Retrofit issue with Coroutine

hi

i use retrofit 2.6.0, with kotlin coroutines 1.2.1,

1. so when use Flow an exception appear:

@GET("/") suspend fun fetch(): Flow<User>

Exception in thread "main" com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `kotlinx.coroutines.flow.Flow`

1. i have this case

GET("/{id}") suspend fun loadBy(@Path("id") id: Int): User? 

if suppose that the back-end return NotFoundException(…) in failure, so the front-end (client) have an exception of can’t cast NotFoundException to User instance (font-end suppose receive a null instance of User in failure) so its possible to use Result<User> instead of User? nullable type however when try to use it i see kotlin not allow Result as return type

1. even if try to use Result<User> instead of User?, an other exception appear:

Exception in thread "main" com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `kotlin.Result` (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator)

[halimpuckjava]

backend-side (developed with NESTJS)

@Entity()
export class User {
    @PrimaryGeneratedColumn('increment') id: number
    @Column({type: 'varchar', length: 20}) name: string
    @CreateDateColumn() createAt?: Date
}

export class UserController {
  constructor(private readonly userRepository: Repository<User>) {}

  @Get(':id')
  findById(@Param('id') id: number): Observable<any> {
     return from(this.userRepository.findOneOrFail(id)).pipe(catchError(err => of(new NotFoundException(`user [id:${id}] not found`))))
  
}

in front-end (retrofit client):

@GET("/{id}") suspend fun findById(@Path("id") id: Int): User?

so when client send request to server to find user by id it expect to return user or null if not found, but in server side when user not found, server return exception of [NotFoundException] as response, so when client receive response (NotFoundException) it can’t cast it to User nullable instance and throw an exception of casting error

[JakeWharton]

You would have to build it yourself. Retrofit only supports suspend functions for coroutines, not Flow.