Clear up few todos, introduce tile processing promise to await single tiles.

src/tile.js

@@ -258,10 +258,12 @@ $.Tile = function(level, x, y, bounds, exists, url, context2D, loadWithAjax, aja
      */
     this.processing = false;
     /**
-     * Remembers last processing time of the tile, 1 if the tile has just been loaded.
+     * Processing promise, resolves when the tile exits processing, or
+     * resolves immediatelly if not in the processing state.
+     * @member {OpenSeadragon.Promise}
      * @private
      */
-    this.lastProcess = 0;
+    this.processingPromise = $.Promise.resolve();
 };
 
 /** @lends OpenSeadragon.Tile.prototype */
@@ -644,7 +646,6 @@ $.Tile.prototype = {
         let ref = this._caches[this._cKey];
         if (ref) {
             // make sure we free drawer internal cache if people change cache key externally
-            // todo make sure this is really needed even after refactoring
             ref.destroyInternalCache();
         }
         this._cKey = value;

src/tilecache.js

@@ -1069,7 +1069,7 @@
                         if ( prevTile.level <= cutoff ||
                             prevTile.beingDrawn ||
                             prevTile.loading ||
-                            prevTile.processing ) { //todo exempt from deletion, or block this routine on data updates
+                            prevTile.processing ) {
                             continue;
                         }
                         if ( !worstTile ) {

src/tiledimage.js

@@ -2132,9 +2132,6 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
         }
 
         function markTileAsReady() {
-            tile.lastProcess = false;
-            tile.processing = false;
-
             const fallbackCompletion = getCompletionCallback();
 
             /**
@@ -2183,8 +2180,7 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
 
 
         if (tileCacheCreated) {
-            const updatePromise = _this.viewer.world.requestTileInvalidateEvent([tile], now, false, true);
+            _this.viewer.world.requestTileInvalidateEvent([tile], now, false, true).then(markTileAsReady);
-            updatePromise.then(markTileAsReady);
         } else {
             // Tile-invalidated not called on each tile, but only on tiles with new data! Verify we share the main cache
             const origCache = tile.getCache(tile.originalCacheKey);
@@ -2199,15 +2195,12 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
                     tile.setCache(t.cacheKey, targetMainCache, true, false);
                     break;
                 } else if (t.processing) {
-                    // TODO consider something nicer, now just wait until subsequent routine finishes
+                    // Await once processing finishes - mark tile as loaded
-                    let interval = setInterval(() => {
+                    t.processingPromise.then(t => {
-                        if (!t.processing && t.getCache()) {
+                        const targetMainCache = t.getCache();
-                            const targetMainCache = t.getCache();
+                        tile.setCache(t.cacheKey, targetMainCache, true, false);
-                            tile.setCache(t.cacheKey, targetMainCache, true, false);
+                        markTileAsReady();
-                            clearInterval(interval);
+                    });
-                            markTileAsReady();
-                        }
-                    }, 50);
                     return;
                 }
             }

src/world.js

@@ -275,7 +275,7 @@ $.extend( $.World.prototype, $.EventSource.prototype, /** @lends OpenSeadragon.W
         }
 
         const tileList = [],
-            markedTiles = [];
+            tileFinishResolvers = [];
         for (const tile of tilesToProcess) {
             // We allow re-execution on tiles that are in process but have too low processing timestamp,
             // which must be solved by ensuring subsequent data calls in the suddenly outdated processing
@@ -289,11 +289,14 @@ $.extend( $.World.prototype, $.EventSource.prototype, /** @lends OpenSeadragon.W
             }
 
             for (let t of tileCache._tiles) {
-                // Mark all related tiles as processing and cache the references to unmark later on,
+                // Mark all related tiles as processing and register callback to unmark later on
-                // last processing is set to old processing (null if finished)
-                t.lastProcess = t.processing;
                 t.processing = tStamp;
-                markedTiles.push(t);
+                t.processingPromise = new $.Promise((resolve) => {
+                    tileFinishResolvers.push(() => {
+                        t.processing = false;
+                        resolve(t);
+                    });
+                });
             }
 
             tileCache.__invStamp = tStamp;
@@ -350,7 +353,7 @@ $.extend( $.World.prototype, $.EventSource.prototype, /** @lends OpenSeadragon.W
                         cache: workingCache,
                         targetKey: newCacheKey,
                         setAsMainCache: true,
-                        tileAllowNotLoaded: false //todo what if called from load event?
+                        tileAllowNotLoaded: tile.loading
                     });
                 } else if (restoreTiles) {
                     // If we requested restore, perform now
@@ -425,9 +428,8 @@ $.extend( $.World.prototype, $.EventSource.prototype, /** @lends OpenSeadragon.W
         });
 
         return $.Promise.all(jobList).then(() => {
-            for (let tile of markedTiles) {
+            for (let resolve of tileFinishResolvers) {
-                tile.lastProcess = false;
+                resolve();
-                tile.processing = false;
             }
             this.draw();
         });