Detect & Fix Memory Leaks in Swift

Written by Team Kodeco

A memory leak is a type of software bug that occurs when an application dynamically allocates memory but fails to deallocate it properly. This can lead to an increase in memory usage over time, resulting in poor performance and eventually causing the application to crash.

Here is an example of a simple memory leak in Swift:

class Child {
  var name: String
  var parent: Parent?
  init(name: String, parent: Parent) {
    self.name = name
    self.parent = parent
  }
  deinit {
    print("Child \(name) is being deinitialized")
  }
}

class Parent {
  var name: String
  var children: [Child] = []
  init(name: String) {
    self.name = name
  }
  func addChild(name: String) {
    let child = Child(name: name, parent: self)
    children.append(child)
  }
  deinit {
    print("Parent \(name) is being deinitialized")
  }
}

var parent: Parent? = Parent(name: "Sandy")
parent!.addChild(name: "Patrick")
parent!.addChild(name: "Emily")
parent!.addChild(name: "Joanna")
parent = nil
// No output: memory leak!

In the above example, the Parent class creates Child objects and keeps them in an array. However, when the parent object is set to nil, it doesn’t get deallocated, because the children each hold a strong reference to the parent. This results in a memory leak.

To fix this, you need to create a weak reference from the child to the parent. A weak reference does not increase the reference count of the object it references, and is set to nil when the object is deallocated.

class Child {
  weak var parent: Parent?
  var name: String
  init(name: String, parent: Parent) {
    self.name = name
    self.parent = parent
  }
  deinit {
    print("Child \(name) is being deinitialized")
  }
}

// ...

var parent: Parent? = Parent(name: "Sandy")
parent!.addChild(name: "Patrick")
parent!.addChild(name: "Emily")
parent!.addChild(name: "Joanna")
parent = nil
// Output: "Parent Sandy is being deinitialized"
//         "Child Patrick is being deinitialized"
//         "Child Emily is being deinitialized"
//         "Child Joanna is being deinitialized"

Manually scanning code to detect memory leaks can be tedious and error-prone. A more effective method is to use Xcode’s Instruments tool, specifically the Leaks instrument. This tool can automatically detect memory leaks and help you identify the source of the leaks in your code, making it easier to fix the issue.