Answer.

Key-path (key path) is a mechanism for safe access to object properties "by reference". They provide full type-safe access for referencing properties:

struct Person {
    var name: String
    var age: Int
}

let kp = \Person.name
let john = Person(name: "John", age: 30)
print(john[keyPath: kp]) // "John"

WritableKeyPath allows not only reading but also modifying properties:

var person = Person(name: "Foo", age: 22)
let namePath: WritableKeyPath<Person, String> = \Person.name
person[keyPath: namePath] = "Bar"

Usage:

• Used for generalized work with models (e.g., during sorting, filtering, data binding with UI).
• Convenient for building user-friendly reflection without losing type safety.

Nuances:

• KeyPath is only for properties that can be accessed by the compiler.
• KeyPath cannot store methods, only properties.
• For WritableKeyPath, the object (struct) must be var.

Trick Question.

Question: "Can class properties be changed through a regular KeyPath (or is WritableKeyPath required)? What is the difference between KeyPath and ReferenceWritableKeyPath?"

Answer: To change a property value via key path, a WritableKeyPath is required. For classes, there is also ReferenceWritableKeyPath, which allows changing properties by key-path for reference types.

class User {
    var score = 0
}
let user = User()
let path: ReferenceWritableKeyPath<User, Int> = \User.score
user[keyPath: path] = 42 // OK

Examples of real errors due to lack of knowledge of the nuances of the topic.

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Story

In a large table-driven UI project, regular KeyPath and WritableKeyPath were confused — an attempt was made to change a model property via KeyPath, resulting in a runtime exception.

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Story

When generating generic forms, KeyPath was used to bind values, but there was an attempt to reflect methods as keypaths, which led to compilation failure and code complexity.

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Story

A developer forgot to add ReferenceWritableKeyPath for classes, which hindered bidirectional binding between UI and the data model: values were updated in the UI, but the model remained unchanged.