add StreamBuilderSample

Samples/UIWidgetsSamples_2019_4/Assets/WidgetsSample/StreamBuilderSample.cs

+using System;
+using Unity.UIWidgets.async;
+using Unity.UIWidgets.engine;
+using Unity.UIWidgets.widgets;
+
+namespace UIWidgetsSample
+{
+    public class StreamBuilderSample : UIWidgetsPanel
+    {
+        protected override void main()
+        {
+            ui_.runApp(
+                new MyStreamBuilderWidget()
+            );
+        }
+    }
+
+    class MyStreamBuilderWidget : StatelessWidget
+    {
+        private Stream<int> counter()
+        {
+            return Stream<int>.periodic(new TimeSpan(0, 0, 0, 1), i =>
+            {
+                return i * 3;
+            }).take(5);
+        }
+        public override Widget build(BuildContext context)
+        {
+            return new WidgetsApp(
+                title: "Text Fields",
+                home: new StreamBuilder<int>(
+                    stream: counter(),
+                    builder: (BuildContext sub_context, AsyncSnapshot<int> snapshot) =>
+                    {
+                        if (snapshot.hasError)
+                            return new Text($"Error: {snapshot.error}");
+                        switch (snapshot.connectionState) {
+                            case ConnectionState.none:
+                                return new Text("没有Stream");
+                            case ConnectionState.waiting:
+                                return new Text("等待数据...");
+                            case ConnectionState.active:
+                                return new Text($"active: {snapshot.data}");
+                            case ConnectionState.done:
+                                return new Text("Stream已关闭");
+                        }
+                        return null; // unreachable
+                    }
+                ),
+                pageRouteBuilder: (settings, builder) =>
+                    new PageRouteBuilder(
+                        settings: settings,
+                        pageBuilder: (Buildcontext, animation, secondaryAnimation) => builder(context)
+                    )
+            );
+        }
+    }
+}

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

+fileFormatVersion: 2
+guid: 97f298138594499f8355a91dab4721fc
+timeCreated: 1629451124