Let the application manage an anchoring cache

This is maybe too verbose for now but at least it's explicit and
doesn't assume that anchoring implementations can be fully
responsible for cache invalidation.

h/static/scripts/annotator/anchoring/pdf.coffee

@@ -27,18 +27,15 @@ getPageTextContent = (pageIndex) ->
   .then((textContent) -> (item.str for item in textContent.items).join(''))
 
 
-# XXX: This will break if the viewer changes documents
-_pageOffsetCache = {}
-
-getPageOffset = (pageIndex) ->
+getPageOffset = (pageIndex, cache = {}) ->
   index = -1
 
-  if _pageOffsetCache[pageIndex]?
-    return Promise.resolve(_pageOffsetCache[pageIndex])
+  if cache[pageIndex]?
+    return Promise.resolve(cache[pageIndex])
 
   next = (offset) ->
     if ++index is pageIndex
-      _pageOffsetCache[pageIndex] = offset
+      cache[pageIndex] = offset
       return Promise.resolve(offset)
 
     return getPageTextContent(index)
@@ -137,7 +134,7 @@ exports.anchor = (selectors, options = {}) ->
       pageSearches = for pageIndex in [0...pagesCount]
         page = getPage(pageIndex)
         content = getPageTextContent(pageIndex)
-        offset = getPageOffset(pageIndex)
+        offset = getPageOffset(pageIndex, options.cache)
         Promise.all([content, offset, page]).then (results) ->
           [content, offset, page] = results
           pageOptions = {root: {textContent: content}}
@@ -181,7 +178,7 @@ exports.describe = (range, options = {}) ->
   start = seek(iter, range.start)
   end = seek(iter, range.end) + start + range.end.textContent.length
 
-  return getPageOffset(startPageIndex).then (pageOffset) ->
+  return getPageOffset(startPageIndex, options.cache).then (pageOffset) ->
     # XXX: range covers only one page
     start += pageOffset
     end += pageOffset

h/static/scripts/annotator/guest.coffee

@@ -40,6 +40,7 @@ module.exports = class Guest extends Annotator
     super
 
     this.anchors = []
+    this.anchoringCache = {}
 
     cfOptions =
       on: (event, handler) =>
@@ -159,7 +160,10 @@ module.exports = class Guest extends Annotator
     annotation.target ?= []
 
     locate = (target) ->
-      options = {ignoreSelector: '[class^="annotator-"]'}
+      options = {
+        cache: self.anchoringCache
+        ignoreSelector: '[class^="annotator-"]'
+      }
       return new Promise(raf)
       .then(-> anchoring.anchor(target.selector, options))
       .then((range) -> {annotation, target, range})
@@ -214,11 +218,16 @@ module.exports = class Guest extends Annotator
     return annotation
 
   createAnnotation: (annotation = {}) ->
+    self = this
+
     ranges = @selectedRanges ? []
     @selectedRanges = null
 
     getSelectors = (range) ->
-      options = {ignoreSelector: '[class^="annotator-"]'}
+      options = {
+        cache: self.anchoringCache
+        ignoreSelector: '[class^="annotator-"]'
+      }
       return anchoring.describe(range, options)
 
     setDocumentInfo = ({metadata, uri}) ->