ellipsis method
字符串中间截取
Implementation
String ellipsis([int? length]) {
if (isEmpty) return '';
if (length == null || length < 0 || this.length <= length) return this;
length = (length / 2).floor() - 3;
return this.substring(0, length + 1) +
'...' +
this.substring(this.length - length - 1);
}