resolves #29

index.d.ts

@@ -24,6 +24,10 @@ declare class GlobalPureComponent<P = {}, S = {}> extends React.PureComponent<P,
   setGlobal(newGlobal: NewGlobal, callback?: GlobalCallback): Promise<void> | void;
 }
 
+type GlobalCallback = (state: GlobalState) => NewGlobal;
+
+type GlobalCallbackRemover = () => void;
+
 type GlobalReducer = (state: GlobalState, ...args: any[]) => NewGlobal;
 
 type GlobalPropertySetter<T> = (value: T) => void;
@@ -46,10 +50,12 @@ type NewGlobalFunction = (global: GlobalState) => NewGlobal;
 
 interface ReactN {
   (Component: React.ComponentType): typeof GlobalComponent;
+  addCallback(callback: GlobalCallback): GlobalCallbackRemover;
   addReducer(name: string, reducer: GlobalReducer): void;
   Component: typeof GlobalComponent;
   default: ReactN;
   PureComponent: typeof GlobalPureComponent;
+  removeCallback(callback: GlobalCallback): void;
   resetGlobal(): void;
   setGlobal(newGlobal: NewGlobal, callback?: GlobalCallback): Promise<void> | void;
   useGlobal(): [ GlobalState, GlobalStateSetter ];

src/global-state-manager.js

@@ -9,6 +9,13 @@ class GlobalStateManager {
     this.reset();
   }
 
+  addCallback(callback) {
+    this._callbacks.add(callback);
+    return () => {
+      this.removeCallback(callback);
+    };
+  }
+
   // Map component instance to a state property.
   addPropertyListener(property, propertyListener) {
     if (this._propertyListeners.has(property)) {
@@ -17,7 +24,7 @@ class GlobalStateManager {
     else {
       this._propertyListeners.set(property, new Set([ propertyListener ]));
     }
-  };
+  }
 
   // Begin a transaction.
   beginTransaction() {
@@ -49,9 +56,20 @@ class GlobalStateManager {
       propertyListener();
     }
 
+    // Call each global callback.
+    for (const callback of this._callbacks) {
+
+      // Delay these under after the current transaction has deleted?
+      this.setAny(callback(this.stateWithReducers));
+    }
+
     this._transactions.delete(transactionId);
   }
 
+  removeCallback(callback) {
+    this._callbacks.remove(callback);
+  }
+
   // Unmap a component instance from all state properties.
   removePropertyListener(propertyListener) {
 
@@ -68,6 +86,7 @@ class GlobalStateManager {
 
   // Reset the global state.
   reset() {
+    this._callbacks = new Set();
     this._propertyListeners = new Map();
     this._state = Object.create(null);
     this._transactionId = 0;
@@ -106,7 +125,10 @@ class GlobalStateManager {
   setAny(any) {
 
     // No changes, e.g. getDerivedGlobalFromProps.
-    if (any === null) {
+    if (
+      any === null ||
+      typeof any === 'undefined'
+    ) {
       return;
     }
 

src/helpers/add-callback.js

@@ -0,0 +1,5 @@
+const globalStateManager = require('../global-state-manager');
+
+module.exports = function addCallback(f) {
+  return globalStateManager.addCallback(f);
+};

src/helpers/remove-callback.js

@@ -0,0 +1,5 @@
+const globalStateManager = require('../global-state-manager');
+
+module.exports = function removeCallback(f) {
+  return globalStateManager.removeCallback(f);
+};

src/index.js

@@ -2,15 +2,18 @@ const React = require('react');
 const { ReactNComponent, ReactNPureComponent } = require('./components');
 const ReactN = require('./decorator');
 const {
-  addReducer, getGlobal, resetGlobal, setGlobal, useGlobal, withGlobal
+  addCallback, addReducer, getGlobal, removeCallback, resetGlobal, setGlobal,
+  useGlobal, withGlobal
 } = require('./helpers/index');
 
 Object.assign(ReactN, React, {
+  addCallback,
   addReducer,
   Component: ReactNComponent,
   default: ReactN,
   getGlobal,
   PureComponent: ReactNPureComponent,
+  removeCallback,
   resetGlobal,
   setGlobal,
   useGlobal,

tests/helpers/add-callback.spec.js

@@ -0,0 +1,9 @@
+describe('addCallback', () => {
+
+  it('should add a callback');
+
+  describe('the added callback', () => {
+    it('should execute on state change');
+  });
+
+});

tests/helpers/remove-callback.spec.js

@@ -0,0 +1,9 @@
+describe('removeCallback', () => {
+
+  it('should remove a callback');
+
+  describe('the removed callback', () => {
+    it('should not execute on state change');
+  });
+
+});