Deinitialization

Deinitialization in Swift allows an instance of a class to free up any resources it has assigned before the instance is deallocated. Structures and enumerations are value types in Swift and are deallocated automatically and don’t use deinitializers. Only class instances have deinitializers.

Syntax

Deinitializers are written using the deinit keyword, similar to how initializers are written using the init keyword. They don't take any parameters and cannot be called directly. Instead, deinitializers are automatically called right before an instance is deallocated. You can't override a superclass's deinitializer.

class SomeClass {
   // Initializers, properties, methods go here
   deinit {
       // Perform the deinitialization
   }
}

Example

Consider a class that opens a file when it's created and writes to it when it's destroyed:

class FileWriter {
   var file: String
   
   init(filename: String) {
       file = filename
       // code to create or open the file
       print("\(file) opened.")
   }
   
   deinit {
       // code to save or close the file
       print("\(file) closed.")
   }
}
// Create an optional FileWriter instance
var writer: FileWriter? = FileWriter(filename: "data.txt")
// ... do some operations
// When setting the instance to nil, the deinitializer will be called
writer = nil  // Outputs: "data.txt closed."

In the above example, the FileWriter class has a deinitializer that gets called when the instance is about to be deallocated, ensuring that the file is properly closed.

Automatic Reference Counting (ARC)

Swift uses Automatic Reference Counting (ARC) to keep track of and manage your app’s memory usage. When an instance of a class is no longer referred to by any strong references, Swift automatically deallocates it, and right before that, the deinit method is called.

However, it's crucial to understand that cycles of strong references can prevent instances from being deallocated. These cycles can be broken using weak or unowned references. It's a common use case where deinitializers come into play, especially when dealing with listeners, observers, or other types of resources that need to be manually released.

Tips

• Only one deinit per class is allowed.
• Superclass deinitializers are inherited by their subclasses and are automatically called by Swift, even if a subclass doesn't provide its own deinitializer.
• Because the superclass deinitializer is called automatically, there's no need to call super.deinit() in your deinitializer.

Overall, deinitializers are a crucial part of resource management in Swift. They allow you to handle cleanup tasks before a class instance is deallocated, ensuring that system resources like files, network connections, or even memory are used efficiently.