模式

switch (number) {
  // Constant pattern matches if 1 == number.
  case 1:
    print('one');
}

const a = 'a';
const b = 'b';
switch (obj) {
  // List pattern [a, b] matches obj first if obj is a list with two fields,
  // then if its fields match the constant subpatterns 'a' and 'b'.
  case [a, b]:
    print('$a, $b');
}

var numList = [1, 2, 3];
// List pattern [a, b, c] destructures the three elements from numList...
var [a, b, c] = numList;
// ...and assigns them to new variables.
print(a + b + c);

switch (list) {
  case ['a' || 'b', var c]:
    print(c);
}

// Declares new variables a, b, and c.
var (a, [b, c]) = ('str', [1, 2]);

var (a, b) = ('left', 'right');
(b, a) = (a, b); // Swap.
print('$a $b'); // Prints "right left".

switch (obj) {
  // Matches if 1 == obj.
  case 1:
    print('one');

  // Matches if the value of obj is between the
  // constant values of 'first' and 'last'.
  case >= first && <= last:
    print('in range');

  // Matches if obj is a record with two fields,
  // then assigns the fields to 'a' and 'b'.
  case (var a, var b):
    print('a = $a, b = $b');

  default:
}

var isPrimary = switch (color) {
  Color.red || Color.yellow || Color.blue => true,
  _ => false
};

switch (shape) {
  case Square(size: var s) || Circle(size: var s) when s > 0:
    print('Non-empty symmetric shape');
}

switch (pair) {
  case (int a, int b):
    if (a > b) print('First element greater');
  // If false, prints nothing and exits the switch.
  case (int a, int b) when a > b:
    // If false, prints nothing but proceeds to next case.
    print('First element greater');
  case (int a, int b):
    print('First element not greater');
}

Map<String, int> hist = {
  'a': 23,
  'b': 100,
};

for (var MapEntry(key: key, value: count) in hist.entries) {
  print('$key occurred $count times');
}

for (var MapEntry(:key, value: count) in hist.entries) {
  print('$key occurred $count times');
}

var info = userInfo(json);
var name = info.$1;
var age = info.$2;

var (name, age) = userInfo(json);

final (:name, :age) =
    getData(); // For example, return (name: 'doug', age: 25);

final Foo myFoo = Foo(one: 'one', two: 2);
var Foo(:one, :two) = myFoo;
print('one $one, two $two');

sealed class Shape {}

class Square implements Shape {
  final double length;
  Square(this.length);
}

class Circle implements Shape {
  final double radius;
  Circle(this.radius);
}

double calculateArea(Shape shape) => switch (shape) {
      Square(length: var l) => l * l,
      Circle(radius: var r) => math.pi * r * r
    };

var json = {
  'user': ['Lily', 13]
};
var {'user': [name, age]} = json;

if (json is Map<String, Object?> &&
    json.length == 1 &&
    json.containsKey('user')) {
  var user = json['user'];
  if (user is List<Object> &&
      user.length == 2 &&
      user[0] is String &&
      user[1] is int) {
    var name = user[0] as String;
    var age = user[1] as int;
    print('User $name is $age years old.');
  }
}

if (json case {'user': [String name, int age]}) {
  print('User $name is $age years old.');
}