Replacing @ContributesIntoSet bindings for testing

[scottjasso]

I'm having difficulty in replacing @ContributesIntoSet bindings for my tests. It seems at @Contributes* bindings are "strict", meaning that they conflict with other bindings directly in the DependencyGraph.Factory. This is an unexpected difference from kotlin-inject-anvil, where providing an explicit Set<T> in the @MergeComponent would take precedence over @ContributesBinding(multiple=true) bindings.

However, this same conflict does not happen for regular @Inject bindings -- providing a different instance in the DependencyGraph.Factory does take precedence.

Is this expected? Is there an easier way to replace @ContributesIntoSet bindings than my workaround?

Example

I have a basic plugin system in my app, with each plugin using @ContributesIntoSet:

@DependencyGraph(AppScope::class)
interface AppGraph {
    val myApplication: Application
}

@Inject
class Application(private val plugins: Set<Plugin>) { ... }

@ContributesIntoSet(AppScope::class)
@Inject
class MyPlugin : Plugin { ... }

In my tests, I want to be able to test with a custom set of plugins. So I have this:

@DependencyGraph(AppScope::class)
interface TestAppGraph {
    val myApplication: Application
    
    @DependencyGraph.Factory
    fun interface Factory {
        fun create(@Provides plugins: Set<Plugin>): TestAppGraph
    }
}

However, that gives me an error about duplicate bindings for Set<Plugin>:

e: Test.kt:29:11 [Metro/DuplicateBinding] Duplicate binding for kotlin.collections.Set<com.scottjasso.Plugin>
├─ Binding 1: Unknown source location, this may be generated by the compiler.
└─ Here's some additional information we have for the binding:
   ├─ Binding type: Multibinding
   └─ Binding information: Multibinding(typeKey=kotlin.collections.Set<com.scottjasso.Plugin>, isSet=true, isMap=false, bindingId=kotlin.collections.Set<com.scottjasso.Plugin>, allowEmpty=false, sourceBindings=[@dev.zacsweers.metro.internal.MultibindingElement("com.scottjasso.Plugin", "1573222671") com.scottjasso.Plugin])

├─ Binding 2: Test.kt:34:20

I could use @DependencyGraph(..., excludes=[...exclude all plugins here...]) but then I have to hardcode all of those plugins, which seems to defeat the purpose of aggregation.

Workaround

Instead, I need to add a wrapper class:

@Inject
class PluginContainer(val plugins: Set<Plugin>)

@Inject
class Application(pluginContainer: PluginContainer) { 
    private val plugins = pluginContainer.plugins
    ...
 }

and change the factory create method:

fun create(@Provides plugins: PluginContainer): NavTestComponent

And that works just fine. This overrides the default PluginContainer. But it's somewhat odd to need this wrapper class in the application, just to support testing.

[ZacSweers]

This is an unexpected difference from kotlin-inject-anvil, where providing an explicit Set in the @MergeComponent would take precedence over @ContributesBinding(multiple=true) bindings.

This is working as intended. kotlin-inject is more lenient here but Metro does not leave room for ambiguity (for example - what if you contribute two different ones? who wins?)

However, this same conflict does not happen for regular @Inject bindings -- providing a different instance in the DependencyGraph.Factory does take precedence.

This is expected, @Inject classes are discovered lazily and are not looked up at all if a direct provider exists already within the graph.