Dart语言指南（一） 顶

// Define a function.
printNumber(num aNumber) {
  print('The number is $aNumber.'); // Print to console.
}

// This is where the app starts executing.
main() {
  var number = 42; // Declare and initialize a variable.
  printNumber(number); // Call a function.
}

var name = 'Bob';

int lineCount;
assert(lineCount == null);
// Variables (even if they will be numbers) are initially null.

String name = 'Bob';

final name = 'Bob'; // Or: final String name = 'Bob';
// name = 'Alice';  // Uncommenting this causes an error

const bar = 1000000;       // Unit of pressure (dynes/cm2)
const atm = 1.01325 * bar; // Standard atmosphere

// Note: [] 创建一个空列表.
// const [] creates an empty, immutable list (EIA).
var foo = const [];   // foo is currently an EIA.
final bar = const []; // bar will always be an EIA.
const baz = const []; // baz is a compile-time constant EIA.

// You can change the value of a non-final, non-const variable,
// even if it used to have a const value.
foo = [];

// You can't change the value of a final or const variable.
// bar = []; // Unhandled exception.
// baz = []; // Unhandled exception.

var x = 1;
var hex = 0xDEADBEEF;
var bigInt = 34653465834652437659238476592374958739845729;

var y = 1.1;
var exponents = 1.42e5;

// String -> int
var one = int.parse('1');
assert(one == 1);

// String -> double
var onePointOne = double.parse('1.1');
assert(onePointOne == 1.1);

// int -> String
String oneAsString = 1.toString();
assert(oneAsString == '1');

// double -> String
String piAsString = 3.14159.toStringAsFixed(2);
assert(piAsString == '3.14');

assert((3 << 1) == 6);  // 0011 << 1 == 0110
assert((3 >> 1) == 1);  // 0011 >> 1 == 0001
assert((3 | 4)  == 7);  // 0011 | 0100 == 0111

const msPerSecond = 1000;
const secondsUntilRetry = 5;
const msUntilRetry = secondsUntilRetry * msPerSecond;

var s1 = 'Single quotes work well for string literals.';
var s2 = "Double quotes work just as well.";
var s3 = 'It\'s easy to escape the string delimiter.';
var s4 = "It's even easier to use the other delimiter.";

var s = 'string interpolation';

assert('Dart has $s, which is very handy.' ==
       'Dart has string interpolation, ' +
       'which is very handy.');
assert('That deserves all caps. ' +
       '${s.toUpperCase()} is very handy!' ==
       'That deserves all caps. ' +
       'STRING INTERPOLATION is very handy!');

var s1 = 'String ' 'concatenation'
         " works even over line breaks.";
assert(s1 == 'String concatenation works even over '
             'line breaks.');

var s2 = 'The + operator '
         + 'works, as well.';
assert(s2 == 'The + operator works, as well.');

var s1 = '''
You can create
multi-line strings like this one.
''';

var s2 = """This is also a
multi-line string.""";

var s = r"In a raw string, even \n isn't special.";

// 以下变量可以插入到一个const修饰的字符串中。因为他们在编译时是可计算的.
const aConstNum = 0;
const aConstBool = true;
const aConstString = 'a constant string';

// 以下变量不能被插入到一个const修饰的字符串中。因为他们在编译时是不可计算的.
var aNum = 0;
var aBool = true;
var aString = 'a string';
const aConstList = const [1, 2, 3];

const validConstString = '$aConstNum $aConstBool $aConstString';
// const invalidConstString = '$aNum $aBool $aString $aConstList';//出错

var name = 'Bob';
if (name) {
  // Prints in JavaScript, not in Dart.
  print('You have a name!');
}

if (1) {
  print('JS prints this line.');
} else {
  print('Dart in production mode prints this line.');
  // However, in checked mode, if (1) throws an
  // exception because 1 is not boolean.
}

// Check for an empty string.
var fullName = '';
assert(fullName.isEmpty);

// Check for zero.
var hitPoints = 0;
assert(hitPoints <= 0);

// Check for null.
var unicorn;
assert(unicorn == null);

// Check for NaN.
var iMeantToDoThis = 0 / 0;
assert(iMeantToDoThis.isNaN);

var list = [1, 2, 3];

var list = [1, 2, 3];
assert(list.length == 3);
assert(list[1] == 2);

list[1] = 1;
assert(list[1] == 1);

var constantList = const [1, 2, 3];
// constantList[1] = 1; // Uncommenting this causes an error.

var gifts = {
// Keys      Values
  'first' : 'partridge',
  'second': 'turtledoves',
  'fifth' : 'golden rings'
};

var nobleGases = {
// Keys  Values
  2 :   'helium',
  10:   'neon',
  18:   'argon',
};

var gifts = new Map();
gifts['first'] = 'partridge';
gifts['second'] = 'turtledoves';
gifts['fifth'] = 'golden rings';

var nobleGases = new Map();
nobleGases[2] = 'helium';
nobleGases[10] = 'neon';
nobleGases[18] = 'argon';

var gifts = {'first': 'partridge'};
gifts['fourth'] = 'calling birds'; // Add a key-value pair

var gifts = {'first': 'partridge'};
assert(gifts['first'] == 'partridge');

var gifts = {'first': 'partridge'};
assert(gifts['fifth'] == null);

var gifts = {'first': 'partridge'};
gifts['fourth'] = 'calling birds';
assert(gifts.length == 2);

final constantMap = const {
  2: 'helium',
  10: 'neon',
  18: 'argon',
};

// constantMap[2] = 'Helium'; // Uncommenting this causes an error.

main() {
  var clapping = '\u{1f44f}';
  print(clapping);
  print(clapping.codeUnits);
  print(clapping.runes.toList());

  Runes input = new Runes(
      '\u2665  \u{1f605}  \u{1f60e}  \u{1f47b}  \u{1f596}  \u{1f44d}');
  print(new String.fromCharCodes(input));
}

#radix
#bar

bool isNoble(int atomicNumber) {
  return _nobleGases[atomicNumber] != null;
}

isNoble(atomicNumber) {
  return _nobleGases[atomicNumber] != null;
}

bool isNoble(int atomicNumber) => _nobleGases[atomicNumber] != null;

enableFlags(bold: true, hidden: false);

/// Sets the [bold] and [hidden] flags to the values
/// you specify.
enableFlags({bool bold, bool hidden}) {
  // ...
}

String say(String from, String msg, [String device]) {
  var result = '$from says $msg';
  if (device != null) {
    result = '$result with a $device';
  }
  return result;
}

assert(say('Bob', 'Howdy') == 'Bob says Howdy');

assert(say('Bob', 'Howdy', 'smoke signal') ==
    'Bob says Howdy with a smoke signal');

/// Sets the [bold] and [hidden] flags to the values you
/// specify, defaulting to false.
void enableFlags({bool bold = false, bool hidden = false}) {
  // ...
}

// bold will be true; hidden will be false.
enableFlags(bold: true);

String say(String from, String msg,
    [String device = 'carrier pigeon', String mood]) {
  var result = '$from says $msg';
  if (device != null) {
    result = '$result with a $device';
  }
  if (mood != null) {
    result = '$result (in a $mood mood)';
  }
  return result;
}

assert(say('Bob', 'Howdy') ==
    'Bob says Howdy with a carrier pigeon');

void doStuff(
    {List<int> list = const [1, 2, 3],
    Map<String, String> gifts = const {
      'first': 'paper',
      'second': 'cotton',
      'third': 'leather'
    }}) {
  print('list:  $list');
  print('gifts: $gifts');
}

void main() {
  querySelector("#sample_text_id")
    ..text = "Click me!"
    ..onClick.listen(reverseText);
}

// Run the app like this: dart args.dart 1 test
void main(List<String> arguments) {
  print(arguments);

  assert(arguments.length == 2);
  assert(int.parse(arguments[0]) == 1);
  assert(arguments[1] == 'test');
}

printElement(element) {
  print(element);
}

var list = [1, 2, 3];

// Pass printElement as a parameter.
list.forEach(printElement);

var loudify = (msg) => '!!! ${msg.toUpperCase()} !!!';
assert(loudify('hello') == '!!! HELLO !!!');

var list = ['apples', 'oranges', 'grapes', 'bananas', 'plums'];
list.forEach((i) {
  print(list.indexOf(i).toString() + ': ' + i);
});

void main() {
  var list = ['apples', 'bananas', 'oranges'];
  list.forEach((item) {
    print('${list.indexOf(item)}: $item');
  });
}

list.forEach((i) => print(list.indexOf(i).toString() + ': ' + i));

var topLevel = true;

main() {
  var insideMain = true;

  myFunction() {
    var insideFunction = true;

    nestedFunction() {
      var insideNestedFunction = true;

      assert(topLevel);
      assert(insideMain);
      assert(insideFunction);
      assert(insideNestedFunction);
    }
  }
}

/// Returns a function that adds [addBy] to the
/// function's argument.
Function makeAdder(num addBy) {
  return (num i) => addBy + i;
}

main() {
  // Create a function that adds 2.
  var add2 = makeAdder(2);

  // Create a function that adds 4.
  var add4 = makeAdder(4);

  assert(add2(3) == 5);
  assert(add4(3) == 7);
}

foo() {}               // A top-level function

class A {
  static void bar() {} // A static method
  void baz() {}        // An instance method
}

main() {
  var x;

  // Comparing top-level functions.
  x = foo;
  assert(foo == x);

  // Comparing static methods.
  x = A.bar;
  assert(A.bar == x);

  // Comparing instance methods.
  var v = new A(); // Instance #1 of A
  var w = new A(); // Instance #2 of A
  var y = w;
  x = w.baz;

  // These closures refer to the same instance (#2),
  // so they're equal.
  assert(y.baz == x);

  // These closures refer to different instances,
  // so they're unequal.
  assert(v.baz != w.baz);
}

a++
a + b
a = b
a == b
a ? b: c
a is T

// 1: () 提高可读性.
if ((n % i == 0) && (d % i == 0))

// 2: 难以阅读，但是等价.
if (n % i == 0 && d % i == 0)

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

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

var a, b;

a = 0;
b = ++a;        // b获得a的增量值.
assert(a == b); // 1 == 1

a = 0;
b = a++;        // 在b得到a的值后，a自增.
assert(a != b); // 1 != 0

a = 0;
b = --a;        // 在b得到a的值前，a自减.
assert(a == b); // -1 == -1

a = 0;
b = a--;        // 在b得到a的值后，a自增.
assert(a != b); // -1 != 0

assert(2 == 2);
assert(2 != 3);
assert(3 > 2);
assert(2 < 3);
assert(3 >= 3);
assert(2 <= 3);

if (emp is Person) { // 类型检查
  emp.firstName = 'Bob';
}

(emp as Person).firstName = 'Bob';

a = value;   // 为a分配值
b ??= value; // 如果b为null,为b分配值;
             // 否则, b维持原值

var a = 2;           // 使用 = 赋值
a *= 3;              // 赋值和乘操作: a = a * 3
assert(a == 6);

if (!done && (col == 0 || col == 3)) {
  // ...Do something...
}

final value = 0x22;
final bitmask = 0x0f;

assert((value & bitmask)  == 0x02);  // 与
assert((value & ~bitmask) == 0x20);  // 与非
assert((value | bitmask)  == 0x2f);  // 或
assert((value ^ bitmask)  == 0x2d);  // 异或
assert((value << 4)       == 0x220); // 左位移
assert((value >> 4)       == 0x02);  // 右位移

var finalStatus = m.isFinal ? 'final' : 'not final';

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

// ?: 操作符使用稍长版本.
String toString() => msg == null ? super.toString() : msg;

//使用if-else 语句 代码非常长.
String toString() {
  if (msg == null) {
    return super.toString();
  } else {
    return msg;
  }
}

querySelector('#button') // 得到一个对象.
  ..text = 'Confirm'   // 使用它的成员.
  ..classes.add('important')
  ..onClick.listen((e) => window.alert('Confirmed!'));

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();

// 不能运行
var sb = new StringBuffer();
sb.write('foo')..write('bar');

if (isRaining()) {
  you.bringRainCoat();
} else if (isSnowing()) {
  you.wearJacket();
} else {
  car.putTopDown();
}

var message = new StringBuffer("Dart is fun");
for (var i = 0; i < 5; i++) {
  message.write('!');
}

var callbacks = [];
for (var i = 0; i < 2; i++) {
  callbacks.add(() => print(i));
}
callbacks.forEach((c) => c());

candidates.forEach((candidate) => candidate.interview());

var collection = [0, 1, 2];
for (var x in collection) {
  print(x);
}

while (!isDone()) {
  doSomething();
}

do {
  printLine();
} while (!atEndOfPage());

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();
}

candidates.where((c) => c.yearsExperience >= 5)
          .forEach((c) => c.interview());

var command = 'OPEN';
switch (command) {
  case 'CLOSED':
    executeClosed();
    break;
  case 'PENDING':
    executePending();
    break;
  case 'APPROVED':
    executeApproved();
    break;
  case 'DENIED':
    executeDenied();
    break;
  case 'OPEN':
    executeOpen();
    break;
  default:
    executeUnknown();
}

var command = 'OPEN';
switch (command) {
  case 'OPEN':
    executeOpen();
    // ERROR: Missing break causes an exception!!

  case 'CLOSED':
    executeClosed();
    break;
}

var command = 'CLOSED';
switch (command) {
  case 'CLOSED': // Empty case falls through.
  case 'NOW_CLOSED':
    // Runs for both CLOSED and NOW_CLOSED.
    executeNowClosed();
    break;
}

var command = 'CLOSED';
switch (command) {
  case 'CLOSED':
    executeClosed();
    continue nowClosed;
    // Continues executing at the nowClosed label.

nowClosed:
  case 'NOW_CLOSED':
    // Runs for both CLOSED and NOW_CLOSED.
    executeNowClosed();
    break;
}

// 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'));

assert(urlString.startsWith('https'),
    'URL ($urlString) should start with "https".');

throw new FormatException('Expected at least 1 section');

throw 'Out of llamas!';

distanceTo(Point other) =>
    throw new UnimplementedError();

try {
  breedMoreLlamas();
} on OutOfLlamasException {
  buyMoreLlamas();
}

try {
  breedMoreLlamas();
} on OutOfLlamasException {
  // A specific exception
  buyMoreLlamas();
} on Exception catch (e) {
  // Anything else that is an exception
  print('Unknown exception: $e');
} catch (e) {
  // No specified type, handles all
  print('Something really unknown: $e');
}

  ...
} on Exception catch (e) {
  print('Exception details:\n $e');
} catch (e, s) {
  print('Exception details:\n $e');
  print('Stack trace:\n $s');
}

final foo = '';

void misbehave() {
  try {
    foo = "You can't change a final variable's value.";
  } catch (e) {
    print('misbehave() partially handled ${e.runtimeType}.');
    rethrow; // Allow callers to see the exception.
  }
}

void main() {
  try {
    misbehave();
  } catch (e) {
    print('main() finished handling ${e.runtimeType}.');
  }
}

try {
  breedMoreLlamas();
} finally {
  // Always clean up, even if an exception is thrown.
  cleanLlamaStalls();
}

try {
  breedMoreLlamas();
} catch(e) {
  print('Error: $e');  // Handle the exception first.
} finally {
  cleanLlamaStalls();  // Then clean up.
}