public FlatMapFunction<M, OM> getTransformFn() {
    return this.transformFn;
  }
Constructor for a {@link StreamOperatorSpec}.

@param transformFn  the transformation function
@param opCode  the {@link OpCode} for this {@link StreamOperatorSpec}
@param opId  the unique ID for this {@link StreamOperatorSpec}
samza-core/src/main/java/org/apache/samza/operators/spec/StreamOperatorSpec.java