Till Chen
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Till Chen

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Dart Notes

Till Chen
   1735 words   9 minutes 

Introduction

  1. A basic Dart program:

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    printInteger(int aNumber) {
    print('The number is $aNumber');
}

main() {
    var number = 42;
    printInteger(number);
}

  2. Everything that can be placed in a variable is an object. Even numbers, function, and null are objects. All objects inherit from the Object class.

  3. Dart is strongly typed. But type annotations are optional thanks to type inference. When we want to say explicitly that no type is expected, use the type dynamic.

  4. Dart supports generic types like List<int> or List<dynamic> (a list of objects of any type).

  5. Unlike Java, Dart doesn’t have public, protected, and private. Prefix an underscore _ makes it private to the library. It generally means that the identifier is visible only inside the file (not just the class).

Variables

  1. Use var for local variables instead of type annotations.

  2. The default value for uninitialized variables is null.

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    int lineCount;
assert(lineCount == null);

  3. final and const:

    • A final variable can be set only once. Use final if we don’t know the value at compile time.

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      final name = 'Bob'; // Without a type annotation
final String nickname = 'Bobby'; // Or this

    • Use const for variables that are compile-time constants. If it’s at the class level, make it static const. We can also use const for constant values. We can change the value of a non-final, non-const variable, even if it used to have a const value.

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      var foo = const [];
final bar = const [];
const baz = []; // Equivalent to `const []`
foo = [1, 2, 3]; // Was const []

Built-in Types

Numbers

  1. Both int and double are subtypes of num.

  2. Conversion between a string and a number:

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    int one = int.parse('1');
double onePointOne = double.parse('1.1');
String oneAsString = 1.toString();
String piAsString = 3.14159.toStringAsFixed(2); // 3.14

Strings

  1. Both single or double quotes are fine. It’s easy to escape the delimiter: 'It\'s easy.'

  2. String interpolation: ${expression}. If the expression is an identifier, we can skip {}.

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    var s = 'string interpolation';
print('Dart has $s');

  3. Like in Python, create a multi-line string using triple quote ''' or """.

  4. Strings use UTF-16.

Booleans

  1. Only true and false have the type bool, which means we have to check the values explicitly (unlike Python).

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    var fullName = '';
assert(fullName.isEmpty);

Lists

  1. Example:

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    var list = [1, 2, 3];
assert(list.length == 3);
var constantList = const [1, 2, 3];
// constantList[1]  = 1; // Uncommenting this causes an error.

  2. Spread operator (...) and null-aware spread operator (....?) provide a concise way to insert all elements into a collection:

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    var list = [1, 2, 3];
var list2 = [0, ...list]; // [0, 1, 2, 3]
var list3;
var list4 = [0, ...?list3]; // list3 might be null. Avoid exceptions.

  3. Collection if and collection for (similar to list comprehension in Python):

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    var nav = [
    'Home',
    'Furniture',
    'Plants',
    if (promoActive) 'Outlet'
];
var listOfInts = [1, 2, 3];
var listOfStrings = [
    '#0',
    for (var i in listOfInts) `#$i`
];

  4. forEach() can replace a for loop; where() can be used to filter:

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

Sets

  1. Example:

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    var halogens = {'fluorine', 'chlorine', 'bromine', 'iodine', 'astatine'};
var names = <String>{}; // Empty set
// Set<String> names = {}; // Works too
// var names = {}; // Creates a map, not a set.
var elements = <String>{};
elements.add('flourine');
elements.addAll(halogens);

Maps

  1. Example:

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    var nobleGases = {
    2: 'helium',
    10: 'neon',
    18: 'argon'
};
// Or we can use the constructor
var nobleGases = Map(); // The new keyword is optional
nobleGases[2] = 'helium';
nobleGases[10] = 'neon';
nobleGases[18] = 'argon';

  2. The map returns a null if the key doesn’t exist.

Runes and Grapheme Clusters

  1. In Dart, runes expose the Unicode code points of a string.

  2. Because a Dart string is a sequence of UTF-16 code units, the usual way to express a code point is \uXXXX, where XXXX is a 4-digit hexadecimal value. For more or less than 4 hex digits, place the value in curly brackets.

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    import 'package:characters/characters.dart';

var hi = 'Hi 🇩🇰';
print('The las character: ${hi.characters.last}');

Symbols

  1. A Symbol object represents an operator or identifier declared in a Dart program. We might never need to use symbols, but they’re invaluable for APIs that refer to identifiers by name, because minification changes identifier names but not identifier symbols.

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    #radix
#bar

Functions

  1. Dart is a true object-oriented language. Even functions are objects and have a type.

  2. Although Effective Dart recommends type annotations for public APIs, the function still works if you omit the types.

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    bool isNoble(int atomicNumber) {
    return _nobleGases[atomicNumber] != null;
}
// Or the below
isNoble(atomicNumber) {
    return _nobleGases[atomicNumber] != null;
}
// Or the arrow syntax
bool isNoble(int atomicNumber) => _nobleGases[atomicNumber] != null;

  3. .. is a cascade, which enables us to perform multiple operations on the members of a single object:

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    querySelector('id')
  ..text = 'Click me!'
  ..onClick.listen(reverseText);

  4. All functions return a value. If no return value is specified, it returns null.

Optional Parameters

  1. Optional parameters can be either named or positional, but not both.

Named Parameters

  1. Example:

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    void enableFlags({bool bold, bool hidden}) {...}

enableFlags(bold: true, hidden: false);

  2. Although named parameters are a kind of optional parameter, we can used @required to make it mandatory:

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    const Scrollbar({Key key, @required Widget child})

Positional parameters

  1. Wrapping a set of parameters in [] makes them optional positional:

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    String say(String from, String msg, [String device]) {...}

Anonymous Functions

  1. Example:

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    var list = ['apples', 'bananas', 'oranges'];
list.forEach((item) {
    print('${list.indexOf(item)}: $item');
});
// Or the arrow notation
list.forEach(
    (item) => print('${list.indexOf(item)}: $item'));

Operators

  1. ~/ is the floor division. The rest are the same as the ones in Java.

  2. as, is, and is!:

    • as: Typecast. Use it iff we are sure the object is of that type: (emp as Person).firstName = 'Bob'
    • is True if the object has the type. is! False if the object has the type.

  3. b ??= value assigns if b is null; otherwise b stays the same.

  4. String playerName(String name) => name ?? 'Guest'; gives name 'Guest' if name is null.

  5. ?. conditional access like in Swift/Kotlin. foo?.bar is null if foo is null.

Exceptions

  1. Example:

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    throw FormatException('Expected at least 1 section');
try {
    breadMoreLlamas();
} on OutOfLlamasException {
    buyMoreLlamas();
} on Exception catch (e, s) {
    print('Unknown exception: $e');
    print('Stack trace $s');
} catch (e) {
    // handles all
    print('Something really unknown: $e');
    rethrow; // Allow callers to see the exception.
} finally {
    // Always executes.
}

Classes

  1. All instance variables generate an implicit getter method. Non-final instance variables also generate an implicit setter. Just do foo.x.

  2. Constructor syntactic sugar:

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    class Point {
    double x, y;

    Point(this.x, this.y);
}

  3. Named constructors for extra clarity:

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    Point.origin() {
    x = 0;
    y = 0;
}

  4. Invoke the super constructor almost like in C++:

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    Employee.fromJson(Map data) : super.fromJson(data) {
    ...
}

  5. Redirecting constructors: Point.alongXAxis(double x) : this(x, 0) delegates to the main constructor.

  6. Use factory for a constructor that doesn’t always create a new instance of the class. It may return an instance from a cache or return an instance of a subtype.

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    abstract class Shape {
  factory Shape(String type) {
    if (type == 'circle') return Circle(2);
    if (type == 'square') return Square(2);
    throw "Can't create $type.";
  }
  num get area;
}

class Circle implements Shape {
  final num radius;
  Circle(this.radius);
  num get area => pi * pow(radius, 2);
}

class Square implements Shape {
  final num side;
  Square(this.side);
  num get area => pow(side, 2);
}

final circle = Shape('circle');
final square = Shape('square');

  7. Getters and setters:

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    // We can define explicit getters and setters using get and set
int _speed = 0;

get speed => _speed; // Read-only
double get right => left + width;
set right(double value) => left = value - width

  8. Abstract classes and methods:

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    abstract class AbstractContainer {
    void updateChildren(); // Abstract method
}

  9. Every class implicitly defines an interface containing all instance members of the class and of any interfaces it implements. Like in Java, we have extends, implements, and @override. (But Dart doesn’t have the interface keyword.)

  10. Liked in C++, operators can be overriden:

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    Vector operator +(Vector v) => Vector(x + v.x, y + v.y);

  11. Mixins: a way of reusing a class’s code in multiple class hierarchies. Use with:

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    class Musician extends Performer with Musical {
    ...
}
// To implement, declare no constructors
mixin Musical { // mixin Musical on Musician to restrict the types that can use the mixin
    ...
}

Enum

  1. Example:

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    enum Color {red, green, blue}

assert(Color.red.index == 0);
List<Color> colors = Color.values;

Asynchrony

  1. To use await, code must be in an async function:

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    Future checkVersion() async { // It returns a Future object
    var version = await lookUpVersion();
}
// Future is then-able, so can always say
checkVersion().then((returnValue) => foo());

  2. Use await for () to handle a Stream (wait for all of the streams results). But we should not use this for UI event listeners, because UI frameworks send endless streams of events.

Functional Programming

  1. Example:

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    String scream(int length) => "A${'a' * length}h!";

main() {
  final values = [1, 2, 3, 5, 10, 50];
  for (var length in values) {
    print(scream(length));
  }
  // Is the same as
  values.map(scream).forEach(print);
  // More
  values.skip(1).take(3).map(scream).forEach(print);
}

References

Updated on June 13, 2020
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