Android开发中Dart语言7个很酷的特点

var paint = Paint();
paint.color = Colors.black;
paint.strokeCap = StrokeCap.round;
paint.strokeWidth = 5.0;
//above block of code when optimized
var paint = Paint()
  ..color = Colors.black
  ..strokeCap = StrokeCap.round
  ..strokeWidth = 5.0;

// This class is declared abstract and thus
// can't be instantiated.
abstract class AbstractContainer {
  // Define constructors, fields, methods...
  void updateChildren(); // Abstract method.
}

class Logger {
  String name;
  Logger(this.name);
  factory Logger.fromJson(Map<String, Object> json) {
    return Logger(json['name'].toString());
  }
}

class Points {
  final double x;
  final double y;
  //unnamed constructor
  Points(this.x, this.y);
  // Named constructor
  Points.origin(double x,double y)
      : x = x,
        y = y;
  // Named constructor
  Points.destination(double x,double y)
      : x = x,
        y = y;
}

class Musician {}
//creating a mixin
mixin Feedback {
  void boo() {
    print('boooing');
  }
  void clap() {
    print('clapping');
  }
}
//only classes that extend or implement the Musician class
//can use the mixin Song
mixin Song on Musician {
  void play() {
    print('-------playing------');
  }
  void stop() {
    print('....stopping.....');
  }
}
//To use a mixin, use the with keyword followed by one or more mixin names
class PerformSong extends Musician with Feedback, Song {
  //Because PerformSong extends Musician,
  //PerformSong can mix in Song
  void awesomeSong() {
    play();
    clap();
  }
  void badSong() {
    play();
    boo();
  }
}
void main() {
  PerformSong().awesomeSong();
  PerformSong().stop();
  PerformSong().badSong();
}

typedef IntList = List<int>;
List<int> i1=[1,2,3]; // normal way.
IntList i2 = [1, 2, 3]; // Same thing but shorter and clearer.
//type alias can have type parameters
typedef ListMapper<X> = Map<X, List<X>>;
Map<String, List<String>> m1 = {}; // normal way.
ListMapper<String> m2 = {}; // Same thing but shorter and clearer.

//extension to convert a string to a number
extension NumberParsing on String {
  int customParseInt() {
    return int.parse(this);
  }
  double customParseDouble() {
    return double.parse(this);
  }
}
void main() {
  //various ways to use the extension
  var d = '21'.customParseDouble();
  print(d);
  var i = NumberParsing('20').customParseInt();
  print(i);
}

String joinWithCommas(int a, [int? b, int? c, int? d, int e = 100]) {
  var total = '$a';
  if (b != null) total = '$total,$b';
  if (c != null) total = '$total,$c';
  if (d != null) total = '$total,$d';
  total = '$total,$e';
  return total;
}
void main() {
  var result = joinWithCommas(1, 2);
  print(result);
}

import 'dart:async';
Future doSomething() {
  return Future.delayed(Duration(seconds: 5));
}
void main() async {
  //the function is fired and awaited till completion
  await doSomething();
  // Explicitly-ignored
  //The function is fired and forgotten
  unawaited(doSomething());
}