What is lazy property?

Ans : 

When to use lazy initialization is when the initial value for a property is not known until after the object is initialized.

For example, if you have a Person class and a personalizedGreeting property. The personalizedGreeting property can be lazily instantiated after the object is created so it can contain the name of the person.

class Person {      
  var name: String
    
  lazy var personalizedGreeting: String = {
    return "Hello, \(self.name)!"
  }()
    
  init(name: String) {
    self.name = name
  }
}

When you initialize a person, their personal greeting hasn’t been created yet:

let person = Person(name: "John Doe") // person.personalizedGreeting is nil But when you attempt to print out the personalized greeting, it’s calculated on-the-fly:

NSLog(person.personalizedGreeting)

Bonus Tip : You do need to declare your lazy property using the var keyword, not the let keyword, because constants must always have a value before initialization completes.

Benefit of lazy property increase performance in terms of speed.

Difference between thread-safe and non-thread-safe in iOS

Ans : 

Thread-Unsafe -> If any object allow to modify by more than one thread at the same time.  (non-atomic property is thread-unsafe. Comments are welcomed)

Thread-safe -> If any object not allow to modify by more than one thread at the same time.Immutable objects are generally thread-safe. (atomic property attribute type. Comments are  welcomed)

In general, immutable classes like NSArray, let are thread-safe, while their mutable variants like NSMutableArray,var are thread-unsafe.

Difference between the atomic and nonatomic attributes?

Ans : 

Atomic property give guarantee that valid value will be return. But valid does not mean that correct value.

This also not mean that atomic property are thread safe. Different threads can attempt to read and write value at same time. So one of two value will be return - value before change or changed value.

So atomic property is suffering from performance hit due to locking and unlocking before and after get or set value.

Non atomic property has no guarantee regarding correct value, a partially correct value or may be garbage value.
This is not thread safe this enhanced speed of access property.

Atomic property lock while setting value,

While non atomic property does not lock while setting value.