Dart dot dot ( .. ) operator
1. Dot dot ( .. ) operator
In the Dart programming language, the dot dot ( .. ) operator is also interpreted as "cascade notation". It allows you to not repeat the same target if you want to call several methods on the same object.
Example:
dotdot_ex1.dart
void main() {
var list1 = [];
list1..add('One')..add('Two')..add('Three');
print(list1); // [One, Two, Three]
// Same as:
var list2 = [];
list2.add('One');
list2.add('Two');
list2.add('Three');
print(list2); // [One, Two, Three]
}
Example: Calling multiple methods of the same object, using the dot dot operator makes your code shorter.
dotdot_ex2.dart
void main() {
var list1 = [];
list1..add('One')..add('Two')..addAll(['Three', 'Four', 'Five']);
print(list1); // [One, Two, Three, Four, Five]
// Same as:
var list2 = [];
list2.add('One');
list2.add('Two');
list2.addAll(['Three', 'Four', 'Five']);
print(list2); // [One, Two, Three, Four, Five]
// Same as:
var list3 = []..add('One')..add('Two')..addAll(['Three', 'Four', 'Five']);
print(list3); // [One, Two, Three, Four, Five]
}
In languages that do not support the dot dot operator, to call a sequence of methods, the methods must return an object. Such as:
without_dotdot_ex1.dart
class Calculator {
double _accumulator = 0;
Calculator(double startValue) {
_accumulator = startValue;
}
Calculator add(double val) {
_accumulator += val;
return this; // Return this object.
}
Calculator subtract(double val) {
_accumulator -= val;
return this; // Return this object.
}
double result() {
return _accumulator;
}
}
void main() {
var calc = Calculator(100);
var result = calc.add(100).subtract(50).subtract(25).result();
print(result); // 125
}
The dot dot operator helps you call a series of methods that don't necessarily return an object. We rewrite the above example.
with_dotdot_ex1.dart
class Calculator {
double _accumulator = 0;
Calculator(double startValue) {
_accumulator = startValue;
}
void add(double val) {
_accumulator += val;
}
void subtract(double val) {
_accumulator -= val;
}
double result() {
return _accumulator;
}
}
void main() {
var calc = Calculator(100);
calc..add(100)..subtract(50)..subtract(25);
var result = calc.result();
print(result); // 125
}
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