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• Threads : Threads are the independent code of execution within the same program. With the help of threads, multiple parts of an application can be executed simultaneously without affecting the speed of execution of the rest of the application.

• Dispatch Queues : Dispatch Queues handle the tasks in First-In-First-Out(FIFO) order. It helps in executing tasks concurrently and asynchronously.

• Operation Queues : Operation Queues is an object-oriented way of managing concurrency. They are built on the topmost layer of Grand Control Dispatch (GCD) system,  providing a high-level abstraction among queueing models. Operation Queues are invoked according to the priorities and queues. 

An Optional is a type in Swift which takes care of how to handle NIL values in Swift. In other words, an Optional can hold either any value or may contain NIL value. 

To create an Optional in Swift, add a question mark (?) after any data type -

var number: Int? = 10

Swift provide the concept of defer statements to execute program code just before transferring control outside the scope in which the defer statements are declared. The statements inside the defer get executed independent of program execution. 

Swift provides the concept of lazy property which is a property which does not compute its initial value until the first time it is used. 

The lazy property must always be declared with the var keyword as its initial value is obtained only after the intialization of an instance is completed.

Grand Central Dispatch (GCD) is a low-level Application Programming Interface(API) designed to execute tasks in multicore hardware environment. Tasks like pulling data from the network, displaying the information on the UI, reading instructions from the users when executed together make the system heavy which might lead to system halt. GCD handles multiple tasks by managing a “DispatchQueue” which is tied to the application’s main thread. The DispatchQueue executes the tasks asynchronously and concurrently in the order in which they are added to the queue.

Automatic Reference Counting (ARC) is used for applications’ memory management. ARC is like the Automatic Garbage Collection of Java, which initializes memory to apps when needed and deallocates it when it no longer requires it. ARC keeps a record of how many constants, variables, and properties that each class object has. When there is at least one active reference to a class object, ARC will not deallocate that instance. 

Along with managing static typing, Swift also has the support for rich metadata of the types, which provides the facility to inspect and manipulate the values arbitrarily at run-time. Swift provides a Mirror initializer which accepts a random value. As a result, the Mirror instance provides Swift freelance developers with the information about the value it contains and also about its children. A child contains an optional value and a label. Mirror can then be used on child values to successfully travers the whole object graph without knowing about the types at compile-time.

Mirror also provides the custom representation in accordance with CustomReflectable protocol. For example, A Dictionary uses it to display its key-value pairs as the labeled children while an Array uses Mirror to display its children as unlabeled children.

The core difference between weak self and unowned self is that the weak reference is optional while the unowned reference is non-optional. The weak self can have nil value inside the closure at some point of time while if access to the unowned reference containing NIL value is made, then the whole program will be crashed.

However, both weak references and unowned references don't increase the retain counts of the objects they are referring.

If you are trying to hire a Swift  Developer. Here is the Swift Developer Job Description that you can use in your hiring.