What is the memory management mechanism in Swift? How to avoid retain cycles?

What is the memory management mechanism in Swift? What is ARC (Automatic Reference Counting)? How to avoid retain cycles?

Swift uses Automatic Reference Counting (ARC) to automatically manage application memory. ARC tracks and manages the memory used by your application, automatically freeing memory when instances are no longer needed.

How ARC Works:

• Each time a new reference to a class instance is created, ARC increases the instance's reference count
• When a reference is removed, ARC decreases the reference count
• When the reference count reaches zero, ARC frees the instance's memory
• ARC only applies to class type instances; structs and enums are value types and don't participate in reference counting

Strong Reference Cycles:

• Two or more class instances hold strong references to each other, causing reference counts to never reach zero
• Common scenarios: mutual references between classes, closures capturing class instances
• Example:

  swift
  class Person {
      var apartment: Apartment?
  }
  
  class Apartment {
      var tenant: Person?
  }
  
  let person = Person()
  let apartment = Apartment()
  person.apartment = apartment
  apartment.tenant = person

Methods to Resolve Retain Cycles:

1. Weak References:

   • Declared using weak keyword
   • Does not increase reference count
   • Weak references automatically become nil when the referenced object is deallocated
   • Must be declared as optional type
   • Suitable when the reference can be nil
   • Example:

     swift
     class Apartment {
         weak var tenant: Person?
     }

2. Unowned References:

   • Declared using unowned keyword
   • Does not increase reference count
   • Unowned references don't automatically become nil when the referenced object is deallocated
   • Cannot be declared as optional type
   • Suitable when the referenced object has a longer lifetime
   • Example:

     swift
     class Customer {
         let creditCard: CreditCard
         init(creditCard: CreditCard) {
             self.creditCard = creditCard
         }
     }
     
     class CreditCard {
         unowned let customer: Customer
     }

3. Retain Cycles in Closures:

   • Use capture lists [weak self] or [unowned self]
   • Example:

     swift
     class HTMLElement {
         let name: String
         lazy var asHTML: () -> String = { [weak self] in
             guard let self = self else { return "" }
             return "<\(self.name)>"
         }
     }

Best Practices:

1. Use weak or unowned in class properties to avoid strong reference cycles
2. Use capture lists in closures to handle retain cycles
3. Use weak when the reference can be nil
4. Use unowned when the referenced object has a longer lifetime
5. Use Instruments tool to detect memory leaks