title: How to use Vue watch to observe a form item value?
body: <pre><code>&lt;el-form-item label="range"
                v-if="isShowDatepicker"&gt;
      &lt;el-date-picker type="datetimerange" 
                                      style="width: 100%"
                                      v-model="timeRange"
                                      range-separator="to"
                                      start-placeholder="start"
                                      end-placeholder="end"&gt;&lt;/el-date-picker&gt;
  &lt;/el-form-item&gt;
  </code></pre>
  
  <p>I have a form that has an item to input <code>date</code> and <code>time</code> (just like the code above), and I want to obtain the value after it has changed. I want to use Vue <code>watch</code> to accomplish this task. However, I don't know how to name the variable in <code>watch</code>. I've tried this:</p>
  
  <pre><code>"SearchForm.timeRange": function (val) {
       console.log(val)
  }
  </code></pre>