start method
void
start()
This Method Starts the Countdown Timer
Implementation
void start() {
if (_isReverse) {
_state._controller.reverse(
from: _initialDuration == 0 ? 1 : 1 - (_initialDuration / _duration));
} else {
_state._controller.forward(
from: _initialDuration == 0 ? 0 : (_initialDuration / _duration));
}
}