1
String? name  // Nullable type. Can be `null` or string.
2

3
String name   // Non-nullable type. Cannot be `null` but can be string.

1
int? lineCount;
2
assert(lineCount == null);

1
late String description;
2

3
void main() {
4
  description = 'Feijoada!';
5
  print(description);
6
}

1
late String test = testLateVariable();
2

3
late String test2 = "test2 late";
4

5
void main() async {
6
  print('Compute π using the Monte Carlo method.');
7
  print(test);
8
  print(test2);
9
}
10

11
String testLateVariable() {
12
  return "test late variable";
13
}

1
final person=new Person();
2
person=new People();//bad
3

4
person.name="John";//right，如果初始化的值是类，那么更改类是不行，但是类里面的值是可以，只要类里面值不是常量或者final修饰
5

6
const person=new Person();
7
person=new People();//bad
8
person.name="John"//bad

1
//块级作用域，或者方法作用域内
2
main() {
3
  var _ = 1;
4
  int _ = 2;
5
}
6

7
//for循环
8
for (var _ in list) {}
9

10
//catch子句
11
try {
12
  throw '!';
13
} catch (_) {
14
  print('oops');
15
}
16

17
//泛型和函数类型参数
18
class T<_> {}
19
void genericFunction<_>() {}
20

21
takeGenericCallback(<_>() => true);
22

23
//函数参数
24
Foo(_, this._, super._, void _()) {}
25

26
list.where((_) => true);
27

28
void f(void g(int _, bool _)) {}
29

30
typedef T = void Function(String _, String _);

1
// Parentheses improve readability.
2
if ((n % i == 0) && (d % i == 0)) {
3
  // ...
4
}
5

6
// Harder to read, but equivalent.
7
if (n % i == 0 && d % i == 0) {
8
  // ...
9
}

1
assert(2 + 3 == 5);
2
assert(2 - 3 == -1);
3
assert(2 * 3 == 6);
4
assert(5 / 2 == 2.5); // Result is a double
5
assert(5 ~/ 2 == 2); // Result is an int
6
assert(5 % 2 == 1); // Remainder
7

8
assert('5/2 = ${5 ~/ 2} r ${5 % 2}' == '5/2 = 2 r 1');

1
int a;
2
int b;
3

4
a = 0;
5
b = ++a; // Increment a before b gets its value.
6
assert(a == b); // 1 == 1
7

8
a = 0;
9
b = a++; // Increment a after b gets its value.
10
assert(a != b); // 1 != 0
11

12
a = 0;
13
b = --a; // Decrement a before b gets its value.
14
assert(a == b); // -1 == -1
15

16
a = 0;
17
b = a--; // Decrement a after b gets its value.
18
assert(a != b); // -1 != 0

1
assert(2 == 2);
2
assert(2 != 3);
3
assert(3 > 2);
4
assert(2 < 3);
5
assert(3 >= 3);
6
assert(2 <= 3);

1
//强制转换结果
2
(employee as Person).firstName = 'Bob';
3

4
//判断类型
5
if (employee is Person) {
6
  // Type check
7
  employee.firstName = 'Bob';
8
}

1
String playerName(String? name) => name ?? 'Guest';

1
var paint = Paint()
2
  ..color = Colors.black
3
  ..strokeCap = StrokeCap.round
4
  ..strokeWidth = 5.0;
5

6
//不要被上面的换行迷惑了，实际上就是一行代码，不过，上面易读性更高
7
var paint = Paint()..color = Colors.black..strokeCap = StrokeCap.round..strokeWidth = 5.0;
8

9

10
//上面的写法相当于下面的
11

12
var paint = Paint();
13
paint.color = Colors.black;
14
paint.strokeCap = StrokeCap.round;
15
paint.strokeWidth = 5.0;

1
//object?..variableName = 'value' 意思是如果object为空，才对成员变量“variableName”进行赋值，如果object不为空，那么“variableName”就不会赋值
2
document.querySelector('#confirm') // Get an object.
3
  ?..textContent =
4
      'Confirm' // Use its members.
5
  ..classList.add('important')
6
  ..onClick.listen((e) => window.alert('Confirmed!'))
7
  ..scrollIntoView();
8

9

10
//也可以采用下面的写法
11

12
final button = document.querySelector('#confirm');
13
button?.textContent = 'Confirm';
14
button?.classList.add('important');
15
button?.onClick.listen((e) => window.alert('Confirmed!'));
16
button?.scrollIntoView();
17

18

19
//更多的写法
20

21
final addressBook =
22
    (AddressBookBuilder()
23
          ..name = 'jenny'
24
          ..email = 'jenny@example.com'
25
          ..phone =
26
              (PhoneNumberBuilder()
27
                    ..number = '415-555-0100'
28
                    ..label = 'home')
29
                  .build())
30
        .build();

1
var sb = StringBuffer();
2
sb.write('foo')
3
  ..write('bar'); // Error: method 'write' isn't defined for 'void'.