BehaviorDelegate fails for suspend functions

retrofit-mock’s BehaviorDelegate throws an exception when attempting to return for a suspend function.

For example

interface SomeApi {
  @GET("foo")
  suspend fun getFoo(): Foo
}

will throw when attempting to use:

val delegate: BehaviorDelegate<SomeApi> = mockRetrofit.create(SomeApi::class.java)
val foo: Foo = delegate.returningResponse(Foo()).getFoo()

I’d submit a failing test for this, but it would involve me pulling in Kotlin into the retrofit-mock module which I’m not sure is desirable?

Exception thrown looks like:

java.lang.IllegalArgumentException: Could not locate call adapter for class java.lang.Object.
 Tried:
  * retrofit2.CompletableFutureCallAdapterFactory
  * retrofit2.DefaultCallAdapterFactory
   at retrofit2.Retrofit.nextCallAdapter(Retrofit.java:241)
   at retrofit2.Retrofit.callAdapter(Retrofit.java:205)
   at retrofit2.mock.BehaviorDelegate$1.invoke(BehaviorDelegate.java:64)
   at java.lang.reflect.Proxy.invoke(Proxy.java:1006)
   at $Proxy0.getGames(Unknown Source)
   at au.com.gridstone.debugdrawer.sampleapp.MockGamesApi.getGames(MockGamesApi.kt:17)
   at au.com.gridstone.debugdrawer.sampleapp.GamesViewModel$refresh$1.invokeSuspend(GamesViewModel.kt:43)
   at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
   at kotlinx.coroutines.DispatchedTask.run(Dispatched.kt:238)
   at android.os.Handler.handleCallback(Handler.java:883)
   at android.os.Handler.dispatchMessage(Handler.java:100)
   at android.os.Looper.loop(Looper.java:214)
   at android.app.ActivityThread.main(ActivityThread.java:7319)
   at java.lang.reflect.Method.invoke(Native Method)
   at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:934)

[JakeWharton]

Yep this needs to be special-cased the same way it was inside regular Retrofit.

[vrickey123]

I took a look at the logs and I believe I have found the root cause and special casing that Jake mentioned earlier.

It looks like in HttpServiceMethod we check for requestFactory.isKotlinSuspensionFunction to determine which adapterType to use; i.e. the coroutine Response Type or a Generic Return Type.

The adapterType is then used by a private method called createCallAdapter() to return an adapter appropriate for a couroutine Response or a Generic Response.

https://github.com/square/retrofit/blob/master/retrofit/src/main/java/retrofit2/HttpServiceMethod.java#L43-L66

In the Mock Retrofit BehaviorDelegate, we naively use the Generic Response Type to create a Call Adapter which is why everyone sees java.lang.IllegalArgumentException: Could not locate call adapter for class java.lang.Object in their logs.

https://github.com/square/retrofit/blob/master/retrofit-mock/src/main/java/retrofit2/mock/BehaviorDelegate.java#L57-L61

Rather than expose the private method for createCallAdapter, can we think of a reason why the requestFactory.isKotlinSuspensionFunction is not being hit or cannot be used by Mock Retrofit? This is where someone else’s knowledge of the library will come in handy.