Render older loaded tiles while waiting for new tiles

test/spec/ol/imagetile.test.js

@@ -60,7 +60,73 @@ describe('ol.ImageTile', function() {
 
       tile.load();
     });
+  });
 
+  describe('interimChain', function() {
+    var head, renderTile;
+    beforeEach(function() {
+      var tileCoord = [0, 0, 0];
+      var src = 'spec/ol/data/osm-0-0-0.png';
+      var tileLoadFunction = ol.source.Image.defaultImageLoadFunction;
+      head = new ol.ImageTile(tileCoord, ol.Tile.State.IDLE, src, null, tileLoadFunction);
+      ol.getUid(head);
+
+      var addToChain = function(tile, state) {
+        var next = new ol.ImageTile(tileCoord, state, src, null, tileLoadFunction);
+        ol.getUid(next);
+        tile.interimTile = next;
+        return next;
+      };
+      var tail = addToChain(head,ol.Tile.State.IDLE); //discard, deprecated by head
+      tail = addToChain(tail,ol.Tile.State.LOADING); //keep, request already going
+      tail = addToChain(tail,ol.Tile.State.IDLE); //discard, deprecated by head
+      tail = addToChain(tail,ol.Tile.State.LOADED); //keep, use for rendering
+      renderTile = tail; //store this tile for later tests
+      tail = addToChain(tail,ol.Tile.State.IDLE);  //rest of list outdated by tile above
+      tail = addToChain(tail,ol.Tile.State.LOADED);
+      tail = addToChain(tail,ol.Tile.State.LOADING);
+      tail = addToChain(tail,ol.Tile.State.LOADED);
+
+    });
+
+    it('shrinks tile chain correctly', function(done) {
+      var chainLength = function(tile) {
+        var c = 0;
+        while (tile) {
+          ++c;
+          tile = tile.interimTile;
+        }
+        return c;
+      };
+
+      expect(chainLength(head)).to.be(9);
+      head.refreshInterimChain();
+      expect(chainLength(head)).to.be(3);
+      done();
+    });
+
+    it('gives the right tile to render', function(done) {
+      expect(head.getInterimTile()).to.be(renderTile);
+      head.refreshInterimChain();
+      expect(head.getInterimTile()).to.be(renderTile);
+      done();
+    });
+
+    it('discards everything after the render tile', function(done) {
+      head.refreshInterimChain();
+      expect(renderTile.interimTile).to.be(null);
+      done();
+    });
+
+    it('preserves order of tiles', function(done) {
+      head.refreshInterimChain();
+      while (head.interimTile !== null) {
+        //use property of ol.getUid returning increasing id's.
+        expect(ol.getUid(head) < ol.getUid(head.interimTile));
+        head = head.interimTile;
+      }
+      done();
+    });
   });
 
 });