第4章 协程挂起函数Mock：coEvery与coVerify

挂起函数（suspend function）是Kotlin协程的核心单元，但在单元测试中直接调用挂起函数需要协程作用域，且传统Mock框架无法处理挂起函数的协程上下文。MockK提供了coEvery和coVerify两个扩展函数，专门针对suspend函数进行打桩（stub）和验证（verify），自动处理协程的挂起与恢复，无需手动构造Continuation实例。

4.1 打桩suspend函数：coEvery

使用coEvery对挂起函数设置预期行为，包括正常返回值或抛出异常。所有配置必须在coEvery块内完成。

基本语法

coEvery { mockObject.suspendMethod(args) } returns expectedValue
coEvery { mockObject.suspendMethod(args) } throws SomeException()
coEvery { mockObject.suspendMethod(args) } answers { arg1 + arg2 }

完整示例

interface UserRepository {
    suspend fun fetchUser(id: Int): User
    suspend fun deleteUser(id: Int)
}

class UserService(private val repo: UserRepository) {
    suspend fun getUserName(id: Int): String {
        val user = repo.fetchUser(id)
        return user.name
    }
}

@Test
fun `test coEvery returns`() = runBlockingTest {
    val mockRepo = mockk<UserRepository>()
    val user = User(1, "Alice")
    coEvery { mockRepo.fetchUser(1) } returns user

    val service = UserService(mockRepo)
    val name = service.getUserName(1)
    assertEquals("Alice", name)
}

@Test
fun `test coEvery throws`() = runBlockingTest {
    val mockRepo = mockk<UserRepository>()
    coEvery { mockRepo.fetchUser(any()) } throws RuntimeException("DB error")

    val service = UserService(mockRepo)
    assertFailsWith<RuntimeException> { service.getUserName(1) }
}

4.2 验证suspend函数调用：coVerify

coVerify用于断言挂起函数是否被调用、调用次数及参数，必须在协程作用域（如runBlockingTest）内执行。

常用验证模式

coVerify { mockObject.suspendMethod(1) }                          // 至少调用一次
coVerify(exactly = 2) { mockObject.suspendMethod(1) }            // 精确次数
coVerify(atLeast = 1, atMost = 3) { mockObject.suspendMethod(1) }
coVerify(inverse = true) { mockObject.suspendMethod(any()) }     // 从未调用

验证顺序与参数匹配

coVerifySequence {
    mockRepo.fetchUser(1)
    mockRepo.deleteUser(1)
}

coVerify {
    mockRepo.fetchUser(match { it > 0 })
}

4.3 coEvery与coVerify功能对比

4.4 挂起函数Mock完整流程

下图展示了从定义接口、创建Mock、打桩、执行到验证的协程测试路径：

4.5 常见错误与调试

1. “No overloads for ‘coEvery’ with these arguments”
   原因：目标函数不是suspend函数。检查函数声明中是否包含suspend关键字。
2. “coVerify requires coroutine context”
   解决方法：将断言移动到runBlockingTest或runBlocking块内。
3. “验证通过但实际并未调用”
   常见于coEvery中使用了any()但验证时用精确参数。使用coVerify { mock.suspendFun(any()) }匹配任意参数。
4. 挂起函数返回值类型为Unit时：coEvery { mock.suspendVoid() } just Runs 表示正常结束。

4.6 实战：完整用例（异常+验证）

data class Order(val id: Int, val amount: Double)
interface PaymentGateway {
    suspend fun charge(order: Order): Boolean
}

class PaymentService(private val gateway: PaymentGateway) {
    suspend fun processOrder(orderId: Int, amount: Double): Boolean {
        val order = Order(orderId, amount)
        return gateway.charge(order)
    }
}

@Test
fun `test payment fails and verify`() = runBlockingTest {
    val mockGateway = mockk<PaymentGateway>()
    coEvery { mockGateway.charge(any()) } throws IllegalStateException("Insufficient funds")

    val service = PaymentService(mockGateway)
    assertFailsWith<IllegalStateException> { service.processOrder(1, 100.0) }

    coVerify(exactly = 1) { mockGateway.charge(Order(1, 100.0)) }
}

@Test
fun `test payment success and verify multiple invocations`() = runBlockingTest {
    val mockGateway = mockk<PaymentGateway>()
    coEvery { mockGateway.charge(any()) } returns true

    val service = PaymentService(mockGateway)
    val result1 = service.processOrder(1, 50.0)
    val result2 = service.processOrder(2, 75.0)

    assertTrue(result1)
    assertTrue(result2)
    coVerify(exactly = 2) { mockGateway.charge(any()) }
}