Types are now bidirectional

It is still possible to define types as functions (Buffer => string for 
state topics, string => Buffer for command topics). Otherwise types have 
from and to properties.
(Fixes #101)

src/config/flowtypes.js

@@ -1,16 +1,22 @@
 // @flow
 import type { Icon } from "config/icon";
 
-export type TopicType = (msg: Buffer) => string;
+export type TopicType = {
+  from: (msg: Buffer) => string,
+  to: (newstate: string) => Buffer
+};
 
-export type StateCommand = {
+export type StateTopicType = TopicType | ((msg: Buffer) => string);
+export type CommandTopicType = TopicType | ((newstate: string) => Buffer);
+
+export type StateCommand<T> = {
   name: string,
-  type: TopicType
+  type: T
 }
 
 export type Topic = {
-  state?: StateCommand,
-  command?: StateCommand,
+  state?: StateCommand<StateTopicType>,
+  command?: StateCommand<CommandTopicType>,
   defaultValue: string
 };
 export type Topics = Map<string, Topic>;
@@ -114,6 +120,17 @@ export type Space = {
   mqtt: string
 };
 
+export type Layer = {
+  image: string,
+  name: string,
+  baseLayer?: boolean,
+  defaultVisibility: "visible" | "hidden",
+  opacity?: number,
+  bounds: {
+    topLeft: Point,
+    bottomRight: Point
+  }
+};
 export type Config = {
   space: Space,
   topics: Topics | Array<Topics>,

src/config/types.js

@@ -1,13 +1,22 @@
 // @flow
 import type { TopicType } from "config/flowtypes";
 import at from "lodash/at";
+import set from "lodash/set";
 
-export const string: TopicType = (msg: Buffer) => msg.toString();
+export const string: TopicType = {
+  from: (msg: Buffer) => msg.toString(),
+  to: (msg: string) => Buffer.from(msg)
+};
 
 export const json = (path: string, innerType?: TopicType): TopicType => {
-  const parseAgain = innerType == null ? (x) => x.toString() : innerType;
-  return (msg) => parseAgain(Buffer.from(
-    at(JSON.parse(msg.toString()), path)[0].toString()));
+  const parseAgain = innerType?.from ?? ((x) => x.toString());
+  const parseFirst = innerType?.to ?? ((x) => Buffer.from(x));
+  return {
+    from: (msg) => parseAgain(Buffer.from(
+      at(JSON.parse(msg.toString()), path)[0].toString())),
+    to: (msg) => Buffer.from(
+      JSON.serialize(set({}, path, parseFirst(msg).toString())))
+  };
 };
 
 export type TypeOptionParam = { otherwise?: string, [string]: string };
@@ -16,13 +25,17 @@ export const option = (values: TypeOptionParam): TopicType => {
     if (values.otherwise != null) {
       return values.otherwise;
     } else {
-      throw new Error(
-        `Value ${x.toString()} cannot be mapped by the option parameters given`
-      );
+      return x;
     }
   };
   const mapVal = (x) => (values[x] != null ? values[x] : defaultValue(x));
-  return (x) => mapVal(x.toString());
+  return {
+    from: (x) => mapVal(x.toString()),
+    to: (x) => Buffer.from(mapVal(x))
+  };
 };
 
-export const jsonArray = (msg: Buffer) => JSON.parse(msg.toString()).join(", ");
+export const jsonArray = {
+  from: (msg: Buffer) => JSON.parse(msg.toString()).join(", "),
+  to: (msg: string) => Buffer.from(`[${msg}]`)
+};