Add navigation guards to liveSocket

This PR introduces a new option that can be specified when creating a
livesocket instance: `navigation`.

Navigation is an object that can define two functions: `beforeEach` and
`afterEach`. The `beforeEach` function is called before each navigation
event and can for example be used to store custom scroll positions. If
the `beforeEach` function returns `false`, the navigation will be aborted.
This can be used to implement a warning when the user tries to navigate
while a form is still being edited. The `afterEach` function is called
after a navigation event has been completed. It can be used to restore
the scroll position that was stored in the `beforeEach` function.

assets/js/phoenix_live_view/live_socket.js

@@ -158,6 +158,7 @@ export default class LiveSocket {
     this.sessionStorage = opts.sessionStorage || window.sessionStorage
     this.boundTopLevelEvents = false
     this.domCallbacks = Object.assign({onNodeAdded: closure(), onBeforeElUpdated: closure()}, opts.dom || {})
+    this.navigationCallbacks = Object.assign({beforeEach: closure(true), afterEach: closure()}, opts.navigation || {})
     this.transitions = new TransitionSet()
     window.addEventListener("pagehide", _e => {
       this.unloaded = true
@@ -687,6 +688,13 @@ export default class LiveSocket {
       }, 100)
     })
     window.addEventListener("popstate", event => {
+      if(this.navigationCallbacks["beforeEach"](window.location.href) === false){
+        // we want to stay at the current location; push the current location back
+        // and then the old location where we're coming from
+        Browser.pushState("push", history.state || {}, window.location.href)
+        Browser.pushState("push", history.state || {}, this.currentLocation.href)
+        return
+      }
       if(!this.registerNewLocation(window.location)){ return }
       let {type, id, root, scroll} = event.state || {}
       let href = window.location.href
@@ -695,11 +703,13 @@ export default class LiveSocket {
       this.requestDOMUpdate(() => {
         if(this.main.isConnected() && (type === "patch" && id === this.main.id)){
           this.main.pushLinkPatch(href, null, () => {
+            this.navigationCallbacks["afterEach"](href)
             this.maybeScroll(scroll)
           })
         } else {
           this.replaceMain(href, null, () => {
             if(root){ this.replaceRootHistory() }
+            this.navigationCallbacks["afterEach"](href)
             this.maybeScroll(scroll)
           })
         }
@@ -757,6 +767,8 @@ export default class LiveSocket {
   }
 
   pushHistoryPatch(href, linkState, targetEl){
+    if(this.navigationCallbacks["beforeEach"](href) === false) return
+
     if(!this.isConnected() || !this.main.isMain()){ return Browser.redirect(href) }
 
     this.withPageLoading({to: href, kind: "patch"}, done => {
@@ -773,9 +785,12 @@ export default class LiveSocket {
     Browser.pushState(linkState, {type: "patch", id: this.main.id}, href)
     DOM.dispatchEvent(window, "phx:navigate", {detail: {patch: true, href, pop: false}})
     this.registerNewLocation(window.location)
+    this.navigationCallbacks["afterEach"](window.location.href)
   }
 
   historyRedirect(href, linkState, flash){
+    if(this.navigationCallbacks["beforeEach"](href) === false) return
+
     if(!this.isConnected() || !this.main.isMain()){ return Browser.redirect(href, flash) }
 
     // convert to full href if only path prefix
@@ -790,6 +805,7 @@ export default class LiveSocket {
           Browser.pushState(linkState, {type: "redirect", id: this.main.id, scroll: scroll}, href)
           DOM.dispatchEvent(window, "phx:navigate", {detail: {href, patch: false, pop: false}})
           this.registerNewLocation(window.location)
+          this.navigationCallbacks["afterEach"](window.location.href)
         }
         done()
       })