关于Dart List，你必须知道的几个方法

forEach()

遍历List中的每一个元素

List list = [19, 20, 12];
list.forEach((f) => print('$f is ${f > 18 ? 'bigger' : 'smaller'}' + ' than 18'));

打印结果

19 is bigger than 18
20 is bigger than 18
12 is smaller than 18

map()

map方法也是遍历List中的每一个元素。它与forEach的区别在于，map会返回一个新的List对象。

List date = ['09-12', '10-10', '12-25'];
print('date is $date');
List newDate = date.map((f) => '2019-' + f).toList();
print('newDate is $newDate');

打印结果

date is [09-12, 10-10, 12-25]
newDate is [2019-09-12, 2019-10-10, 2019-12-25]

sort()

根据传入的函数，给List排序

  List nums = [9, 4, 7, 10];
  nums.sort((a, b) => a - b);
  print('after sort nums is $nums');

  // 在unicode编码表中 C在c之前
  List strs = ['angela', 'danny', 'ben', 'Catherine'];
  strs.sort((a, b) => a.toLowerCase().compareTo(b.toLowerCase()));
  print(strs);

打印结果

after sort nums is [4, 7, 9, 10]
[angela, ben, Catherine, danny]

reduce(), fold()

依次访问List中所有的元素，并将List中所有的元素根据传入的函数压缩成一个数值。

  List list = [1, 2, 3, 4, 5];
  var value = list.reduce((a, b) => a * b);
  print('value is $value');

  var value1 = list.fold(10, (a, b) => a * b);
  print('value1 is $value1');

打印结果：

value is 120
value1 is 1200

every()

遍历List中每一个元素，验证是否每一个元素都满足传入函数的要求

List students = ['danny', 'david', 'erik'];
bool startWithD = students.every((s) => s.startsWith('d'));
print('startWithD is $startWithD');

打印结果：

startWithD is false

take(), skip()

take从0下标开始，裁剪List到传入下标
skip从传入下标开始，裁剪List到最后一个元素

  var fiboNumbers = [1, 2, 3, 5, 8, 13, 21];
  var firstTwo = fiboNumbers.take(2);
  print('firstTwo is $firstTwo');

  var lastTwo = fiboNumbers.skip(5);
  print('lastTwo is $lastTwo');

打印结果：

firstTwo is (1, 2)
lastTwo is (13, 21)

where()

返回满足传入函数的集合

  List> users = [
    {'name': 'John', 'age': 18},
    {'name': 'Jane', 'age': 21},
    {'name': 'Mary', 'age': 23}
  ];

  var over21s = users.where((user) => user['age'] > 21);
  print('there are ${over21s.length} uses over 21 years old');

打印结果：

there are 1 over 21 years old

expand()

遍历List中每一个元素，根据传入函数将所遍历元素扩展。将最后所有得出的结果合并为一个新的List。

  List list = [1, 2, 3];
  var newList1 = list.expand((each) => [each, each + 1]).toList();
  print('newList1 is $newList1');

  List list1 = [
    [1, 2],
    [2, 3]
  ];
  var newList2 = list1.expand((each) => each).toList();
  print('newList2 is $newList2');

打印结果：

newList1 is [1, 2, 2, 3, 3, 4]
newList2 is [1, 2, 2, 3]