无法使用Kotlin和Dagger注入多重绑定

时间:2019-03-29 08:03:10

标签: kotlin dagger-2

我有以下定义:

@Module
class WeaverDataModule {
    // Provide the three pumps from providers
    // All of them still explicitly mark 'Pump' as their return type
    @Provides @IntoSet fun providesPump(thermosiphon: Thermosiphon) : Pump = thermosiphon
    @Provides @IntoSet fun providesAnotherPump(suctionBased: SuctionBased) : Pump = suctionBased
    @Provides @IntoSet fun providesGenericPump(genericPump: GenericPump) : Pump = genericPump
}

@Component(modules = [WeaverDataModule::class])
interface WeaverData {
    // Get the CoffeeMaker
    fun coffeeMaker(): CoffeeMaker
    // Get the list of pumps
    fun getPumps() : Set<Pump>
}

interface Pump

// The three pumps
class Thermosiphon @Inject constructor(val heater: Heater) : Pump
class SuctionBased @Inject constructor() : Pump
class GenericPump @Inject constructor() : Pump

// Some random heater
class Heater @Inject constructor()

在我的代码中,当我执行以下操作时:

val cm = DaggerWeaverData.builder().build().getPumps()

我确实得到了预期的三个泵。但是,当我尝试将其注入其他类时:

class CoffeeMaker @Inject constructor(
    private val heater: Heater,
    private val pump: Set<Pump>
) {
    fun makeCoffee() =
        "Making coffee with heater ${heater::class.java} and using pumps" +
                " ${pump.map { it::class.java }.joinToString(",")}"
}

我收到以下错误:

e: .../WeaverData.java:7: error: [Dagger/MissingBinding] java.util.Set<? extends weaver.Pump> cannot be provided without an @Provides-annotated method.                    
public abstract interface WeaverData {
                ^
      java.util.Set<? extends weaver.Pump> is injected at
          weaver.CoffeeMaker(…, pump)
      weaver.CoffeeMaker is provided at
          weaver.WeaverData.coffeeMaker()

我也尝试注入Collection<Pump>,但仍然遇到类似的错误。在dagger docs on multibinding中,示例(在Java中)显示以下内容:

class Bar {
  @Inject Bar(Set<String> strings) {
    assert strings.contains("ABC");
    assert strings.contains("DEF");
    assert strings.contains("GHI");
  }
}

这正是我在做什么。对于基于构造函数的注入,它在Kotlin中运行良好,因为以下代码可以按预期编译并运行:

class CoffeeMaker @Inject constructor(
    private val heater: Heater
) {
    fun makeCoffee() =
        "Making coffee with heater ${heater::class.java}"
}

所以我对如何使这种多重绑定起作用感到困惑。

1 个答案:

答案 0 :(得分:0)

因此,您需要做的是:

class CoffeeMaker @Inject constructor(
    private val heater: Heater,
    private val pumps: Set<@JvmSuppressWildcards Pump>
) {
    fun makeCoffee() =
        "Making coffee with heater ${heater::class.java} with pumps ${pumps.map { it::class.java }.joinToString(",")}"
}

这是因为Set在Kotlin中定义为Set<out E>,而Java则将其翻译为Set<? extends Pump>。从类型理论的角度来看,Set<? extends Pump>Set<Pump>不同,因此Dagger(可能)拒绝将Set<Pump>视为Set<? extends Pump>的注射剂,这是公平而正确的行为。

我们遇到的问题是,对于这些集合中的任何一个,由于默认情况下它们都是不可变的,因此类型为Set<X>的声明将转换为Set<? extends X>,因为不可变的集合仅具有对已解析的引用。收益类型,因此是协变的。为了验证该理论,以下内容也适用:

class CoffeeMaker @Inject constructor(
    private val heater: Heater,
    private val pumps: MutableSet<Pump>
) {
    fun makeCoffee() =
        "Making coffee with heater ${heater::class.java} with pumps ${pumps.map { it::class.java }.joinToString(",")}"
}

请注意MutableSet的使用,其定义为MutableSet<E> : Set<E> ...。这可能不是我们应该使用的东西,因为我怀疑这个集合实际上是可变的。因此,我们需要的是kotlin编译器将Set<out E>视为Set<E>(在这种情况下,可分配性是有效的,反之则不行)。为此,我们使用@JvmSuppressWildcards批注。我希望这可以帮助其他面临类似问题的人。