Dart

Dart

1. 基础

1.1 变量

1. var
2. final 和 const

####1.2 Built-in-types

• numbers
• strings
• booleans
• lists (also known as arrays)
• sets
• maps
• runes 符号 (for expressing Unicode characters in a string)
• symbols

####

2. 特性

1. 混用继承

   class A {
   }
   class B {
   }
      
   class C extends A with B {
   }
   

2. 接口实现

   class A implements B {
   }
   abstract class B {
     void describe();
   }
   

3. 异步Async

   const oneSecond = Duration(seconds: 1);
   Future<void> printWithDelay(String message) async {
     await Future.delayed(oneSecond);
     print(message);
   }
   

4. Async*

   Stream<String> report(Spacecraft craft, Iterable<String> objects) async* {
     for (var object in objects) {
       await Future.delayed(oneSecond);
       yield '${craft.name} flies by $object';
     }
   }
   

5. 异常

   // 抛出异常
   if (astronauts == 0) {
     throw StateError('No astronauts.');
   }
   // try on or catch(both)
   try {
     breedMoreLlamas();
   } on OutOfLlamasException {
     // A specific exception
     buyMoreLlamas();
   } on Exception catch (e) {
     // Anything else that is an exception
     print('Unknown exception: $e');
   } catch (e) {
     // No specified type, handles all
     print('Something really unknown: $e');
   }
   

6. ?? ?.

   int a;
   print(a ??= 3); // print 3
   print(a ?? 5); // print 3
   print(null ?? 'This is null'); // print 'This is null'
      
   myObject?.someProperty
   /*<=>*/
   (myObject != null) ? myObject.someProperty : 'myObject is null'
   

7. Collection—list, set, map

   final aListOfStrings = ['one', 'two', 'three'];
   final aSetOfStrings = {'one', 'two', 'three'};
   final aMapOfStringsToInts = {
     'one': 1,
     'two': 2,
     'three': 3,
   };
   final aListOfInts = <int>[];
   final aSetOfInts = <int>{};
   final aMapOfIntToDouble = <int, double>{};
   

8. =>

   bool hasEmpty = aListOfStrings.any((s) => s.isEmpty);
   /*<=>*/
   bool hasEmpty = aListOfStrings.any((s) {
     return s.isEmpty;
   });
   

9. 串联调用

   var button = querySelector('#confirm');
   button.text = 'Confirm';
   button.classes.add('important');
   button.onClick.listen((e) => window.alert('Confirmed!'));
   /*<=>*/
   querySelector('#confirm')
   ..text = 'Confirm'
   ..classes.add('important')
   ..onClick.listen((e) => window.alert('Confirmed!'));
   

10. factory

    class Square extends Shape {}
    class Circle extends Shape {}
    class Shape {
      Shape();
      factory Shape.fromTypeName(String typeName) {
        if (typeName == 'square') return Square();
        if (typeName == 'circle') return Circle();
        print('I don\'t recognize $typeName');
        return null;
      }
    }
    

11. 重定向构造函数, 没有重装

    class Automobile {
      String make;
      String model;
      int mpg;
      // The main constructor for this class.
      Automobile(this.make, this.model, this.mpg);
      // Delegates to the main constructor.
      Automobile.hybrid(String make, String model) : this(make, model, 60);
      // Delegates to a named constructor
      Automobile.fancyHybrid() : this.hybrid('Futurecar', 'Mark 2');
    }
    

2. 异步 Future + async + await

1. 异步场景
   1. 从网上获取数据Fetching data over a network
   2. 读写DB Writing to a database
   3. 从文件中读数据 Reading data from a file

2. 与同步接口对比

   // Synchronous
   String createOrderMessage () {
     var order = getUserOrder();
     return 'Your order is: $order';
   }
      
   Future<String> getUserOrder() {
     // Imagine that this function is
     // more complex and slow.
     return 
       Future.delayed(
         Duration(seconds: 4), () => 'Large Latte'); 
   }
      
   // Synchronous
   main() {
     print('Fetching user order...');
     print(createOrderMessage());
   }
      
   // 'Fetching user order...'
   // 'Your order is: Instance of _Future<String>'
   

   // Asynchronous
   Future<String> createOrderMessage () async {
     var order = await getUserOrder();
     return 'Your order is: $order';
   }
      
   Future<String> getUserOrder() {
     // Imagine that this function is
     // more complex and slow.
     return
      Future.delayed(
        Duration(seconds: 4), () => 'Large Latte');
   }
      
   // Asynchronous
   main() async {
     print('Fetching user order...');
     print(await createOrderMessage());
   }
      
   // 'Fetching user order...'
   // 'Your order is: Large Latte'