Dart cheatsheet codelab

https://dart.dev/codelabs/dart-cheatsheet

Dart cheatsheet codelab

 

Null-aware operators

dart에서는 null 일 수도 있는 값들을 다루는 handy operators가 있다.

하나는 ??= assignment operator : 현재 null일 경우만 값이 할당 된다.

int a; // The initial value of a is null.
a ??= 3;
print(a); // <-- Prints 3.
 
a ??= 5;
print(a); // <-- Still prints 3. a는 null이 아니므로! 5는 대입되지 않음

 

다른 하나는 ?? left가 null이 아니면 왼쪽, null이면 오른쪽 값을 반환

print(1 ?? 3); // <-- Prints 1.
print(null ?? 12); // <-- Prints 12.

 

Conditional property access

null 일 수도 있는 object의 property나 method에 접근을 보호하려면 dot(.) 앞에 (?)를 넣으면 됨

myObject?.someProperty
 
// 동일함
// (myObject != null) ? myObject.someProperty : null
 
myObject?.someProperty?.someMethod()

 

Collection literals

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

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

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

 

Code example

// Assign this a list containing 'a', 'b', and 'c' in that order:
final aListOfStrings = ['a', 'b', 'c'];
 
// Assign this a set containing 3, 4, and 5:
final aSetOfInts = {3, 4, 5};
 
// Assign this a map of String to int so that aMapOfStringsToInts['myKey'] returns 12:
final aMapOfStringsToInts = { 'myKey' : 12 };
 
// Assign this an empty List<double>:
final anEmptyListOfDouble = <double>[];
 
// Assign this an empty Set<String>:
final anEmptySetOfString = <String>{};
 
// Assign this an empty Map of double to int:
final anEmptyMapOfDoublesToInts = <double, int>{};

 

Arrow syntax

=> 오른쪽을 값을 return 하는 function 

bool hasEmpty = aListOfStrings.any((s) {
  return s.isEmpty;
});

 

Code example

class MyClass {
  int _value1 = 2;
  int _value2 = 3;
  int _value3 = 5;
  
  // Returns the product of the above values:
  int get product => _value1*_value2*_value3;
  
  // Adds 1 to _value1:
  void incrementValue1() => _value1++;
  
  // Returns a string containing each item in the
  // list, separated by commas (e.g. 'a,b,c'): 
  String joinWithCommas(List<String> strings) => strings.join(',');
}

 

Cascades

To perform a sequence of operations on the same object, use cascades (..). We’ve all seen an expression like this:

myObject.someMethod()
 
// It invokes someMethod() on myObject, 
// and the result of the expression is the return value of someMethod().
// Here’s the same expression with a cascade:
myObject..someMethod()
 
/*
Although it still invokes someMethod() on myObject, 
the result of the expression isn’t the return value — 
it’s a reference to myObject! Using cascades, 
you can chain together operations 
that would otherwise require separate statements. 
For example, consider this code:
*/
var button = querySelector('#confirm');
button.text = 'Confirm';
button.classes.add('important');
button.onClick.listen((e) => window.alert('Confirmed!'));
 
// With cascades, the code becomes much shorter, 
// and you don’t need the button variable:
querySelector('#confirm')
..text = 'Confirm'
..classes.add('important')
..onClick.listen((e) => window.alert('Confirmed!'));