feat: call scrollBehavior with app context (#1804)

* Call scrollBehavior with app context

This allows us to listen for events in the scroll behavior hook, thus enabling us to resolve a promise returned by the hook upon triggering of some custom event (for example one emited on the $root by an after-leave transition event).

* Update scrollBehavior example

* change scrollBehavior context to router

examples/scroll-behavior/app.js

@@ -25,36 +25,41 @@ const scrollBehavior = function (to, from, savedPosition) {
     // savedPosition is only available for popstate navigations.
     return savedPosition
   } else {
-    return new Promise(resolve => {
-      const position = {}
-      let delay = 500
-      // new navigation.
-      // scroll to anchor by returning the selector
-      if (to.hash) {
-        position.selector = to.hash
+    const position = {}
+
+    // scroll to anchor by returning the selector
+    if (to.hash) {
+      position.selector = to.hash
 
-        // specify offset of the element
-        if (to.hash === '#anchor2') {
-          position.offset = { y: 100 }
-        }
+      // specify offset of the element
+      if (to.hash === '#anchor2') {
+        position.offset = { y: 100 }
+      }
 
-        if (document.querySelector(to.hash)) {
-          delay = 0
-        }
+      if (document.querySelector(to.hash)) {
+        return position
       }
+
+      // if the returned position is falsy or an empty object,
+      // will retain current scroll position.
+      return false
+    }
+
+    return new Promise(resolve => {
       // check if any matched route config has meta that requires scrolling to top
       if (to.matched.some(m => m.meta.scrollToTop)) {
         // coords will be used if no selector is provided,
         // or if the selector didn't match any element.
         position.x = 0
         position.y = 0
       }
+
       // wait for the out transition to complete (if necessary)
-      setTimeout(() => {
-        // if the returned position is falsy or an empty object,
+      this.app.$root.$once('triggerScroll', () => {
+        // if the resolved position is falsy or an empty object,
         // will retain current scroll position.
         resolve(position)
-      }, delay)
+      })
     })
   }
 }
@@ -82,9 +87,14 @@ new Vue({
         <li><router-link to="/bar#anchor">/bar#anchor</router-link></li>
         <li><router-link to="/bar#anchor2">/bar#anchor2</router-link></li>
       </ul>
-      <transition name="fade" mode="out-in">
+      <transition name="fade" mode="out-in" @after-leave="afterLeave">
         <router-view class="view"></router-view>
       </transition>
     </div>
-  `
+  `,
+  methods: {
+    afterLeave () {
+      this.$root.$emit('triggerScroll')
+    }
+  }
 }).$mount('#app')

src/util/scroll.js

@@ -39,7 +39,7 @@ export function handleScroll (
   // wait until re-render finishes before scrolling
   router.app.$nextTick(() => {
     const position = getScrollPosition()
-    const shouldScroll = behavior(to, from, isPop ? position : null)
+    const shouldScroll = behavior.call(router, to, from, isPop ? position : null)
 
     if (!shouldScroll) {
       return