Flutter开发之Dart语法基础

var name = 'name';
dynamic name1 = 'name1';
String name2 = 'name2';

// 变量的赋值
name = 'a';
name1 = 'a1';
name2 = 'a2';

var name;
dynamic name1;
String name2;

String name2 = 'name2';

final age = 10;
final int age1 = 20;

// final修饰的变量不能重新赋值, 会报错
age = 20;
age1 = 30;

// const
const m1 = 12;
const double m2 = 23;
const m3 = m1 + m2;

// final修饰的变量不能重新赋值, 会报错
m1 = 10;
m2 = 1.02;

assert(2 + 3 == 5);
assert(2 - 3 == -1);
assert(2 * 3 == 6);
assert(5 / 2 == 2.5);   // 结果是double类型
assert(5 ~/ 2 == 2);    // 结果是integer类型 
assert(5 % 2 == 1);     // 余数

var a = 0, b = 0;

b = a++;
print('a = $a, b = $b'); //a = 1, b = 0
b = ++a;
print('a = $a, b = $b'); //a = 2, b = 2


b = a--;
print('a = $a, b = $b'); //a = 1, b = 2
b = --a;
print('a = $a, b = $b'); //a = 0, b = 0

external bool identical(Object a, Object b);

if (emp is Person) { // Type check
  emp.firstName = 'Bob';
}

// 上面代码可简化为
(emp as Person).firstName = 'Bob';

// 给 a 变量赋值
a = value;  

// 复合赋值操作符
a += b;  // 等价于a = a + b;

// 如果 b 是 null，则赋值给 b；
// 如果不是 null，则 b 的值保持不变
b ??= value; 
     
// 如下所示:     
  var s;
  print(s);  // null
  print(s ?? 'str');  // str
  s ??= 'string';
  print(s);  // string

condition ? expr1 : expr2
// 如果 condition 是 true，执行 expr1 (并返回执行的结果)； 否则执行 expr2 并返回其结果

expr1 ?? expr2
// 如果 expr1 是 non-null，返回其值； 否则执行 expr2 并返回其结果

String toString() => msg ?? super.toString();

// 上面的代码等价于
String toString() => msg == null ? super.toString() : msg;

// 等价于
String toString() {
  if (msg == null) {
    return super.toString();
  } else {
    return msg;
  }
}

querySelector('#button') // Get an object.
  ..text = 'Confirm'   // Use its members.
  ..classes.add('important')
  ..onClick.listen((e) => window.alert('Confirmed!'));

// 上面代码等价于
var button = querySelector('#button');
button.text = 'Confirm';
button.classes.add('important');
button.onClick.listen((e) => window.alert('Confirmed!'));

final addressBook = (new AddressBookBuilder()
      ..name = 'jenny'
      ..email = 'jenny@example.com'
      ..phone = (new PhoneNumberBuilder()
            ..number = '415-555-0100'
            ..label = 'home')
          .build())
    .build();

if (a == 0) {
  print('a = 0');
} else if (a == 1) {
  print('a = 1');
} else {
  print('a = 2');
}

var arr = [0, 1, 2];

// for循环
for (var i = 0; i < arr.length; i++) {
  print(arr[i]);
}

// for-in循环
for (var x in arr) {
  print(x);
}

// while 循环在执行循环之前先判断条件是否满足：
while (c == 0) {
  print('c = $c');
}

// 而do-while循环是先执行循环代码再判断条件：
do {
  print('c = $c');
} while (c == 0);

while (true) {
  if (shutDownRequested()) break;
  processIncomingRequests();
}

for (int i = 0; i < candidates.length; i++) {
  var candidate = candidates[i];
  if (candidate.yearsExperience < 5) {
    continue;
  }
  candidate.interview();
}

var command = 'OPEN';
switch (command) {
  case 'CLOSED':
    print('CLOSED');
    break;
  case 'APPROVED':
    print('APPROVED');
    // break;
    // 这里非空的case, 没有break会报错
  case 'DENIED':
    // 这里空的case, 可以不要break
  case 'OPEN':
    print('OPEN');
    continue nowClosed;
//如果你需要实现这种继续到下一个 case 语句中继续执行，则可以 使用 continue 语句跳转到对应的标签（label）处继续执行：
nowClosed:
  case 'PENDING':
    print('PENDING');
    break;
  default:
    print('default');
}

// Make sure the variable has a non-null value
assert(text != null);

// Make sure the value is less than 100
assert(number < 100);

// Make sure this is an https URL
assert(urlString.startsWith('https'));