Add beforeHistoryChange router event. (#1360)

lib/router/index.js

@@ -15,7 +15,7 @@ const SingletonRouter = {
 // Create public properties and methods of the router in the SingletonRouter
 const propertyFields = ['components', 'pathname', 'route', 'query']
 const coreMethodFields = ['push', 'replace', 'reload', 'back', 'prefetch']
-const routerEvents = ['routeChangeStart', 'routeChangeComplete', 'routeChangeError']
+const routerEvents = ['routeChangeStart', 'beforeHistoryChange', 'routeChangeComplete', 'routeChangeError']
 
 propertyFields.forEach((field) => {
   // Here we need to use Object.defineProperty because, we need to return

lib/router/router.js

@@ -168,6 +168,7 @@ export default class Router extends EventEmitter {
       return false
     }
 
+    this.emit('beforeHistoryChange', as)
     this.changeState(method, url, as, options)
     const hash = window.location.hash.substring(1)
 

readme.md

@@ -311,6 +311,7 @@ Here's a list of supported events:
 - `routeChangeStart(url)` - Fires when a route starts to change
 - `routeChangeComplete(url)` - Fires when a route changed completely
 - `routeChangeError(err, url)` - Fires when there's an error when changing routes
+- `beforeHistoryChange(url)` - Fires just before changing the browser's history
 - `appUpdated(nextRoute)` - Fires when switching pages and there's a new version of the app
 
 > Here `url` is the URL shown in the browser. If you call `Router.push(url, as)` (or similar), then the value of `url` will be `as`.