addListener method
下面是对ValueNotifier的简单封装
Implementation
void addListener(ValueCallback<int> callback) {
if (_listenerMap[callback] != null) {
_notifier.removeListener(() {
_listenerMap[callback]!;
});
}
voidCallback() {
callback.call(_notifier.value.value);
}
_listenerMap[callback] = voidCallback;
_notifier.addListener(voidCallback);
}