Fixing rendering crash when a last prim was switching from hidden to visible

 #1925

canvas2D/src/Engine/babylon.modelRenderCache.ts

@@ -6,6 +6,7 @@
     export class GroupInstanceInfo {
         constructor(owner: Group2D, mrc: ModelRenderCache, partCount: number) {
             this._partCount = partCount;
+            this._primCount = 0;
             this.owner = owner;
             this.modelRenderCache = mrc;
             this.modelRenderCache.addRef();
@@ -18,11 +19,26 @@
             this.opaqueDirty = this.alphaTestDirty = this.transparentDirty = this.transparentOrderDirty = false;
         }
 
+        public incPrimCount() {
+            ++this._primCount;
+        }
+
         public dispose(): boolean {
             if (this._isDisposed) {
                 return false;
             }
 
+            // Disposed is called when a primitive instance is disposed, so we decrement the counter
+            --this._primCount;
+
+            // If the counter is still greater than 0 there's still other primitives using this GII
+            if (this._primCount > 0) {
+                return false;
+            }
+
+            // We're going to dispose this GII, first remove it from the dictionary
+            this.owner._renderableData._renderGroupInstancesInfo.remove(this.modelRenderCache.modelKey);
+
             if (this.modelRenderCache) {
                 this.modelRenderCache.dispose();
                 this.modelRenderCache = null;
@@ -130,6 +146,7 @@
         }
 
         private _partCount: number;
+        private _primCount: number;
         private _strides: number[];
         private _usedShaderCategories: string[];
         private _opaqueData: GroupInfoPartData[];

canvas2D/src/Engine/babylon.renderablePrim2d.ts

@@ -451,28 +451,19 @@
                 gii = part.groupInstanceInfo;
                 part.groupInstanceInfo = null;
             }
-            if (gii && !gii.isDisposed) {
-                let usedCount = 0;
+
+            if (gii) {
                 if (gii.hasOpaqueData) {
-                    let od = gii.opaqueData[0];
-                    usedCount += od._partData.usedElementCount;
                     gii.opaqueDirty = true;
                 }
                 if (gii.hasAlphaTestData) {
-                    let atd = gii.alphaTestData[0];
-                    usedCount += atd._partData.usedElementCount;
                     gii.alphaTestDirty = true;
                 }
                 if (gii.hasTransparentData) {
-                    let td = gii.transparentData[0];
-                    usedCount += td._partData.usedElementCount;
                     gii.transparentDirty = true;
                 }
 
-                if (usedCount === 0 && gii.modelRenderCache!=null) {
-                    this.renderGroup._renderableData._renderGroupInstancesInfo.remove(gii.modelRenderCache.modelKey);
-                    gii.dispose();
-                }
+                gii.dispose();
 
                 if (this._modelRenderCache) {
                     this._modelRenderCache.dispose();
@@ -480,6 +471,7 @@
                 }
 
             }
+
             this._instanceDataParts = null;
         }
 
@@ -597,6 +589,9 @@
                 part.groupInstanceInfo = gii;
             }
 
+            // Increment the primitive count as one more primitive is using this GroupInstanceInfo
+            gii.incPrimCount();
+
             return gii;
         }