Dart Language Overview By Example II

Information drawn from

• Dart.dev cheatsheet

String interpolation

To put the value of an expression inside a string, use ${expression}. If the expression is an identifier, you can omit the {}.

Here are some examples of using string interpolation:

’${3 + 2}’ ——–> ‘5’ ‘${“word”.toUpperCase()}’ ——–> ‘WORD’ ‘$myObject’ ——–> The value of myObject.toString()

Conditional property access

To guard access to a property or method of an object that might be null, put a question mark (?) before the dot (.):

myObject?.someProperty

The preceding code is equivalent to the following:

(myObject != null) ? myObject.someProperty : null

You can chain multiple uses of ?. together in a single expression:

myObject?.someProperty?.someMethod()

The preceding code returns null (and never calls someMethod()) if either myObject or myObject.someProperty is null.

Collection literals

Dart has built-in support for lists, maps, and sets. You can create them using literals:

final aListOfStrings = ['one', 'two', 'three'];
final aSetOfStrings = {'one', 'two', 'three'};
final aMapOfStringsToInts = {
  'one': 1,
  'two': 2,
  'three': 3,
};

Dart’s type inference can assign types to these variables for you. In this case, the inferred types are List<String>, Set<String>, and Map<String, int>.

Or you can specify the type yourself:

final aListOfInts = <int>[];
final aSetOfInts = <int>{};
final aMapOfIntToDouble = <int, double>{};

Specifying types is handy when you initialize a list with contents of a subtype, but still want the list to be List:

final aListOfBaseType = <BaseType>[SubType(), SubType()];

Getters and setters

You can define getters and setters whenever you need more control over a property than a simple field allows.

For example, you can make sure a property’s value is valid:

class MyClass {
  int _aProperty = 0;

  int get aProperty => _aProperty;

  set aProperty(int value) {
    if (value >= 0) {
      _aProperty = value;
    }
  }
}

You can also use a getter to define a computed property:

class MyClass {
  List<int> _values = [];

  void addValue(int value) {
    _values.add(value);
  }

  // A computed property.
  int get count {
    return _values.length;
  }
}

Optional positional parameters

Dart has two kinds of function parameters: positional and named.

Positional parameters are the kind you’re likely familiar with:

int sumUp(int a, int b, int c) {
  return a + b + c;
}
// ···
  int total = sumUp(1, 2, 3);

With Dart, you can make these positional parameters optional by wrapping them in brackets:

int sumUpToFive(int a, [int b, int c, int d, int e]) {
  int sum = a;
  if (b != null) sum += b;
  if (c != null) sum += c;
  if (d != null) sum += d;
  if (e != null) sum += e;
  return sum;
}
// ···
  int total = sumUpToFive(1, 2);
  int otherTotal = sumUpToFive(1, 2, 3, 4, 5);

Optional positional parameters are always last in a function’s parameter list. Their default value is null unless you provide another default value:

int sumUpToFive(int a, [int b = 2, int c = 3, int d = 4, int e = 5]) {
// ···
}
// ···
  int newTotal = sumUpToFive(1);
  print(newTotal); // <-- prints 15

Optional named parameters

Using a curly brace syntax, you can define optional parameters that have names.

void printName(String firstName, String lastName, {String suffix}) {
  print('$firstName $lastName ${suffix ?? ''}');
}
// ···
  printName('Avinash', 'Gupta');
  printName('Poshmeister', 'Moneybuckets', suffix: 'IV');

As you might expect, the value of these parameters is null by default, but you can provide default values:

void printName(String firstName, String lastName, {String suffix = ''}) {
  print('$firstName $lastName $suffix');
}

A function can’t have both optional positional and optional named parameters.

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Last update on 23 Apr 2020

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