Cognitive Complexity

Cognitive Complexity is a measure of how difficult a unit of code is to intuitively understand. Unlike Cyclomatic Complexity, which determines how difficult your code will be to test, Cognitive Complexity tells you how difficult your code will be to read and comprehend.

A method's cognitive complexity is based on a few simple rules:

• Code is not considered more complex when it uses shorthand that the language provides for collapsing multiple statements into one
• Code is considered more complex for each "break in the linear flow of the code"
• Code is considered more complex when "flow breaking structures are nested"

Further reading

• Cognitive Complexity docs
• Cognitive Complexity: A new way of measuring understandability

forced-type-cast

Avoid using the forced form of the type cast operator (as!) because Swift is not able to determine at compile time if the type conversion will succeed. In the event of an unsuccessful conversion, a runtime error will be triggered. The conditional form of the type cast operator (as?) is safer and should be used when possible.

Preferred

if let movie = item as? Movie {
    print("Movie: '\(movie.name)', dir. \(movie.director)")
}

Not Preferred

let movie = item as! Movie
print("Movie: '\(movie.name)', dir. \(movie.director)")

forced-type-cast

Avoid using the forced form of the type cast operator (as!) because Swift is not able to determine at compile time if the type conversion will succeed. In the event of an unsuccessful conversion, a runtime error will be triggered. The conditional form of the type cast operator (as?) is safer and should be used when possible.

Preferred

if let movie = item as? Movie {
    print("Movie: '\(movie.name)', dir. \(movie.director)")
}

Not Preferred

let movie = item as! Movie
print("Movie: '\(movie.name)', dir. \(movie.director)")

trailing-whitespace

Flag whitespace after the last non-whitespace character on each line until the newline.

Preferred

let number = 42¬

Not Preferred

let number = 42••¬

trailing-whitespace

Flag whitespace after the last non-whitespace character on each line until the newline.

Preferred

let number = 42¬

Not Preferred

let number = 42••¬

trailing-whitespace

Flag whitespace after the last non-whitespace character on each line until the newline.

Preferred

let number = 42¬

Not Preferred

let number = 42••¬

function-whitespace

Every function and method declaration should have one blank line before and after itself. An exception to this rule are functions that are declared at the start of a file (only need one blank line after their declaration) or at the end of a file (only need one blank line before their declaration). Comments immediately before a function declaration (no blank lines between them and the function) are considered to be part of the declaration.

Preferred

func function1() {
  var text = 1
  var text = 2
}

function1()

// a comment
func function2() {
  // something goes here
}

struct SomeStruct {

  func function3() {
    // something goes here
  }

  func function4() {
    // something else goes here
  };

}

func function5() {
  // something goes here
}

Not Preferred

func function1() {
  var text = 1
  var text = 2
}
function1()
// a comment
func function2() {
  // something goes here
}

struct SomeStruct {
  func function3() {
    // something goes here
  }

  func function4() {
    // something else goes here
  };
}
func function5() {
  // something goes here
}

trailing-whitespace

Flag whitespace after the last non-whitespace character on each line until the newline.

Preferred

let number = 42¬

Not Preferred

let number = 42••¬

trailing-whitespace

Flag whitespace after the last non-whitespace character on each line until the newline.

Preferred

let number = 42¬

Not Preferred

let number = 42••¬