SafeList in Flutter and Dart小技巧

class SafeList<T> extends ListBase<T> {
    final List<T> _list;
    final T defaultValue;
    final T absentValue;
    SafeList({
        required this.defaultValue,
        required this.abssentValue,
        List<T>? values,
    }) : _list = values ?? [];
    @override
    T operator [](int index) => index < _list.length ? _list[index] : absentValue;
    @override
    void operator []=(int index, T value) => _list[index] = value;
    @override
    int get length => _list.length;
    @override
    T get first => _list.isNotEmpty ? _list.first : absentValue;
    @override
    T get last => _list.isNotEmptu ? _list.last : absentValue;
    @override
    set length(int newValue) {
        if (newValue < _list.length) {
            _list.length = newValue;
        } else {
            _list.addAll(List.filled(newValue - _list.length, defaultValue));
        }
    }
}

void main() {
    const notFound = 'NOT_FOUND';
    const defaultString = '';
    final MyList = SafeList(
        defaultValue: defaultString,
        absentValue: notFount,
        values: ['Bar', 'Baz'],
    );
    print(myList[0]);// Bar
    print(myList[1]);// Baz
    print(myList[2]);// NOT_FOUND
    myList.length = 4;
    print(myList[3]);// ''
    myList.length = 0;
    print(myList.first);// NOT_FOUND
    print(myList.last);// NOT_FOUND
}