Merge pull request #285 from Unity-Technologies/zxw/fix_backdropfilter

fix typo in assertion and add a backdropfilter sample

com.unity.uiwidgets/Runtime/rendering/layer.cs

             Path clipPath = null,
             Clip clipBehavior = Clip.antiAlias
         ) {
-            D.assert(clipPath != null);
+            D.assert(clipBehavior != null);
             D.assert(clipBehavior != Clip.none);
             _clipPath = clipPath;
             _clipBehavior = clipBehavior;

Samples/UIWidgetsSamples_2019_4/Assets/WidgetsSample/BackdropFilterSample.cs

+using uiwidgets;
+using Unity.UIWidgets.engine;
+using Unity.UIWidgets.foundation;
+using Unity.UIWidgets.painting;
+using Unity.UIWidgets.ui;
+using Unity.UIWidgets.widgets;
+using Color = Unity.UIWidgets.ui.Color;
+using ui_ = Unity.UIWidgets.widgets.ui_;
+
+namespace UIWidgetsSample {
+    public class BackdropFilterSample : UIWidgetsPanel {
+        protected override void main() {
+            ui_.runApp(new MyApp());
+        }
+        
+        class MyApp : StatelessWidget
+        {
+            public override Widget build(BuildContext context)
+            {
+                return new WidgetsApp(
+                    color: Color.white,
+                    home: new BackdropFilterApp(),
+                    pageRouteBuilder: (settings, builder) =>
+                        new PageRouteBuilder(
+                            settings: settings,
+                            pageBuilder: (Buildcontext, animation, secondaryAnimation) => builder(context)
+                        )
+                );
+            }
+        }
+
+        class BackdropFilterApp : StatefulWidget {
+            public BackdropFilterApp(Key key = null) : base(key) {
+            }
+
+            public override State createState() {
+                return new BackdropFilterAppState();
+            }
+        }
+
+        class BackdropFilterAppState : State<BackdropFilterApp> {
+            public override Widget build(BuildContext context)
+            {
+                var item = new Container(
+                    width: 350,
+                    height: 300,
+                    decoration: new BoxDecoration(
+                        image: new DecorationImage(
+                            image: new FileImage("shrine_images/0-0.jpg"),
+                            fit: BoxFit.cover
+                        )
+                    ),
+                    child: new BackdropFilter(
+                        filter: ImageFilter.blur(sigmaX: 5f, sigmaY: 5f),
+                        child: new Container(
+                            color: Colors.black.withOpacity(0.1f)
+                        )
+                    )
+                );
+                
+                return new Container(color: Colors.blue, child:
+                    new Center(
+                        child: item
+                    ));
+            }
+        }
+    }
+}

Samples/UIWidgetsSamples_2019_4/Assets/WidgetsSample/BackdropFilterSample.cs.meta

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