Release xrView related memory when xr session stops (iOS) (#246)

Author: ryantrem

Modules/@babylonjs/react-native/ios/EngineViewManager.mm

@@ -26,13 +26,6 @@ - (instancetype)init:(RCTBridge*)_bridge {
         super.translatesAutoresizingMaskIntoConstraints = false;
         super.colorPixelFormat = MTLPixelFormatBGRA8Unorm_sRGB;
         super.depthStencilPixelFormat = MTLPixelFormatDepth32Float;
-
-        xrView = [[MTKView alloc] initWithFrame:self.bounds device:self.device];
-        xrView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
-        xrView.userInteractionEnabled = false;
-        xrView.hidden = true;
-        [self addSubview:xrView];
-        [BabylonNativeInterop updateXRView:xrView];
     }
     return self;
 }
@@ -60,14 +53,26 @@ - (void)touchesCancelled:(NSSet<UITouch*>*)touches withEvent:(UIEvent*)event {
 
 - (void)drawRect:(CGRect)rect {
     if ([BabylonNativeInterop isXRActive]) {
-        xrView.hidden = false;
-    } else {
-        xrView.hidden = true;
+        if (!xrView) {
+            xrView = [[MTKView alloc] initWithFrame:self.bounds device:self.device];
+            xrView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
+            xrView.userInteractionEnabled = false;
+            [self addSubview:xrView];
+            [BabylonNativeInterop updateXRView:xrView];
+        }
+    } else if (xrView) {
+        [BabylonNativeInterop updateXRView:nil];
+        [xrView removeFromSuperview];
+        xrView = nil;
     }
 
     [BabylonNativeInterop renderView];
 }
 
+-(void)dealloc {
+    [BabylonNativeInterop updateXRView:nil];
+}
+
 - (void)takeSnapshot {
     // We must take the screenshot on the main thread otherwise we might fail to get a valid handle on the view's image.
     dispatch_async(dispatch_get_main_queue(), ^{