Merge pull request rust-lang#186 from bcardarella/features/bc/track-unique-history-location-state

RFC: Track unique history location state

Author: rwjblue

text/0000-track-unique-history-location-state.md

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+- Start Date: 2016/12/05
+- RFC PR: (leave this empty)
+- Ember Issue: (leave this empty)
+
+# Summary
+
+Track unique history location states
+
+# Motivation
+
+The path alone does not provide enough information. For example, if you
+visit page A, scroll down, then click on a link to page B, then click on
+a link back to page A. Your actual browser history stack is [A, B, A].
+Each of those nodes in the history should have their own unique scroll
+position. In order to record this position we need a UUID 
+for each node in the history.
+
+This API will allow other libraries to reflect upon each location to
+determine unique state. For example,
+[ember-router-scroll](https://github.com/dollarshaveclub/ember-router-scroll)
+is making use of a [modified `Ember.HistoryLocation` object to get this
+behavior](https://github.com/dollarshaveclub/ember-router-scroll/blob/master/addon/locations/router-scroll.js).
+
+Tracking unique state is required when setting the scroll position
+properly based upon where you are in the history stack, as described in
+[Motivation](#motivation)
+
+# Detailed design
+
+Code: [PR#14011](https://github.com/emberjs/ember.js/pull/14011)
+
+We simply unique identifier (UUID) so we can track uniqueness on two
+dimensions. Both `path` and the generated `uuid`. A simple UUID
+generator such as
+https://gist.github.com/lukemelia/9daf074b1b2dfebc0bd87552d0f6a537
+should suffice.
+
+# How We Teach This
+
+We could describe what meta data is generated for each location in the
+history stack. For example, it could look like:
+
+```js
+// visit page A
+
+[
+  { path: '/', uuid: 1 }
+]
+
+// visit page B
+
+[
+  { path: '/about', uuid: 2 },
+  { path: '/', uuid: 1 }
+]
+
+// visit page A
+
+[
+  { path: '/', uuid: 3 },
+  { path: '/about', uuid: 2 },
+  { path: '/', uuid: 1 }
+]
+
+// click back button
+
+[
+  { path: '/about', uuid: 2 },
+  { path: '/', uuid: 1 }
+]
+```
+
+# Drawbacks
+
+* The property access is writable
+
+# Alternatives
+
+The purpose for this behavior is to enable scroll position libraries.
+There are two other solutions in the wild. One is in the guides that
+suggests resetting the scroll position to `(0, 0)` on each new route
+entry. The other is
+[ember-memory-scroll](https://github.com/ef4/memory-scroll) which I
+believe is better suited for tracking scroll positions for components
+rather than the current page.
+
+However, in both cases neither solution provides the experience that
+users have come to expect from server-rendered pages. The browser tracks
+scroll position and restores it when you revisit the page in the history
+stack. The scroll position is unique even if you have multiple instances
+of the same page in the stack.
+
+# Unresolved questions
+
+None at this time.