initTree method
build tree by int provinceId, @param provinceId this is province id @return tree
Implementation
Point initTree(String provinceId) {
String _cacheKey = provinceId;
// 这里为了避免 https://github.com/hanxu317317/city_pickers/issues/68
// if (_cache.has(_cacheKey)) {
// return tree = _cache.get(_cacheKey);
// }
String name = this._provincesData[provinceId]!;
String letter = PinyinHelper.getFirstWordPinyin(name).substring(0, 1);
var root =
new Point(code: provinceId, letter: letter, children: [], name: name);
tree = _buildTree(root, metaInfo[provinceId], metaInfo);
_cache.set(_cacheKey, tree);
return tree;
}