feat: Added isTransitioning property to RouterStore

`isTransitioning` is set to true when the router is in the process of transitioning from one state
to another.

Implements #32

README.md

@@ -454,6 +454,9 @@ The `RouterStore` is the keeper of the `RouterState`. It allows transitioning be
 
 ```jsx
 export class RouterStore {
+    @observable.ref routerState: RouterState;
+    @observable isTransitioning: boolean = false;
+
     constructor(rootStore: any, routes: Route[], notFoundState: RouterState, initialRoute: Route = INITIAL_ROUTE);
     goTo(toState: RouterState): Promise<RouterState>;
     goTo(routeName: string, params?: StringMap, queryParams?: Object): Promise<RouterState>;
@@ -463,6 +466,10 @@ export class RouterStore {
 }
 ```
 
+The `RouterStore` exposes two observable properties:
+- routerState: the state of the router
+- isTransitioning: set to true when the router is in the process of transitioning from one state to another. This property can be used, for example, to display a progress indicator during transitions.
+
 ### HistoryAdapter
 The `HistoryAdapter` is responsible for keeping the browser address bar and the `RouterState` in sync. It also provides a `goBack()` method to go back in history.
 

src/router-store.ts

@@ -74,6 +74,7 @@ export class RouterStore {
     routes: Route[];
     notFoundState: RouterState;
     @observable.ref routerState: RouterState;
+    @observable isTransitioning: boolean = false;
 
     constructor(
         rootStore: any,
@@ -172,6 +173,7 @@ export class RouterStore {
         const { beforeEnter, onEnter } = this.getRoute(toState.routeName);
 
         // Call the transition hook chain
+        this.isTransitioning = true;
         return (
             [beforeExit, beforeEnter, onExit, onEnter]
                 .reduce(
@@ -202,5 +204,6 @@ export class RouterStore {
     @action
     private setRouterState(routerState: RouterState) {
         this.routerState = routerState;
+        this.isTransitioning = false;
     }
 }

test/router-store.test.ts

@@ -62,11 +62,12 @@ const errorState = new RouterState('errorRoute');
 
 describe('RouterStore', () => {
     test('transitions to the desired state', () => {
-        expect.assertions(1);
+        expect.assertions(2);
 
         const routerStore = new RouterStore({}, routes, notFound);
         return routerStore.goTo(home).then(toState => {
             expect(toState.isEqual(home)).toBeTruthy();
+            expect(routerStore.isTransitioning).toBeFalsy();
         });
     });