Fix #24 where DragDropMonitor failes to notify subscribeToStateChange…

… subscribers in some cases

src/DragDropMonitor.js

@@ -23,9 +23,21 @@ export default class DragDropMonitor {
       'handlerIds, when specified, must be an array of strings.'
     );
 
+    let prevStateId = this.store.getState().stateId;
     const handleChange = () => {
-      if (areDirty(this.store.getState().dirtyHandlerIds, handlerIds)) {
-        listener();
+      const state = this.store.getState();
+      const currentStateId = state.stateId;
+      try {
+        const canSkipListener = currentStateId === prevStateId || (
+          currentStateId === prevStateId + 1 &&
+          !areDirty(state.dirtyHandlerIds, handlerIds)
+        )
+
+        if (!canSkipListener) {
+          listener();
+        }
+      } finally {
+        prevStateId = currentStateId;
       }
     };
 

src/reducers/index.js

@@ -2,12 +2,14 @@ import { default as dragOffset } from './dragOffset';
 import { default as dragOperation } from './dragOperation';
 import { default as refCount } from './refCount';
 import { default as dirtyHandlerIds } from './dirtyHandlerIds';
+import { default as stateId } from './stateId';
 
 export default function (state = {}, action) {
   return {
     dirtyHandlerIds: dirtyHandlerIds(state.dirtyHandlerIds, action, state.dragOperation),
     dragOffset: dragOffset(state.dragOffset, action),
     refCount: refCount(state.refCount, action),
-    dragOperation: dragOperation(state.dragOperation, action)
+    dragOperation: dragOperation(state.dragOperation, action),
+    stateId: stateId(state.stateId)
   };
 }

src/reducers/stateId.js

@@ -0,0 +1,3 @@
+export default function stateId(state = 0) {
+    return state + 1;
+}

test/DragDropMonitor.spec.js

@@ -463,6 +463,29 @@ describe('DragDropMonitor', () => {
       expect(raisedChange).to.equal(false);
     });
 
+    it('raises global change event on beginDrag(), even if redux did not notify us for all stat updates.', (done) => {
+      const source = new NormalSource();
+      const sourceId = registry.addSource(Types.FOO, source);
+      const target = new NormalTarget()
+
+      let notified = false;
+      monitor.subscribeToStateChange(
+          () => {
+            if (!notified)
+            {
+              notified = true;
+              // Add target will send an action to redux, which will trigger a new state update.
+              // The redux behaviour then is to send only the latest state to the subscribers
+              // which means that the subscription below will not receive this state.
+              registry.addTarget(Types.FOO, target)
+            }
+          });
+
+      monitor.subscribeToStateChange(
+          () => { done()});
+      backend.simulateBeginDrag([sourceId]);
+    });
+
     it('throws when passing clientOffset without getSourceClientOffset', () => {
       const source = new NormalSource();
       const sourceId = registry.addSource(Types.FOO, source);