Fix tileset style recompilation edge cases

CHANGES.md

@@ -1,5 +1,11 @@
 # Change Log
 
+### 1.76 - 2020-12-01
+
+##### Fixes :wrench:
+
+- Fixed an issue where tileset styles would be reapplied every frame when a tileset has a style and `tileset.preloadWhenHidden` is true and `tileset.show` is false. Also fixed a related issue where styles would be reapplied if the style being set is the same as the active style. [#9223](https://github.com/CesiumGS/cesium/pull/9223)
+
 ### 1.75 - 2020-11-02
 
 ##### Fixes :wrench:

Source/Scene/Cesium3DTileStyleEngine.js

@@ -15,6 +15,9 @@ Object.defineProperties(Cesium3DTileStyleEngine.prototype, {
       return this._style;
     },
     set: function (value) {
+      if (value === this._style) {
+        return;
+      }
       this._style = value;
       this._styleDirty = true;
     },
@@ -25,7 +28,11 @@ Cesium3DTileStyleEngine.prototype.makeDirty = function () {
   this._styleDirty = true;
 };
 
-Cesium3DTileStyleEngine.prototype.applyStyle = function (tileset, passOptions) {
+Cesium3DTileStyleEngine.prototype.resetDirty = function () {
+  this._styleDirty = false;
+};
+
+Cesium3DTileStyleEngine.prototype.applyStyle = function (tileset) {
   if (!tileset.ready) {
     return;
   }
@@ -36,11 +43,6 @@ Cesium3DTileStyleEngine.prototype.applyStyle = function (tileset, passOptions) {
 
   var styleDirty = this._styleDirty;
 
-  if (passOptions.isRender) {
-    // Don't reset until the render pass
-    this._styleDirty = false;
-  }
-
   if (styleDirty) {
     // Increase "time", so the style is applied to all visible tiles
     ++this._lastStyleTime;

Source/Scene/Cesium3DTileset.js

@@ -1892,6 +1892,7 @@ Cesium3DTileset.prototype.postPassesUpdate = function (frameState) {
   cancelOutOfViewRequests(this, frameState);
   raiseLoadProgressEvent(this, frameState);
   this._cache.unloadTiles(this, unloadTile);
+  this._styleEngine.resetDirty();
 };
 
 /**
@@ -2173,7 +2174,7 @@ function updateTileDebugLabels(tileset, frameState) {
 }
 
 function updateTiles(tileset, frameState, passOptions) {
-  tileset._styleEngine.applyStyle(tileset, passOptions);
+  tileset._styleEngine.applyStyle(tileset);
 
   var isRender = passOptions.isRender;
   var statistics = tileset._statistics;

Specs/Scene/Cesium3DTilesetSpec.js

@@ -2866,6 +2866,40 @@ describe(
       );
     });
 
+    it("doesn't re-evaluate style during the next update", function () {
+      return Cesium3DTilesTester.loadTileset(scene, withBatchTableUrl).then(
+        function (tileset) {
+          tileset.show = false;
+          tileset.preloadWhenHidden = true;
+          tileset.style = new Cesium3DTileStyle({ color: 'color("red")' });
+          scene.renderForSpecs();
+
+          var statistics = tileset._statisticsPerPass[Cesium3DTilePass.PRELOAD];
+          expect(statistics.numberOfTilesStyled).toBe(1);
+
+          scene.renderForSpecs();
+          expect(statistics.numberOfTilesStyled).toBe(0);
+        }
+      );
+    });
+
+    it("doesn't re-evaluate style if the style being set is the same as the active style", function () {
+      return Cesium3DTilesTester.loadTileset(scene, withBatchTableUrl).then(
+        function (tileset) {
+          var style = new Cesium3DTileStyle({ color: 'color("red")' });
+          tileset.style = style;
+          scene.renderForSpecs();
+
+          var statistics = tileset._statisticsPerPass[Cesium3DTilePass.RENDER];
+          expect(statistics.numberOfTilesStyled).toBe(1);
+
+          tileset.style = style;
+          scene.renderForSpecs();
+          expect(statistics.numberOfTilesStyled).toBe(0);
+        }
+      );
+    });
+
     function testColorBlendMode(url) {
       return Cesium3DTilesTester.loadTileset(scene, url).then(function (
         tileset