避免滥用 Kotlin 扩展函数

fun String.isPhoneNumber(): Boolean =
    length == 7 && all { it.isDigit() }

fun isPhoneNumber(`$this$: String): Boolean =
    $this$.length == 7 && $this`.all { it.isDigit() }

interface PhoneBook {
    fun String.isPhoneNumber(): Boolean
}
class Fizz : PhoneBook {
    override fun String.isPhoneNumber(): Boolean =
        this.length == 7 && this.all { it.isDigit() }
}

// Bad practice, do not do this
class PhoneBookIncorrect {
    fun verify(number: String): Boolean {
        require(number.isPhoneNumber())
        // ...
    }
    // ...
    fun String.isPhoneNumber(): Boolean =
        this.length == 7 && this.all { it.isDigit() }
}

PhoneBookIncorrect().apply {
    "1234567890".isPhoneNumber()
}

class PhoneBook {
    fun verify(number: String): Boolean {
        require(number.isPhoneNumber())
        // ...
    }
    // ...
}
// This is how we limit extension functions visibility
private fun String.isPhoneNumber(): Boolean =
    this.length == 7 && this.all { it.isDigit() }

class PhoneBook(
    private val phoneNumberVerifier: PhoneNumberVerifier
) {
    fun verify(number: String): Boolean {
        require(number.let(::isPhoneNumber))
    }
    private fun isPhoneNumber(number: String): Boolean =
        phoneNumberVerifier.verify(number)
}

val ref = String::isPhoneNumber
val str = "1234567890"
val boundedRef = str::isPhoneNumber
val refX = PhoneBookIncorrect::isPhoneNumber // ERROR
val book = PhoneBookIncorrect()
val boundedRefX = book::isPhoneNumber // ERROR

class A {
    val a = 10
}
class B {
    val a = 20
    val b = 30
    fun A.test() = a + b // Is it 40 or 50?
}

class A {
    //...
}
class B {
    //...
    fun A.update() ... // Does it update A or B?
}
复制代

class OrderUseCase(
    // ...
) {
    // ...
    private fun CoroutineScope.produceOrders() =
        produce<Order> {
            var page = 0
            do {
                val orders = api
                    .requestOrders(page = page++)
                    .orEmpty()
                for (order in orders) send(order)
            } while (orders.isNotEmpty())
        }
}
interface UserApiTrait {
    fun TestApplicationEngine.requestRegisterUser(
        token: String,
        request: RegisterUserRequest
    ): UserJson? = ...
    fun TestApplicationEngine.requestGetUserSelf(
        token: String
    ): UserJson? = ...
    // ...
}