光标所在位置插入一串字符，并在特定位置设置光标锁定位置

showdownContainer.prototype.insertStrAtCursor = function(textareaObj,insertString){//IE support
if (document.selection) {
textareaObj.focus();
var sel = document.selection.createRange();
sel.text = myValue;
sel.select();
}
//MOZILLA/NETSCAPE support
else if (textareaObj.selectionStart || textareaObj.selectionStart == '0') {
var startPos = textareaObj.selectionStart;
var endPos = textareaObj.selectionEnd;
// save scrollTop before insert
var restoreTop = textareaObj.scrollTop;
textareaObj.value = textareaObj.value.substring(0, startPos) + insertString + textareaObj.value.substring(endPos, textareaObj.value.length);
if (restoreTop > 0) {
// restore previous scrollTop
textareaObj.scrollTop = restoreTop;
}
textareaObj.focus();
textareaObj.selectionStart = startPos + insertString.length;
textareaObj.selectionEnd = startPos + insertString.length;
} else {
textareaObj.value += insertString;
textareaObj.focus();
}
};
/**
* 设置光标位置
* @param textareaObj  文本域元素对象
* @param insertString  待插入的字符串
* @param posLen 定位光标在插入字符串的第几个字符处
*/
showdownContainer.prototype.setCursorPosition = function(textareaObj,insertString,posLen){
var cursorPosition = 0;
if(textareaObj.selectionStart){
cursorPosition = textareaObj.selectionStart;
}
this.insertStrAtCursor(textareaObj,insertString);
textareaObj.focus();
textareaObj.setSelectionRange(textareaObj.value.length,cursorPosition + posLen);
};