Add ability to cancel marker head position observations

spec/app/buffer-spec.coffee

@@ -707,15 +707,26 @@ describe 'Buffer', ->
 
     describe "marker observation", ->
       describe ".observeMarkerHeadPosition(marker, callback)", ->
+        observeHandler = null
+
+        beforeEach ->
+          observeHandler = jasmine.createSpy("observeHandler")
+
         it "calls the given callback whenever the marker's head position changes", ->
           marker = buffer.markRange([[4, 20], [4, 23]])
-          observeHandler = jasmine.createSpy("observeHandler")
           buffer.observeMarkerHeadPosition(marker, observeHandler)
 
           buffer.setMarkerHeadPosition(marker, [6, 2])
           expect(observeHandler).toHaveBeenCalled()
           expect(observeHandler.argsForCall[0][0]).toEqual [6, 2]
 
+        it "allows the observation subscription to be cancelled", ->
+          marker = buffer.markRange([[4, 20], [4, 23]])
+          subscription = buffer.observeMarkerHeadPosition(marker, observeHandler)
+          subscription.cancel()
+          buffer.setMarkerHeadPosition(marker, [6, 2])
+          expect(observeHandler).not.toHaveBeenCalled()
+
     describe "marker updates due to buffer changes", ->
       [marker1, marker2] = []
 

spec/app/display-buffer-spec.coffee

@@ -604,10 +604,14 @@ describe "DisplayBuffer", ->
         expect(displayBuffer.getMarkerBufferRange(marker)).toEqual [[5, 4], [8, 4]]
 
     describe "observation", ->
+      observeHandler = null
+
+      beforeEach ->
+        observeHandler = jasmine.createSpy("observeHandler")
+
       describe ".observeMarkerHeadScreenPosition(marker, callback)", ->
         it "calls the callback whenever the markers head's screen position changes", ->
           marker = displayBuffer.markScreenRange([[5, 4], [5, 10]])
-          observeHandler = jasmine.createSpy("observeHandler")
           displayBuffer.observeMarkerHeadScreenPosition(marker, observeHandler)
           displayBuffer.setMarkerHeadScreenPosition(marker, [8, 20])
           expect(observeHandler).toHaveBeenCalled()
@@ -630,7 +634,6 @@ describe "DisplayBuffer", ->
 
         it "does not call the callback for screen changes that don't change the position of the marker", ->
           marker = displayBuffer.markScreenPosition([3, 4])
-          observeHandler = jasmine.createSpy("observeHandler")
           displayBuffer.observeMarkerHeadScreenPosition(marker, observeHandler)
 
           buffer.insert([3, 0], '...')
@@ -649,3 +652,13 @@ describe "DisplayBuffer", ->
           fold.destroy()
           expect(observeHandler).toHaveBeenCalled()
           expect(observeHandler.argsForCall[0][0]).toEqual [3, 7]
+
+        it "allows observation subscriptions to be cancelled", ->
+          marker = displayBuffer.markScreenRange([[5, 4], [5, 10]])
+          subscription = displayBuffer.observeMarkerHeadScreenPosition(marker, observeHandler)
+          subscription.cancel()
+          buffer.insert([11, 0], '...')
+          expect(observeHandler).not.toHaveBeenCalled()
+
+          displayBuffer.unfoldBufferRow(4)
+          expect(observeHandler).not.toHaveBeenCalled()

src/app/buffer-marker.coffee

@@ -53,6 +53,8 @@ class BufferMarker
 
   observeHeadPosition: (callback) ->
     @headPositionObservers.push(callback)
+    cancel: =>
+      _.remove(@headPositionObservers, callback)
 
   tryToInvalidate: (oldRange) ->
     containsStart = oldRange.containsPoint(@getStartPosition(), exclusive: true)

src/app/display-buffer.coffee

@@ -355,10 +355,18 @@ class DisplayBuffer
     @markerScreenPositionObservers[id] ?= { head: [], tail: [] }
     @cacheMarkerScreenPositions(id) unless @markerScreenPositions[id]
     @markerScreenPositionObservers[id].head.push(callback)
-    @buffer.observeMarkerHeadPosition id, (bufferPosition) =>
+    subscription = @buffer.observeMarkerHeadPosition id, (bufferPosition) =>
       @cacheMarkerScreenPositions(id)
       callback(@screenPositionForBufferPosition(bufferPosition))
 
+    cancel: =>
+      subscription.cancel()
+      { head, tail } = @markerScreenPositionObservers[id]
+      _.remove(head, callback)
+      unless head.length + tail.length
+        delete @markerScreenPositionObservers[id]
+        delete @markerScreenPositions[id]
+
   cacheMarkerScreenPositions: (id) ->
     @markerScreenPositions[id] = { head: @getMarkerHeadScreenPosition(id), tail: @getMarkerTailScreenPosition }