vue3 在el-input的光标处插入文本

点击文本框下方的按钮，将相应的文本插入光标处的实现：

<el-input type="textarea" rows="4" v-model="formula" @blur="handleBlur" clearable></el-input>
 <el-button-group class="short_btn">
            <el-button type="primary" plain @click="appendFormula('+')">+</el-button>
             <el-button type="primary" plain @click="appendFormula('-')">-</el-button>
              <el-button type="primary" plain @click="appendFormula('*')">*</el-button>
               <el-button type="primary" plain @click="appendFormula('/')">/</el-button>
                <el-button type="primary" plain @click="appendFormula('(')">(</el-button>
                 <el-button type="primary" plain @click="appendFormula(')')">)</el-button>
        </el-button-group>

const cursorPos = ref(); // 光标位置
const handleBlur = (e:any)=>{
  cursorPos.value = e.srcElement.selectionStart;
};

const appendFormula = (str: any ) => {
  if (!formula.value) {
    formula = str;
  } else {
    const start = formula.value.substring(0, cursorPos.value);
    const end =formula.value.substring(cursorPos.value);
    formula.value = `${start}${str}${end}`;
  }
};