feature: 增加异步相关

Author: Guo-Zhang

dart_language/async.md

@@ -1,6 +1,6 @@
 # 异步
 
-同步写法：
+同步写法
 
 ```dart
 // This example uses the Google Books API to search
@@ -19,30 +19,8 @@ http.get(url).then((result)=>print(result.body));
 
 修改自[DartPad提供的样例](https://dartpad.cn/?id=4a68e553746602d851ab3da6aeafc3dd)。
 
-## Flutter
+## Future vs Stream
 
-https://api.flutter.dev/flutter/widgets/FutureBuilder-class.html
+单次 vs 多次
 
-```dart
-Widget buildOutput(input){
-  return FutureBuilder(
-    // https://api.flutter.dev/flutter/widgets/FutureBuilder-class.html
-    future: executor.execute(input),
-    builder: (BuildContext context, AsyncSnapshot<dynamic> snapshot) {
-    if (snapshot.connectionState == ConnectionState.done) {
-        // 请求已结束
-        if (snapshot.hasError) {
-        // 请求失败，显示错误
-        return Text("Error: ${snapshot.error}");
-        } else {
-        // 请求成功，显示数据
-        return Text("${snapshot.data}");
-        }
-    } else {
-        // 请求未结束，显示正在加载的状态
-        return const Text('代码运行中...');
-    }
-    },
-  );
-}
-```
+详见：https://www.digitalocean.com/community/tutorials/flutter-futures-and-streams-dart

widgets/asnyc.md

@@ -0,0 +1,29 @@
+# 异步组件
+
+## FutureBuilder
+
+https://api.flutter.dev/flutter/widgets/FutureBuilder-class.html
+
+```dart
+Widget buildOutput(input){
+  return FutureBuilder(
+    // https://api.flutter.dev/flutter/widgets/FutureBuilder-class.html
+    future: executor.execute(input),
+    builder: (BuildContext context, AsyncSnapshot<dynamic> snapshot) {
+    if (snapshot.connectionState == ConnectionState.done) {
+        // 请求已结束
+        if (snapshot.hasError) {
+        // 请求失败，显示错误
+        return Text("Error: ${snapshot.error}");
+        } else {
+        // 请求成功，显示数据
+        return Text("${snapshot.data}");
+        }
+    } else {
+        // 请求未结束，显示正在加载的状态
+        return const Text('代码运行中...');
+    }
+    },
+  );
+}
+```