From 73ffda10db762a8337b9a1ea994fdf2d4ad4f53f Mon Sep 17 00:00:00 2001
From: ahocevar <andreas.hocevar@gmail.com>
Date: Fri, 16 Nov 2018 12:19:29 +0100
Subject: [PATCH] Smarter interim tile creation

---
 src/ol/VectorImageTile.js | 28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/src/ol/VectorImageTile.js b/src/ol/VectorImageTile.js
index eaa36142c9..a8531a7dbe 100644
--- a/src/ol/VectorImageTile.js
+++ b/src/ol/VectorImageTile.js
@@ -6,7 +6,7 @@ import Tile from './Tile.js';
 import TileState from './TileState.js';
 import {createCanvasContext2D} from './dom.js';
 import {listen, unlistenByKey} from './events.js';
-import {getHeight, getIntersection, getWidth} from './extent.js';
+import {containsExtent, getHeight, getIntersection, getWidth} from './extent.js';
 import EventType from './events/EventType.js';
 import {loadFeaturesXhr} from './featureloader.js';
 import {VOID} from './functions.js';
@@ -103,6 +103,8 @@ class VectorImageTile extends Tile {
      */
     this.sourceTileListenerKeys_ = [];
 
+    this.sourceTilesLoaded = false;
+
     /**
      * Use only source tiles that are loaded already
      * @type {boolean}
@@ -148,16 +150,24 @@ class VectorImageTile extends Tile {
         this.finishLoading_();
       }
 
-      if (zoom <= tileCoord[0] && this.state != TileState.LOADED) {
-        while (zoom > tileGrid.getMinZoom()) {
+      if (!this.sourceTilesLoaded && !useLoadedOnly) {
+        let bestZoom = -1;
+        for (const key in sourceTiles) {
+          const sourceTile = sourceTiles[key];
+          if (sourceTile.getState() === TileState.LOADED) {
+            const sourceTileCoord = sourceTile.tileCoord;
+            const sourceTileExtent = sourceTileGrid.getTileCoordExtent(sourceTileCoord);
+            if (containsExtent(sourceTileExtent, extent) && sourceTileCoord[0] > bestZoom) {
+              bestZoom = sourceTileCoord[0];
+            }
+          }
+        }
+        if (bestZoom !== -1) {
           const tile = new VectorImageTile(tileCoord, state, sourceRevision,
             format, tileLoadFunction, urlTileCoord, tileUrlFunction,
             sourceTileGrid, tileGrid, sourceTiles, pixelRatio, projection,
-            tileClass, VOID, --zoom);
-          if (tile.state == TileState.LOADED) {
-            this.interimTile = tile;
-            break;
-          }
+            tileClass, VOID, bestZoom);
+          this.interimTile = tile;
         }
       }
     }
@@ -168,6 +178,7 @@ class VectorImageTile extends Tile {
    * @inheritDoc
    */
   disposeInternal() {
+    this.getInterimTile = super.getInterimTile;
     this.state = TileState.ABORT;
     this.changed();
     if (this.interimTile) {
@@ -312,6 +323,7 @@ class VectorImageTile extends Tile {
     if (loaded == this.tileKeys.length) {
       this.loadListenerKeys_.forEach(unlistenByKey);
       this.loadListenerKeys_.length = 0;
+      this.sourceTilesLoaded = true;
       this.setState(TileState.LOADED);
     } else {
       this.setState(empty == this.tileKeys.length ? TileState.EMPTY : TileState.ERROR);