Reduce observation to single observeMarker method on display buffer

It will call the observer callbacks with the buffer and screen positions
of the head and tell each time any of these values is changed.

src/app/cursor.coffee

@@ -12,7 +12,7 @@ class Cursor
   needsAutoscroll: null
 
   constructor: ({@editSession, @marker}) ->
-    @editSession.observeMarkerHeadPosition @marker, (e) =>
+    @editSession.observeMarker @marker, (e) =>
       @needsAutoscroll ?= @isLastCursor() and !e.bufferChanged
       @trigger 'moved', e
       @editSession.trigger 'cursor-moved', e

src/app/display-buffer-marker.coffee

@@ -1,7 +1,13 @@
+Range = require 'range'
 _ = require 'underscore'
 
 module.exports =
 class DisplayBufferMarker
+  observers: null
+  bufferMarkerSubscription: null
+  previousHeadScreenPosition: null
+  previousTailScreenPosition: null
+
   constructor: ({@id, @displayBuffer}) ->
     @buffer = @displayBuffer.buffer
 
@@ -18,7 +24,7 @@ class DisplayBufferMarker
     @buffer.setMarkerRange(@id, bufferRange, options)
 
   getHeadScreenPosition: ->
-    @headScreenPosition ?= @displayBuffer.screenPositionForBufferPosition(@getHeadBufferPosition(), wrapAtSoftNewlines: true)
+    @displayBuffer.screenPositionForBufferPosition(@getHeadBufferPosition(), wrapAtSoftNewlines: true)
 
   setHeadScreenPosition: (screenPosition, options) ->
     screenPosition = @displayBuffer.clipScreenPosition(screenPosition, options)
@@ -31,7 +37,8 @@ class DisplayBufferMarker
     @buffer.setMarkerHeadPosition(@id, bufferPosition)
 
   getTailScreenPosition: ->
-    @getMarker(@id).getTailScreenPosition()
+    if tailBufferPosition = @getTailBufferPosition()
+      @displayBuffer.screenPositionForBufferPosition(tailBufferPosition, wrapAtSoftNewlines: true)
 
   setTailScreenPosition: (screenPosition, options) ->
     screenPosition = @displayBuffer.clipScreenPosition(screenPosition, options)
@@ -49,44 +56,60 @@ class DisplayBufferMarker
   clearTail: ->
     @buffer.clearMarkerTail(@id)
 
-  observeHeadPosition: (callback) ->
-    unless @headObservers
-      @observeBufferMarkerHeadPosition()
-      @displayBuffer.markers[@id] = this
-      @headObservers = []
-    @headObservers.push(callback)
-    cancel: => @unobserveHeadPosition(callback)
+  observe: (callback) ->
+    @observeBufferMarkerIfNeeded()
+    @observers.push(callback)
+    cancel: => @unobserve(callback)
 
-  unobserveHeadPosition: (callback) ->
-    _.remove(@headObservers, callback)
-    @unsubscribe() unless @headObservers.length
+  unobserve: (callback) ->
+    _.remove(@observers, callback)
+    @unobserveBufferMarkerIfNeeded()
 
-  observeBufferMarkerHeadPosition: ->
-    @getHeadScreenPosition()
-    @bufferMarkerHeadSubscription =
-      @buffer.observeMarkerHeadPosition @id, (e) =>
-        bufferChanged = e.bufferChanged
-        oldBufferPosition = e.oldPosition
-        newBufferPosition = e.newPosition
-        @refreshHeadScreenPosition({bufferChanged, oldBufferPosition, newBufferPosition})
+  observeBufferMarkerIfNeeded: ->
+    return if @observers
+    @observers = []
+    @previousHeadScreenPosition = @getHeadScreenPosition()
+    @previousTailScreenPosition = @getTailScreenPosition()
+    @bufferMarkerSubscription =
+      @buffer.observeMarker @id, ({oldHeadPosition, newHeadPosition, oldTailPosition, newTailPosition, bufferChanged}) =>
+        @notifyObservers
+          oldHeadBufferPosition: oldHeadPosition
+          newHeadBufferPosition: newHeadPosition
+          oldTailBufferPosition: oldTailPosition
+          newTailBufferPosition: newTailPosition
+          bufferChanged: bufferChanged
+    @displayBuffer.markers[@id] = this
 
-  refreshHeadScreenPosition: ({bufferChanged, oldBufferPosition, newBufferPosition}={}) ->
-    unless bufferChanged
-      oldBufferPosition ?= @getHeadBufferPosition()
-      newBufferPosition ?= oldBufferPosition
-    oldScreenPosition = @getHeadScreenPosition()
-    @headScreenPosition = null
-    newScreenPosition = @getHeadScreenPosition()
-
-    unless newScreenPosition.isEqual(oldScreenPosition)
-      @notifyHeadObservers({ oldBufferPosition, newBufferPosition, oldScreenPosition, newScreenPosition, bufferChanged })
-
-  notifyHeadObservers: (event) ->
-    observer(event) for observer in @getHeadObservers()
-
-  getHeadObservers: ->
-    new Array(@headObservers...)
-
-  unsubscribe: ->
-    @bufferMarkerHeadSubscription.cancel()
+  unobserveBufferMarkerIfNeeded: ->
+    return if @observers.length
+    @observers = null
+    @bufferMarkerSubscription.cancel()
     delete @displayBuffer.markers[@id]
+
+  notifyObservers: ({oldHeadBufferPosition, oldTailBufferPosition, bufferChanged}) ->
+    oldHeadScreenPosition = @previousHeadScreenPosition
+    newHeadScreenPosition = @getHeadScreenPosition()
+    @previousHeadScreenPosition = newHeadScreenPosition
+
+    oldTailScreenPosition = @previousTailScreenPosition
+    newTailScreenPosition = @getTailScreenPosition()
+    @previousTailScreenPosition = newTailScreenPosition
+
+    return if _.isEqual(newHeadScreenPosition, oldHeadScreenPosition) and _.isEqual(newTailScreenPosition, oldTailScreenPosition)
+
+    oldHeadBufferPosition ?= @getHeadBufferPosition()
+    newHeadBufferPosition = @getHeadBufferPosition()
+    oldTailBufferPosition ?= @getTailBufferPosition()
+    newTailBufferPosition = @getTailBufferPosition()
+
+    for observer in @getObservers()
+      observer({
+        oldHeadScreenPosition, newHeadScreenPosition,
+        oldTailScreenPosition, newTailScreenPosition,
+        oldHeadBufferPosition, newHeadBufferPosition,
+        oldTailBufferPosition, newTailBufferPosition,
+        bufferChanged
+      })
+
+  getObservers: ->
+    new Array(@observers...)

src/app/display-buffer.coffee

@@ -369,12 +369,12 @@ class DisplayBuffer
   isMarkerReversed: (id) ->
     @buffer.isMarkerReversed(id)
 
-  observeMarkerHeadPosition: (id, callback) ->
-    @getMarker(id).observeHeadPosition(callback)
+  observeMarker: (id, callback) ->
+    @getMarker(id).observe(callback)
 
   refreshMarkerScreenPositions: ->
     for marker in @getMarkers()
-      marker.refreshHeadScreenPosition(bufferChanged: false)
+      marker.notifyObservers(bufferChanged: false)
 
   destroy: ->
     @tokenizedBuffer.destroy()

src/app/edit-session.coffee

@@ -489,8 +489,8 @@ class EditSession
   setMarkerTailBufferPosition: (args...) ->
     @displayBuffer.setMarkerTailBufferPosition(args...)
 
-  observeMarkerHeadPosition: (args...) ->
-    @displayBuffer.observeMarkerHeadPosition(args...)
+  observeMarker: (args...) ->
+    @displayBuffer.observeMarker(args...)
 
   placeMarkerTail: (args...) ->
     @displayBuffer.placeMarkerTail(args...)

src/app/selection.coffee

@@ -10,10 +10,7 @@ class Selection
 
   constructor: ({@cursor, @marker, @editSession}) ->
     @cursor.selection = this
-
-    @cursor.on 'moved.selection', ({bufferChange}) =>
-      @screenRangeChanged() unless bufferChange
-
+    @editSession.observeMarker @marker, => @screenRangeChanged()
     @cursor.on 'destroyed.selection', =>
       @cursor = null
       @destroy()
@@ -78,7 +75,6 @@ class Selection
 
   clear: ->
     @editSession.clearMarkerTail(@marker)
-    @screenRangeChanged()
 
   selectWord: ->
     options = {}