stubFinal.swift

// Globally defined helper function that injects a spy
// to record all stub function calls along with their arguments 
func spyCalls<I,O>(of stub: inout (I) -> (O)) -> (ArgRecords<I>) {
 // data structure to track all args
 let argRecords = ArgRecords<I>()
 // keep previously used stub function
 let orginalStub = stub
 // override stub implementation (inout stub argument)
 stub = { [weak weakArgRecords = argRecords] i in
   weakArgRecords?.record(i) // store arg to the data structure
   return orginalStub(i) // call original body implementation
 }
 return argRecords 
}

// Stub declaraion
class DatabaseStub: Database {    
  lazy var addUserAction = niceStub(of: addUser)  
  func addUser(name: String) -> Bool {     
    return addUserAction(name)    
  }
}

// Sample testcase
func testAddingUser_savesToDatabase() {
  // Arrange - introduce addUser's spy
  let addUserArgs = spyCalls(&database.addUserAction)
  // Act
  database.addUser(name: "myuser") // returns defaultly `false`
  // Assert
  XCTAssertEqual(addUserArgs[0], "myuser") // ✅
}