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What things in Swift don't exist in Objective-C?
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Date created
Mar 1, 2020
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Section 1
(50 cards)
What things in Swift don't exist in Objective-C?
Type inference, operator overloading, protocol extensions, string interpolation, namespaces, tuples, optionals, playgrounds, guard, defer, enums with associated values
When do we use NSArray and NSMutableArray?
"NSArray creates static arrays that cannot be mutated. NSMutableArray creates arrays that can be mutated. "
Why are Objective-C based classes often prefixed with letters like XCTestCase, MKMapView?
"Objective-C has no namespacing. Where Swift allows two frameworks to declare classes with the same name, as they can be namespaced such as Framework1.MyClass and Framework2.MyClass, Objective-C cannot achieve this. This means identifiers have to be globally unique. Hence frequent prefixed letters."
What is a global autoreleasepool?
At any time a global autoreleasepool may pick up pointers when, say, receiving objects from methods but not allocating or creating. The global release pool can be drained from time to time. In Cocoa, it's after an NSEvent such as tap. When the release pool drains, references drop which can cause deallocation.
Explain types of protocol.
"Formal protocols act like Swift protocols except you can have optional methods. Otherwise a class must adopt the necessary methods. And subclasses inherit the protocol conformance. Informal protocols are categories on NSObject, which means that most Objects adopt them. Callers of informal protocol methods check that the instance implements it. "
What does @ mean?
It means treat everything coming up as Objective-C not C code. As it's not used in C or C++ as an identifier it's a way to strictly use Objective-C constructs instead, such as using Objective-C string type not C.
What is the responder chain?
"The responder chain is the chain of responders listening for UI events. For example, if you tap a UITextField it becomes the first responder. UIKit dynamically handles events and passes them to the first responder, if the first responder doesn't handle the event it passes it to the next responder. The predefined rules of UIKit tends to forward events from one view to its superview. "
What happens before Objective-C code is built?
Pre-processing. The pre-processer is used for header files and macros (which act like Swift constants but can be more complicated)
What is the use of category in Objective-C?
"A category is like a Swift extension. It extends the functionality of an existing class and you can add your own methods to it. Methods are inherited by subclasses and at like original methods of the class. They can be used to divide code clearly. The disadvantage is you cannot override methods from the original class or other categories. "
What is KVC and KVO? Give an example of using KVC to set value.
"KVC - Key Value Coding allows you to access the property of an object using a specific string. In Objective-C you can use valueForKeyPath to get properties. KVO - Key Value Observing is observing the properties of a given object and responding to those changes. In Objective-C you use addObserver forKeyPath to obsere properties. "
Does Objective-C have function overloading?
No
Explain the difference between atomic and nonatomic synthesized properties.
"Atomic is default Atomic is thread-safe. It means the synthesised getters and setters of a property guarantee a whole value will be returned, i.e. a thread calling a getter while another thread calls the setter, will wait for the set to finish. Nonatomic is not thread-safe but quicker. It means you could get garbage data returned. Ok to use if you can guarantee single thread acess."
What is atomic and non-atomic in Objective C and which one is considered to be a default?
"Atomic is default Atomic is thread-safe. It means the synthesised getters and setters of a property guarantee a whole value will be returned, i.e. a thread calling a getter while another thread calls the setter, will wait for the set to finish. Nonatomic is not thread-safe but quicker. It means you could get garbage data returned. Ok to use if you can guarantee single thread acess."
What should you not do when managing memory?
Dealloc explicitly, as deallocation should be managed by dropping reference count. If you dealloc manually you may end up with dangling pointers
What does dispatch_once do? What is it used for?
It guarantees that a block will only be executed once in the lifetime of the app. It is useful for initialisation of global create singletons in app.
What are declared properties in Objective-C?
"Declared properties are properties declared using the @property identifier. They are properties with getter and setter methods already implemented under the hood. You can set attritibutes such as read or write only, atomic or nonatomic, and strong, weak, retain, assign, copy"
Explain what is @synthesize in Objective-C
"Synthesize is used against properties in Objective-C in order to get automatic getter and setter for the underlying instance variable. If you declare a property in Objective-C Xcode automatically provides a instance varable with the same but a preceding _. For example, @property NString *name gives a property name, getter and setter to the underlying ivar that has the name ""_name"" Adding synthesize is unecssary unless you need custom getters and setters to your instance variable"
Explain when to use NSArray and NSMutableArray. Which one is faster and threadsafe?
"NSArray creates static arrays that cannot be mutated NSMutableArray creates arrays that can be mutated. It is not thread-safe and you have to implement a lock to get thread-safety. Speed is about the same however NSArray has fixed memory size whereas mutable array can change and take more memory. "
What are blocks and how are they used?
Blocks are the equivalent of Swift closures. They are independent blocks of code that can be executed and passed around. They are a first step to asynchronous programming in that you can pass a block of code to an operation that may happen asynchronously, then when that ends the block will be executed.
What is the difference between #import and #include in Objective-C
Import makes sure a file is only imported once, whereas include can include files multiple times.
What is the difference when importing between quotation marks and angular brackets?
"Quotation marks search you local files. Angular brackets search the system files."
Hoe does message work in Objective C?
Message is like method in Swift. You send messages not call methods.
What happens to optionals in Objective-C?
There is no concept of optionals in Objective-C. If you call a nil, nothing happens, so it's worthing checking things aren't null.
How do you manage memory in Objective-C?
"You can enable Automatic Reference Counting, just like Swift. Alternatively you manage it manually, in which case you must follow some rules. When creating you can alloc, copy or new - to create memory for the this object and increment it's reference count. When retain something you increment it's reference count. If you own references you must release them when finished with them.
Mention whether NSObject is a parent class or derived class.
"NSObject is a root object from which all other classes should inherit. Other objects expect to have acces to basic NSObject functionality on other objects, such as init, copy, or description. "
Variables are _ by default and require the keyword _ to be constant?
variable, const, constant
What does @autoreleasepool mean?
I'm going to be allocating lots of memory and when I am done please free it up.
What is instanceType?
Instancetype is a keyword that means `an instance of this class will be returned`. It was introduced to plug problems where initialisers were returning `id`, which is a general pointer to any swift object, that can break type safety. As the compiler won't warn about id being not the type you expected.
What does * do?
It indicates that a thing is a pointer to something in memory
Switches differ to swift in what way?
They have implicit fallthrough meaning you need to break everytime
What is fast enumeration?
It's pretty much a for loop in swift, where you loop through sequence and do things on each element.
What would %@ do in an string in Objective-C?
It is a format specifier. It takes an object and passes into that string to be shown, logged or printed. Remember Objective-C has no String Interpolation?
Is it possible to use ARC and Non-ARC code together in a project?
Yes, you can say to Xcode use ARC, but there is a flag you can set for particular classes or segments of code where you want to manage your own memory
What is the purpose of the .h file in Objective-C?
"This is the Header file. When creating classes in Objective-C you create both a header file and an implementation file. The header file exposes the interface. The implementation file contains the code. This means people can ship header files with `.a` files that contain the actual compiled code, without sharing implementation"
Explain what is #import.
Ensures a file is only ever imported once so you don't have a probeem with recursive imports
What is id?
"id is a pointer to any object in Objective-C. It was originally used to be the data type returned when initialising certain data types like dictionaries and arrays. You can call anything on it, string methods, array methods etc. and the compiler won't mind because of this. Which breaks type safety. Id was also the return type for some initialisers. InstanceType came to replace this so that initialisers could return their actual class type rather than id. "
Can you write setter method for a retain property?
Yes, if you want to write your custom setting logic.
Explain how to call function in Objective-C.
"Square bracket, Class, method name, (plus parameters if needed) close square bracket. Can use self in place of class to call own instance methods"
Is Objective C, a dynamic language? True/False, explain.
"It is dynamic. It is based on sending messages not calling methods. At runtime, the dispatcher sends and messages and only needs to know if the object responded. This means you can inject methods and swizzle methods at runtime. In contrast, Swift is static and compiler needs to know before build what objects can do what methods. @Objc provides a way to gain the dynamism. "
What safety risks does Objective-C present?
"No concept of optionals. Can force one data type into another. Can read array values that don't exist. Switches don't have to be exhaustive. Most things are variables rather than constants"
What are the characteristics of a category?
"A category is like a Swift extension. It extends the functionality of an existing class and you can add your own methods to it. Methods are inherited by subclasses and at like original methods of the class. They can be used to divide code clearly. The disadvantage is you cannot override methods from the original class or other categories. "
Explain what is protocol in Objective-C.
"Protocols are contracts that classes must conform to if they want to implement them. They allow us to abstract the implementation to detail and just worry about the interface. We can swap underlying instances while only exposing the protocol. Classes can conform to multiple protocols. Good for mocking"
What are NSAutoreleasePool? When to use them?
"An autorelease pool allows you to add objects to the pool and mark them as autorelease. There reference count is 1 until the pool is drained such as at the end of the runloop for the pool. They can be used to keep objects in memory while you may setup other objects to retain those objects."
Why is explicitly checking reference count bad?
Because autoreleasepool references don't count
What is delegate? Can delegates be retained?
"Delegates are objects that act on behalf or in coordination with another object when an event happens. For example, a UITextField has a delegate, it tells the delegate if text has started editing or finished editing so that it can do something. Delgates can be retained but typically aren't and are declared as weak to avoid strong reference cycles. Reason being often, object a owns object b, and object b delegates back to object a. So if they strongly referenced each other they'd never be release from memory. "
What does alloc mean?
It means allocate some memory for something
What's the difference between #import <UIKit/UIKit.h> and #import "MyClass.h"?
"The first says search system libraries and import this. The second says search system libraries and my own project and import this. "
Does a thread created using performSelectorInBackground:withObject: creates its own autorelease pool?
No, if you call performSelectorInBackground you spawn your own thread that you have to manage and configure. Such as creating your own autoreleasepool
Explain what dot notation is in Objective-C
"It is syntatic sugar for calling getters and setters of properties. For example, a class Person with property Name could be grabbed with person.name"
What does # mean?
It's a preprocesser directive, telling the preprocessor to do something. For example, it's used to import required headers such as Apple's Foundation
Section 2
(22 cards)