Defining Actors in Swift with Protocols

actor-protocol.swift

protocol Actor : AnyObject, Sendable {
  associatedtype State
  
  isolated var state: State
}

struct BankAccountState {
  var balance: Double
  
  mutating func deposit(amount: Double) {
    assert(amount >= 0)
    balance = balance + amount
  }
  
  mutating func withdraw(amount: Double) throws {
    assert(amount >= 0)
    if amount > state.balance {
      throw BankError.insufficientFunds
    }
    balance = balance - amount
  }
}

class BankAccount : Actor {
  let accountNumber: Int
  var state: BankAccountState
  
  init(accountNumber: Int, initialDeposit: Double) {
    self.accountNumber = accountNumber
    self.state = BankAccountState(balance: initialDeposit)
  }
  
  async var balance: Double { await state.balance }
  
  mutating func transfer(amount: Double, to other: BankAccount) async throws {
    // Safe: this operation is the only one that has access to the actor's isolated
    // state right now, and there have not been any suspension points between
    // the place where we checked for sufficient funds and here.
    try state.withdraw(amount: amount)

    print("Transferring \(amount) from \(accountNumber) to \(other.accountNumber)")

    // Safe: the deposit operation is placed in the `other` actor's mailbox; when
    // that actor retrieves the operation from its mailbox to execute it, the
    // other account's balance will get updated.
    await other.deposit(amount: amount)
  }
  
  mutating func deposit(amount: Double) {
    state.deposit(amount: amount)
  }
}

var bankAccountA = BankAccount(accountNumber: 7, initialDeposit: 200)
var bankAccountB = BankAccount(accountNumber: 42, initialDeposit: 200)

await bankAccountA.deposit(amount: 200)
await bankAccountA.transfer(amount: 100, to: bankAccountB)