面试题与答案

iOS stands for "iPhone Operating System".

It is the operating system for Apple devices, and it is considered the second most popular mobile operating system globally after Android.

This operating system powers many of Apple's products including the iPhone, iPad, and iPod. iOS is widely praised for its intuitive and user-friendly interface.

• The iPhone offers multitasking capabilities. On an iOS device, you can easily switch between apps using the multitasking feature or a multi-finger gesture.
• iOS helps you easily integrate social network interactions with your app by displaying an activity stream and sharing content.
• Apple's iCloud service allows users to store data on the Internet. It offers a high level of encryption and a backup option to ensure the user does not lose data.
• Apple's in-app purchases are available on all platforms, offering users additional services and materials including digital items (iOS, iPad, macOS), subscriptions, and premium content.
• You can see all of your app alerts in the Notification Center in iOS. However, the notification settings can be modified.
• iOS is a closed system. The source code of Apple's apps isn't available for developers, and iPhone and iPad owners can't modify the code on their devices. This makes iOS-powered devices harder to hack.

iOS operates in a Layered structure.

iOS Architecture is comprised of four layers, each of which offers a programming framework for creating applications that operate on top of the hardware. Communication will be enhanced by the layers between the Application Layer and the Hardware Layer. A lower-level layer provides the services that all applications require, while an upper-level layer (or high-level layer) provides graphics and interface-related services.

Properties are basically values that are associated with a class, struct, or enum.  They can be thought of as "sub-variables," i.e., parts of another object.

Example:

struct Building  {    

    var name: String = ""

}

var apartment = Building()

apartment.name = "Prestige"

In the above code, we created a structure called Building. One of its properties is called name, a String whose initial value is empty. 

Stored Properties: This type of property can be used to store constant or variable values as instances and is usually provided by classes and structures.

class Programmer {    

    var progName: String    

    let progLanguage: String    

    var totalExperience = 0    

    var secondLanguage: String?

}

Computed properties: These properties can be used to calculate values instead of storing them and are usually provided by classes, enumerations, and structures.

struct Angle {    

    var degrees: Double = 0    

    var rads: Double {        

        get {            

             return degrees * .pi / 180        

        }        

        set(newRads) {            

              degrees = newRads * 180 / .pi        

        }    

    }

}

Atomic Property: It is the default property and ensures a valid value will be returned from the getter or set by the setter. This ensures that only one thread can access the getter/setter of a given property at a time and that all other threads must wait until the first thread releases the getter/setter. Despite being thread-safe, it is not fast, since it ensures that the process is completely completed.

Non-Atomic Property: With non-atomic properties, multiple threads can access the getter/setter method of a given property at the same time, so the potential for inconsistency between values exists. They come with enhanced access, but no guarantee of the return value.

Android: It is the mobile operating system for Android devices offered by Google LLC (limited liability company) and is focused on touchscreen mobile devices like smartphones and tablets. Several programming languages were used in its development, including C, Java, C++, and others. 

iOS: It is the operating system for Apple devices offered by Apple incorporation and it is considered the second most popular mobile operating system globally after Android. It is primarily designed for Apple mobile devices like the iPhone, iPod Touch, etc. Several programming languages were used in its development, including Objective-C, Swift, C++, and others. 

For more differences, please check here.

Programming languages used for iOS development are:

• HTML5
• .NET
• C
• Swift
• Javascript
• Objective-C

Objects are mainly the variables that are of class types. It can be a function, method, data structure, or variable.

UIKIT framework. It renders drawing models, windows, event handling, and views.

 It is used to add a set of related methods and to add additional methods in the Cocoa framework.

The process of adding new operators and changing existing ones to do various things is known as operator overloading.

+, *, and / symbols are known as operators.

The visual elements that we can see in our applications are known as UI elements. Some of these components, such as buttons and text fields, respond to user interactions, while others, such as images and labels, provide information.

Objective-C is the primary programming language for writing applications for OS X and iOS. It’s an object-oriented programming language with a dynamic runtime that’s a superset of the C programming language. Objective-C takes C’s syntax, primitive types, and flow control statements and adds class and process definition syntax.

NSObject -> UIResponder -> UIView -> UIControl -> UIButton. 

MVC (Model View Controller) is a design pattern that defines how logic will be separated when the user interface is implemented. In iOS, UIView is the base class provided by Apple for all views, and UIViewController is provided to support the Controller, which listens to events in a View and updates it when data changes.

The methods to achieve concurrency are:

• Dispatch queues
• Threads
• Operation Queues

It is used for building software codes to run on iOS for the iPad and iPhone. Cocoa Touch is written in the objective-C language and has a different set of graphical control elements to Cocoa.

iBeacon enables mobile apps to listen to signals from beacons in the physical world and respond accordingly. It comprises two parts: a broadcaster and a receiver.

The process of changing the implementation of an existing domain, Method Swizzling, enables you to write that code that can be executed before or after the original method.

Viewdidload:

• It is called when it is loaded into memory.
• Data can be loaded in Viewdidload if it is fairly static and likely to change.

Viewdidappear:

• It is called when the view is visible and presented on the device.
• You can perform any layout functions or draw in the UI.

• SBJson framework is supported by iOS. It is a JSON parser and generator for Objective-C (Objective-C is the primary programming language we use when writing software for OS X and iOS. It is a superset of the C programming language and provides object-oriented capabilities and a dynamic runtime).
• SBJson provides flexible APIs and additional control that makes JSON handling easy.

• Synchronous can also be defined as In order. When you perform the synchronous operation, all that follows must wait for the operation to complete before proceeding.
• In contrast, “asynchronous” can also be defined as “out of order.” When you do something asynchronously, you can run the following code right away, and the asynchronous process will happen someday. It could be run on a separate thread from the rest of the code. It could easily be rescheduled on the same thread at a later date and you can notify you when it is done.

Here are a few common ways to specify the layout of elements in UIView:

• Using InterfaceBuilder, we can add a XIB file to our project, layout elements within it, and then load XIB in our application code (either automatically, based on naming conventions, or manually). Also, using InterfaceBuilder, we can create a storyboard for our application.
• We can write our own code to use NSLayoutConstraints and have elements in a view arranged by Auto Layout.
• We can create CGRects describing the exact coordinates for each element and pass them to UIView’s (id)initWithFrame:(CGRect)frame method.

When only one instance of a class is created in the application, that class is called a singleton class.

A UIView’s bounds are a rectangle with a size (width, height) and position (x,y) relative to its own coordinate system (0,0).

A UIView’s frame is a rectangle with a scale (width, height) and position (x,y) relative to the superview it is located within.

To prioritize usability, the design process has broken down into 4 steps:

• Consider the user’s perspective when designing the user experience.
• Users, not demographics, are what you should focus on.
• Consider all of the scenarios in which an app might be useful when promoting it.
• Even after the app has been released, continue to improve its functionality.

A class type containing a group of related items under the same umbrella, but it is impossible to create an instance of it.

When the property is called for the first time, an initial value of the lazy stored properties is calculated. In many situations, lazy properties come handy to developers. 

The bundle ID is specified in Xcode, and it defines each App. A single project can have multiple targets and can output multiple apps. Use: it is branded multiple ways and has both free/lite and full/pro versions.

App ID is used to identify one or more apps from a single development team. It is a two-part string with a period(.) separating Team ID and bundle IF search string. The bundle ID search string is supplied by the developer, while Apple supplies the Team ID.  

Before launch, an app is not said to be running. After a brief transition through the inactive state, it moves to the background or the active state when it is launched. 

The certificate types are:

1. Development Certificate
2. Distribution Certificate 
3. Development and Distributing Certificates 

In iOS, ‘assign’ is used to create an orientation from one object to the other without raising the retain count of the source object.

The ‘reuseIdentifier’ is used to group all the similar rows from UITableView.

The three different methods to reduce app size are:

1. Bit code
2. On-demand Resource
3. App Slicing 

The 'defer' keyword provides a block of code executed in cases when execution leaves the current scope. 

KVO means key-value observing, which enables a controller to observe changes to property value.

Unless it’s stated, use UIKit classes only from your application’s main thread or main dispatch queue. This restriction applies in particular to classes derived from UIResponder or that require modifying the user interface of your app in some way.

TVMLKit serves as a bridge between TVML, JavaScript, and your native tvOS software. You can test TVMLKit JS and TVML files from inside your tvOS app using the TVMLKit framework. The JavaScript environment can be used to build TVML objects, styles, views, and view controllers.

Swift is a modern programming language created by Apple for iOS, OS X, watchOS, and tvOS apps that combines the best of C and Objective-C, but without the C compatibility issues. Swift follows secure programming patterns while also incorporating modern features to make programming simpler, more versatile, and enjoyable. Swift is welcoming to novice programmers and feels familiar with Objective-C developers.

The NSError class is a Cocoa class. The knowledge about an error condition is encapsulated in an extendable, object-oriented manner by an NSError object. It includes a predefined error domain, a domain-specific error code, and a user details dictionary with application-specific data.

• KVC (Key-Value Coding) is a method for accessing an object’s properties using strings at runtime rather than needing to know the property names statically at development time.
• KVO (Key-Value Observing) allows a controller or class to monitor changes in a property value. In KVO, an object may request to be informed of any adjustments to a particular property, and the observer is automatically notified if that property’s value changes.

Retain creates a reference from one object to another and increases the retain count of the source object.

Assign creates a reference from one object to another without increasing the source’s retain count.

At runtime, Dynamic Dispatch determines which implementation of a polymorphic procedure, such as a method or a function, to call. This means that when we want to call our methods, such as object methods, we must use this syntax. Swift, on the other hand, does not use dynamic dispatch by default.

The GCD stands for Grand Central Dispatch. It is a low-level API that allows you to manage multiple concurrent operations. It will assist you in increasing the responsiveness of your app by deferring computationally intensive tasks to the context. It’s a simpler concurrency model than locks and threads to deal with.

• To handle all of the work, only a small amount of code is needed.
• Available in both Object C and Swift.
• SQLite and Core Data have slower performance.
• Database files can be shared easily between iOS and Android devices.
• There is no fee, no charge.
• There is no limit to the data amount that can be stored.
• Regardless of huge data sets or massive storage, consistent speed and consistency

SpriteKit is a platform for creating animated 2D objects quickly and easily.

SceneKit is a platform for 3D graphics rendering that was inherited from OS X.

SpriteKit, SceneKit, and Metal are expected to boost a new generation of mobile games that push the boundaries of what the powerful GPUs in iOS devices can do.

The benefits are:

1. There are no nested if let statements
2. An early exit out of the function using return or using break
3. Guard statement safely unwrap optionals. 

Both of these methods are present in AppDelegate.swift file and are used to add functionality to the App when the App is going to be launched. 

A ResponderChain is a hierarchy of objects that have the chance of responding to the received events.