What is encapsulation, and how does Swift's access control support it?

Answer: Encapsulation means bundling data with the operations that act on it, and restricting direct access to that data so internal invariants can't be broken from outside. Swift enforces this through access control levels rather than getter/setter conventions.

Level	Visibility
private	Within the enclosing declaration (and extensions in the same file)
fileprivate	Anywhere in the same file
internal (default)	Anywhere in the same module
public	Any module, but not overridable/subclassable outside
open	Any module, including override/subclass

class BankAccount {
    private(set) var balance: Decimal = 0   // readable outside, writable only inside

    func deposit(_ amount: Decimal) {
        guard amount > 0 else { return }    // invariant protected here
        balance += amount
    }
}

let account = BankAccount()
account.deposit(100)
// account.balance = 1_000_000  // ❌ compile error — can't bypass deposit()