The definition of the subscription root object is slightly different from other root objects. Its resolver function always returns a [Stream](https://docs.rs/futures-core/~0.3/futures_core/stream/trait.Stream.html) or `FieldResult<Stream>`, and the field parameters are usually used as data filtering conditions.
The following example subscribes to an integer stream, which generates one integer per second. The parameter `step` specifies the integer step size with a default of 1.