以 Swift 開發者的角度學習 Dart

// String-typed variable.
String name = 'Bob';

// Immutable String-typed variable.
final String name = 'Bob';

// This is the same as `String name = 'Bob';`
// since Dart infers the type to be String.
var name = 'Bob';

// And this is the same as `final String name = 'Bob';`.
final name = 'Bob';

// Declare a variable first:
String name; 
// Initialize the variable later:
name = 'bob';
// Declare and initialize a variable at once with inference:
var name = 'bob';

final String name;
if (b1) {
  name = 'John';
} else {
  name = 'Jane';
}

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

class StandardAtmosphere {
  static const bar = 1000000; // Unit of pressure (dynes/cm2)
  static const double atm = 1.01325 * bar; // Standard atmosphere
}

var foo = const ['one', 'two', 'three'];
foo.add('four'); // Error: foo contains a constant value.
foo = ['apple', 'pear']; // This is allowed as foo itself isn't constant.
foo.add('orange'); // Allowed as foo no longer contains a constant value.

final foo1 = const [1, 2, 3];
const foo2 = [1, 2, 3]; // Equivalent to `const [1, 2, 3]`
const bar2 = foo2; // OK
const bar1 = foo1; // Compile-time error, `foo1` isn't constant

int intVariable = 3;
double doubleVariable = intVariable.toDouble();

double doubleValue = 3;

int intVariable = 3;
double doubleVariable = 3.0;
print(intVariable == doubleVariable); // true

String c = 'a'; // There isn't a specialized "Character" type
String s1 = 'This is a String';
String s2 = "This is also a String";

final singleQuotes = 'I\'m learning Dart'; // I'm learning Dart
final doubleQuotes = "Escaping the \" character"; // Escaping the " character
final unicode = '\u{1F60E}'; // 😎,  Unicode scalar U+1F60E

var food = 'bread';
var str = 'I eat $food'; // I eat bread
var str = 'I eat ${bakery.bestSeller}'; // I eat bread

// Include the \n characters.
final s1 = r'Includes the \n characters.';
// Also includes the \n characters.
final s2 = r"Also includes the \n characters.";

final s3 = r'''
  The \n characters are also included
  when using raw multiline strings.
  ''';
final s4 = r"""
  The \n characters are also included
  when using raw multiline strings.
  """;

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

a = a ?? b;

a = a ?? b;

// Assign b to a if a is null; otherwise, a stays the same
a ??= b;

int? a = 5;

int b = a; // Not allowed.
int b = a!; // Allowed.

myObject!.someProperty;
myObject!.someMethod();

// Using null safety:
class Coffee {
  late String _temperature;

  void heat() { _temperature = 'hot'; }
  void chill() { _temperature = 'iced'; }

  String serve() => _temperature + ' coffee';
}

class Weather {
  late int _temperature = _readThermometer();
}

late final int a;
a = 1;
a = 2; // Throws a runtime exception because
       // "a" is already initialized.

void main() {
  // main function is the entry point
  print("hello world");
}

try {
  // Create audio player object
  audioPlayer = AVAudioPlayer(soundUrl);
            
  // Play the sound
  audioPlayer.play();
}
catch {
  // Couldn't create audio player object, log the exception
  print("Couldn't create the audio player for file $soundFilename");
}

int multiply(int a, int b) {
  return a * b;
}

int multiply(int a, int b, {int c = 1, int d = 1}) {
  return a * b * c * d;
}

// Calling a function with both required and named parameters
multiply(3, 5); // 15
multiply(3, 5, c: 2); // 30
multiply(3, 5, d: 3); // 45
multiply(3, 5, c: 2, d: 3); // 90

int multiply(int a, int b, { required int c }) {
  return a * b * c;
}
// When calling the function, c has to be provided
multiply(3, 5, c: 2);

int multiply(int a, int b, [int c = 1, int d = 1]) {
  return a * b * c * d;
}
// Calling a function with both required and optional positioned parameters.
multiply(3, 5); // 15
multiply(3, 5, 2); // 30
multiply(3, 5, 2, 3); // 90

typedef int MultiplierFunction(int value);
// Define a function that returns another function
MultiplierFunction multiplyBy(int multiplier) {
  return (int value) {
    return value * multiplier;
  };
}
// Call function that returns new function
MultiplierFunction multiplyByTwo = multiplyBy(2);
// Call the new function
print(multiplyByTwo(3)); // 6

// Multi line anonymous function
[1,2,3].map((element) { 
  return element * 2; 
}).toList(); // [2, 4, 6]

// Single-line anonymous function
[1,2,3].map((element) => element * 2).toList(); // [2, 4, 6]

multiply(int a, int b) => a * b;

multiply(int a, int b) {
  return a * b;
}

Iterable<int> listNumbers(int n) sync* {
  int k = 0;
  while (k < n) yield k++;
}

// Returns an `Iterable<int>` that iterates
// through 0, 1, 2, 3, and 4.
print(listNumbers(5));

Iterable<int> doubleNumbersTo(int n) sync* {
  int k = 0;
  while (k < n) { 
    yield* [k, k]; 
    k++;
  }
}

print(doubleNumbersTo(3)); // Returns an iterable with [0, 0], [1, 1], and [2, 2].

var a = 1;
// Parentheses for conditions are required in Dart.
if (a == 1) {
  print('a == 1');
} else if (a == 2) {
  print('a == 2');
} else {
  print('a != 1 && a != 2');
}

// Curly braces are optional for single line `if` statements.
if (a == 1) print('a == 1');

var list = [0, 1, 2, 3, 4];
for (var i in list) {
  print(i);
}

Map<String, int> dict = {
  'Foo': 1,
  'Bar': 2
};
for (var e in dict.entries) {
  print('${e.key}, ${e.value}');
}
dict.forEach((key, value) {
  print('$key, $value');
});

class Vector {
  final double x;
  final double y;
  final double z;

  Vector operator +(Vector v) {
    return Vector(x: x + v.x, y: y + v.y, z: z+v.z);
  }
}

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

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

a = 0;
b = a++; // Increment a AFTER b gets its value.
assert(a != b); // 1 != 0

(person as Employee).employeeNumber = 4204583;

if (person is Employee) {
  person.employeeNumber = 4204583;
}

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

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

assert((value & bitmask) == 0x02); // AND
assert((value & ~bitmask) == 0x20); // AND NOT
assert((value | bitmask) == 0x2f); // OR
assert((value ^ bitmask) == 0x2d); // XOR
assert((value << 4) == 0x220); // Shift left
assert((value >> 4) == 0x02); // Shift right
assert((-value >> 4) == -0x03); // Shift right // Result may differ on the web

final displayLabel = canAfford ? 'Please pay below' : 'Insufficient funds';

Animal animal = Animal()
  ..name = 'Bob'
  ..age = 5
  ..feed()
  ..walk();

print(animal.name); // "Bob"
print(animal.age); // 5

final List<String> list1 = <String>['one', 'two', 'three']; // Initialize list and specify full type
final list2 = <String>['one', 'two', 'three']; // Initialize list using shorthand type
final list3 = ['one', 'two', 'three']; // Dart can also infer the type

final fruits = ['apple', 'orange', 'pear'];
final fruit = fruits[1];

final fruits = ['apple', 'orange', 'pear'];
fruits.add('peach');
fruits.addAll(['kiwi', 'mango']);

final fruits = List<String>.unmodifiable(['apple', 'orange', 'pear']);

final list = [1, 2, 3];
final list2 = [0, ...list]; // [ 0, 1, 2, 3 ]
assert(list2.length == 4);

List<int>? list;
final list2 = [0, ...?list]; //[ 0 ]
assert(list2.length == 1);

final abc = {'a', 'b', 'c'};

final names = <String>{};
Set<String> alsoNames = {}; // This works, too.
// final names = {}; // Creates an empty map, not a set.

final abc = Set<String>.unmodifiable(['a', 'b', 'c']);

final gifts = {
 'first': 'partridge',
 'second': 'turtle doves',
 'fifth': 'golden rings',
};

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

final gifts = {'first': 'partridge'};
final gift = gifts['first']; // 'partridge'

final gifts = {'first': 'partridge'};
assert(gifts.containsKey('fifth')); // false

final gifts = {'first': 'partridge'};
gifts['second'] = 'turtle'; // Gets added
gifts['second'] = 'turtle doves'; // Gets updated

final gifts = {'first': 'partridge'};
gifts.remove('first');
gifts.removeWhere((key, value) => value == 'partridge');

class Point {
  double x = 0;
  double y = 0;

  Point(double x, double y) {
    // There's a better way to do this in Dart, stay tuned.
    this.x = x;
    this.y = y;
  }
}

// Create a new instance of the Point class
Point p = Point(3, 5);

class Point {
  double x;
  double y;

  // Syntactic sugar for setting x and y
  // before the constructor body runs.
  Point(this.x, this.y);
}

// Create a new instance of the Point class
Point p = Point(3, 5);

class Point {
  ...
  // With an optional positioned parameter
  Point(this.x, [this.y = 0]);
  // With named parameters
  Point({required this.y, this.x = 0});
  // With both positional and named parameters
  Point(int x, int y, {int scale = 1}) {
    ...
  }
  ...
}

class Point {
  ...
  Point(Map<String, double> json)
      : x = json['x']!,
        y = json['y']! {
    print('In Point.fromJson(): ($x, $y)');
  }
  ...
}

class Point {
  double x;
  double y;

  Point(this.x, this.y);

  // Named constructor
  Point.fromJson(Map<String, double> json)
      : x = json['x']!,
        y = json['y']!;
}

class ImmutablePoint {
  final double x, y;

  const ImmutablePoint(this.x, this.y);
}

const ImmutablePoint origin = ImmutablePoint(0, 0);

class Point {
  double x, y;

  // The main constructor for this class.
  Point(this.x, this.y);

  // Delegates to the main constructor.
  Point.alongXAxis(double x) : this(x, 0);
}

class Logger {
  static final Map<String, Logger> _cache =
    <String, Logger>{};
  
  final String name;
  
  // Factory constructor that returns a cached copy,
  // or creates a new one if it's not yet available.
  factory Logger(String name)=> _cache[name] ??= Logger._internal(name);
  // Private constructor used only in this library
  Logger._internal(this.name);
}

void doSomething() { // This is a function
 // Implementation..
}

class Example {
 void doSomething() { // This is a method
   // Implementation..
 }
}

class Rectangle {
  double left, top, width, height;

  Rectangle(this.left, this.top, this.width, this.height);

  // Define two calculated properties: right and bottom.
  double get right => left + width;
  set right(double value) => width = value - left;

  double get bottom => top + height;
  set bottom(double value) => height = value - top;
}

// This class is declared abstract and thus can't be instantiated.
abstract class AbstractContainer {
  void updateChildren(); // Abstract method.

  // Method with default implementation.
  String toString() => "AbstractContainer";
}

abstract class Animal {
  int getLegs();
  void makeNoise();
}

class Dog implements Animal {
  @override
  int getLegs() => 4;

  @override
  void makeNoise() => print('Woof woof');
}

abstract class Animal {
  // Define constructors, fields, methods...
}

class Dog extends Animal {
  // Define constructors, fields, methods...
}

abstract class Animal {}

// Defining the mixins
mixin Flyer {
  fly() => print('Flaps wings');
}
mixin Walker {
  walk() => print('Walks legs');
}
  
class Bat extends Animal with Flyer {}
class Goose extends Animal with Flyer, Walker {}
class Dog extends Animal with Walker {}

// Correct calls
Bat().fly();
Goose().fly();
Goose().walk(); 
Dog().walk();

// Incorrect calls
Bat().walk(); // Not using the Walker mixin
Dog().fly(); // Not using the Flyer mixin
class Animal {
}

mixin Walker {
  walk() => print('Walks legs');
}

// Impossible, as Walker is no longer a class.
class Bat extends Walker {}

class Bird extends Animal with Consumer, Flyer {

extension <name> on <type> {
  (<member definition>)*
}

extension NumberParsing on String {
  int parseInt() {
    return int.parse(this);
  }
}

print('21'.parseInt() * 2); // 42

// Hide "MyExtension" when importing types from
// "path/to/file.dart".
import 'path/to/file.dart' hide MyExtension; 
// Only show "MyExtension" when importing types
// from "path/to/file.dart".
import 'path/to/file.dart' show MyExtension;

// The `shout()` method is only available within this library.
extension _Private on String {
  String shout() => this.toUpperCase();
}

class Person {
  Person(this.fullName);

  final String fullName;
}

extension ExtendedPerson on Person {
  static Person create(String firstName, String lastName) {
    return Person("$firstName $lastName");
  }
}

// To use the factory method, use the name of
// the extension, not the type.
final person = ExtendedPerson.create('John', 'Doe');

class Animal {
  void makeNoise => print('Noise');
}

class Dog implements Animal {
  @override
  void makeNoise() => print('Woof woof');
}

// Defining a method that uses generics.
T transform<T>(T param) {
  // For example,  doing some transformation on `param`...
  return param;
}

// Calling the method. Variable "str" will be
// of type String.
var str = transform('string value');

// Defining a method with multiple generics.
T transform<T, Q>(T param1, Q param2) {
  // ...
}
// Calling the method with explicitly-defined types.
transform<int, String>(5, 'string value');
// Types are optional when they can be inferred.
transform(5, 'string value');

class Cache<T> {
  T getByKey(String key) {}
  void setByKey(String key, T value) {}
}
// Creating a cache for strings.
// stringCache has type Cache<String>
var stringCache = Cache<String>();
// Valid, setting a string value.
stringCache.setByKey('Foo', 'Bar')
// Invalid, int type doesn't match generic.
stringCache.setByKey('Baz', 5)

class NumberManager<T extends num> {
  // ...
}
// Valid
var manager = NumberManager<int>(); 
var manager = NumberManager<double>(); 
// Invalid, neither String nor its parent classes extend num.
var manager = NumberManager<String>();

var objList = [5, 2.0]; // Type: List<num> // Automatic type inference
var objList = <Object>[5, 2.0]; // Type: List<Object> // Explicit type definition
var objSet = <Object>{5, 2.0}; // Sets work identically

// Automatic type inference
var map = {
  'foo': 'bar'
}; // Type: Map<String, String>
// Explicit type definition:
var map = <String, Object>{
  'foo': 'bar'
}; // Type: Map<String, Object>

String str = 'String Value';
Future<String> strFuture = Future<String>.value(str);

// Returns a future of a string, as the method is async
Future<String> fetchString() async {
  // Typically some other async operations would be done here.
  
  Response response = await makeNetworkRequest();
  if (!response.success) {
    throw BadNetwork();
  }

  return 'String Value';
}

String stringFuture = await fetchString();
print(str); // "String Value"

// We can only await futures within an async context.
asyncFunction() async {
  String returnedString = await fetchString();
  print(returnedString); // 'String Value'
}

// We can only await futures within an async context.
Future<void> asyncFunction() async {
  String? returnedString;
  try {
    returnedString = await fetchString();
  } catch (error) {
    print('Future encountered an error before resolving.');
    return;
  }
  print(returnedString);
}

Future<int> sumStream(Stream<int> stream) async {
  var sum = 0;
  try { 
    await for (final value in stream) {
      sum += value;
    }
  } catch (error) {
    print('Stream encountered an error! $err');
  }
  return sum;
}

try {
  await for (final value in stream) { ... }
} catch (err) {
  print('Stream encountered an error! $err');
}

Stream<int> stream = ...
stream.listen((int value) {
  print('A value has been emitted: $value');
});

stream.listen(
  (int value) { ... },
  onError: (err) {
    print('Stream encountered an error! $err');
  },
  onDone: () {
    print('Stream completed!');
  },
);

StreamSubscription subscription = stream.listen(...);
subscription.cancel();

Stream<int> asynchronousNaturalsTo(int n) async* {
  int k = 0;
  while (k < n) yield k++;
}

Stream<int> stream = asynchronousNaturalsTo(5);

/// The number of characters in this chunk when unsplit.
int get length => ...

/// Returns the [int] multiplication result of [a] * [b].
multiply(int a, int b) => a * b;

/** 
 * The number of characters in this chunk when unsplit. 
 * (AVOID USING THIS SYNTAX, USE /// INSTEAD.)
 */
int get length => ...

final foo = 'this is a public property';
final _foo = 'this is a private property';

String bar() {
  return 'this is a public method';
}
String _bar() {
  return 'this is a private method';
}

// Public class
class Foo {
}

// Private class
class _Foo {
},

library animals;

part 'parrot.dart';

class _Animal {
  final String _name;

  _Animal(this._name);
}

part of animals;

class Parrot extends _Animal {
  Parrot(String name) : super(name);

  // Has access to _name of _Animal
  String introduction() {
    return 'Hello my name is $_name';
  }
}