Returns the character range of the selected text within the string currently being edited. (Read Only)
For example: if the keyboard is editing a text "abcdef" and the "cde" substring is selected, the return value is a RangeInt with the RangeInt.start value set to 2 and a RangeInt.length value of 3.
如果光标介于两个字符之间并且没有选择文本，则 RangeInt.length 属性为 0。
如果 canGetSelection 为 false，这将始终返回一个空范围（从 0 开始，长度为 0）。