Kotlin 使用Dagger 2提供函数依赖性

发布于01月06日

我想使用Dagger 2提供一个函数作为依赖项:

@Module
class DatabaseModule {

    @Provides
    @Singleton
    fun provideDatabase(application: Application, betaFilter: (BetaFilterable) -> Boolean): Database {
        return Database(application, BuildConfig.VERSION_CODE, betaFilter)
    }

    @Provides
    @Suppress("ConstantConditionIf")
    fun provideBetaFiler(): (BetaFilterable) -> Boolean {
        return if (BuildConfig.FLAVOR_audience == "regular") {
            { it.betaOnly.not() }
        } else {
            { true }
        }
    }

}

不幸的是，它似乎不起作用:

[dagger.android.AndroidInjector.inject(T)] kotlin.jvm.functions.Function1<? 
super com.app.data.BetaFilterable,java.lang.Boolean> 
cannot be provided without an @Provides-annotated method.

What am I missing here?

推荐答案

Yes, this can be done in Kotlin.

You need to add @JvmSuppressWildcards at the injection site to ensure the signature matches. (source)

为了验证这一点，我写了以下内容:

import dagger.Component
import dagger.Module
import dagger.Provides
import javax.inject.Singleton

class G constructor(val function: Function1<Int, Boolean>)

@Singleton
@Module
class ModuleB {
    @Provides
    fun intToBoolean(): (Int) -> Boolean {
        return { it == 2 }
    }
    @JvmSuppressWildcards
    @Provides fun g(intToBoolean: (Int) -> Boolean): G {
        return G(intToBoolean)
    }
}

@Singleton
@Component(modules = [ModuleB::class])
interface ComponentB {
    fun g(): G
}

val componentB = DaggerComponentB.create()
val g = componentB.g()
println(g.function(2)) // true
println(g.function(3)) // false

Background: Examining @Kiskae's response, it seems the problem is that a parameter of function type in Kotlin becomes contravariant on its own parameter types when the code is converted to Java bytecode. If this doesn't make sense to you, don't worry. It's not necessary to understand it to use the technique I show above.