Checking conditions

有時候可能會遇到要驗證元素全部符合特定條件的時候，這時候並不推薦使用for-in

不推薦使用

for (final item in items) {
  if (item.length < 5) {
    return false;
  }
}
return true;

使用every()可以讓程式更簡潔

return items.every((item) => item.length >= 5);

any() and every()

any():當至少有一個元素符合條件時回傳true
every():當全部元素符合條件時回傳true

void main() {
  const items = ['Salad', 'Popcorn', 'Toast'];

  if (items.any((item) => item.contains('a'))) {
    print('At least one item contains "a"');
  }

  if (items.every((item) => item.length >= 5)) {
    print('All items have length >= 5');
  }
}

output

At least one item contains "a"
All items have length >= 5

Filtering

如果要找到滿足某個條件的所有元素，where()也是好選擇

var evenNumbers = numbers.where((number) => number.isEven);
for (final number in evenNumbers) {
  print('$number is even');
}

以上程式碼是在篩選出全部屬於偶數的元素

takeWhile() and skipWhile()

takeWhile()會傳回元素直到出現第一個不符合的元素就停止，而skipWhile()是直到出現第一個不符合的元素才會return第一個不符合條件的元素和後面所有元素

void main() {
  const numbers = [1, 3, -2, 0, 4, 5];

  var numbersUntilZero = numbers.takeWhile((number) => number != 0);
  print('Numbers until 0: $numbersUntilZero');

  var numbersStartingAtZero = numbers.skipWhile((number) => number != 0);
  print('Numbers starting at 0: $numbersStartingAtZero');
}

output

Numbers until 0: (1, 3, -2)
Numbers starting at 0: (0, 4, 5)

參考資料:
https://dart.dev/codelabs/iterables#filtering