在mailItem htmlbody中获取光标位置

function getCursorPosition() {
  var cursorPos = 0;
  if (window.getSelection) {
    var selection = window.getSelection();
    if (selection.rangeCount) {
      var range = selection.getRangeAt(0);
      var preCaretRange = range.cloneRange();
      preCaretRange.selectNodeContents(mailItem.htmlbody);
      preCaretRange.setEnd(range.endContainer, range.endOffset);
      cursorPos = preCaretRange.toString().length;
    }
  } else if (document.selection && document.selection.createRange) {
    var textRange = document.selection.createRange();
    var preCaretTextRange = document.body.createTextRange();
    preCaretTextRange.moveToElementText(mailItem.htmlbody);
    preCaretTextRange.setEndPoint("EndToEnd", textRange);
    cursorPos = preCaretTextRange.text.length;
  }
  return cursorPos;
}