would read from the value of an autoreleased object.
• If before writing you have erased that which was previously written on paper and then someone wants to read you can still read. How? You will be reading from something similar to Mac OS Trash bin ( as Trash bin is not still 100% erased…it's in a limbo) ---> If ThreadA is to read while ThreadB has already deallocated to write, you would get a value from either the final fully written value by ThreadB or get something from autorelease pool.
• Retain counts are the way in which memory is managed in Objective-C. When you create an object, it has a retain count of 1. When you send an object a retain message, its retain count is incremented by 1. When you send an object a release message, its retain count is decremented by 1. When you send an object an autorelease message, its retain count is decremented by 1 at some stage in the future. If an objectʼs retain count is reduced to 0, it is deallocated.

Atomic doesn't guarantee thread safety, though it's useful for achieving thread safety. Thread Safety is relative to how you write your code/ which thread queue you are reading/writing from. It only guarantees non - crushable multithreading.

nonatomic

• Since there is no such thing like Mac OS Trash Bin, then nobody cares whether or not you always get a value (<-- This could potentially lead to a crash), nor anybody cares if someone tries to read halfway through your writing (although halfway writing in memory is very different from halfway writing on paper, on memory it could give you a crazy stupid value from before, while on paper you only see half of what's been written) --> Doesn't guarantee to not crash, because it doesn't use autorelease mechanism.
• Doesn't guarantee full written values to be read!
• Is faster than atomic

https://stackoverflow.com/a/36920240/10616887

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