Add ViewRegistry.prototype.getNextUpdatePromise

Signed-off-by: Max Brunsfeld <maxbrunsfeld@github.com>

spec/view-registry-spec.coffee

@@ -209,3 +209,21 @@ describe "ViewRegistry", ->
       window.dispatchEvent(new UIEvent('resize'))
 
       expect(events).toEqual ['poll 1', 'poll 2']
+
+  describe "::getNextUpdatePromise()", ->
+    it "returns a promise that resolves at the end of the next update cycle", ->
+      updateCalled = false
+      readCalled = false
+      pollCalled = false
+
+      waitsFor 'getNextUpdatePromise to resolve', (done) ->
+        registry.getNextUpdatePromise().then ->
+          expect(updateCalled).toBe true
+          expect(readCalled).toBe true
+          expect(pollCalled).toBe true
+          done()
+
+        registry.updateDocument -> updateCalled = true
+        registry.readDocument -> readCalled = true
+        registry.pollDocument -> pollCalled = true
+        registry.pollAfterNextUpdate()

src/view-registry.coffee

@@ -195,6 +195,10 @@ class ViewRegistry
   pollAfterNextUpdate: ->
     @performDocumentPollAfterUpdate = true
 
+  getNextUpdatePromise: ->
+    @nextUpdatePromise ?= new Promise (resolve) =>
+      @resolveNextUpdatePromise = resolve
+
   clearDocumentRequests: ->
     @documentReaders = []
     @documentWriters = []
@@ -220,6 +224,9 @@ class ViewRegistry
     # process updates requested as a result of reads
     writer() while writer = @documentWriters.shift()
 
+    @nextUpdatePromise = null
+    @resolveNextUpdatePromise?()
+
   startPollingDocument: ->
     window.addEventListener('resize', @requestDocumentPoll)
     @observer.observe(document, {subtree: true, childList: true, attributes: true})