新增 week 31 draggable demo

mecury_project/example/flutter_widget_of_the_week/lib/main.dart

@@ -41,7 +41,8 @@ class MyApp extends StatelessWidget {
 //      home: Week27(),
 //      home: Week28(),
 //      home: Week29(),
-      home: Week30(),
+//      home: Week30(),
+      home: Week31(),
     );
   }
 }

mecury_project/example/flutter_widget_of_the_week/lib/widget/week31_draggable.dart

@@ -0,0 +1,69 @@
+import 'package:flutter/material.dart';
+
+class Week31 extends StatefulWidget {
+  @override
+  _Week31State createState() => _Week31State();
+}
+
+class _Week31State extends State<Week31> {
+  Color color = Colors.grey;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: Column(
+        children: <Widget>[
+          Expanded(
+            child: Center(
+              child: Draggable<Color>(
+                data: Colors.deepPurpleAccent,
+                feedback: Container(
+                  height: 100,
+                  width: 100,
+                  color: Colors.deepPurpleAccent.withOpacity(0.5),
+                ),
+                child: Container(
+                  height: 100,
+                  width: 100,
+                  color: Colors.deepPurpleAccent,
+                ),
+                childWhenDragging: Container(
+                  height: 100,
+                  width: 100,
+                  color: Colors.grey.withOpacity(0.5),
+                ),
+              ),
+            ),
+          ),
+          Expanded(
+            child: Center(
+              child: DragTarget<Color>(
+                onWillAccept: (color){
+                  return color == Colors.deepPurpleAccent;
+                },
+                builder: (context, colors, rejectedData){
+                  print(colors.toString());
+                  return Container(
+                    height: 200,
+                    width: 200,
+                    color: colors.length > 0 ? colors[0] : color,
+                  );
+                },
+                onAccept: (color){
+                  setState(() {
+                    this.color = color;
+                  });
+                },
+                onLeave: (color){
+                  setState(() {
+                    this.color = color;
+                  });
+                },
+              ),
+            ),
+          )
+        ],
+      ),
+    );
+  }
+}

mecury_project/example/flutter_widget_of_the_week/lib/widget/widgets.dart

@@ -27,4 +27,5 @@ export 'week26_position.dart';
 export 'week27_animated_builder.dart';
 export 'week28_dismissible.dart';
 export 'week29_sizedbox.dart';
-export 'week30_value_listenable_builder.dart';
+export 'week30_value_listenable_builder.dart';
+export 'week31_draggable.dart';