How to get the first dropdown answer's value dynamically?

question

I have defined a custom question type, which name is cascadedropdown,the code is as follows:

ComponentCollection.Instance.add({
  // A unique name; must use lowercase
  name: 'cascadedropdown', // A display name used in the Toolbox
  title: 'cascade dropdown', // A default title for questions created with this question type
  defaultQuestionTitle: 'choose the options',
  elementsJSON: [
    {
      type: 'dropdown',
      name: 'dropdown1',
      title: 'question1',
      choices: [
        {
          text:"apple",
          value:1,
        },
        {
          text:"orange",
          value:2,
        }
      ], 
    },
    {
      type: 'dropdown',
      name: 'dropdown2',
      title: 'question2',
      startWithNewLine: false,
      choices:getSecondLevelChoices(1)
    },
    {
      type: 'dropdown',
      name: 'dropdown3',
      title: 'question3',
      startWithNewLine: false,
      choices:[],
      visible: casecadeQuestionList.length > 2
    },
    {
      type: 'dropdown',
      name: 'dropdown4',
      title: '问题4',
      startWithNewLine: false,
      choices:[],
      visible: casecadeQuestionList.length > 3
    },
  ],
  inheritBaseProps: true
})

on the code above, there are at least two dropdowns in the casecadedropdown question, the second dropdown question’s choices depend on the first dropdown answer. the method getSecondLevelChoices(1), the parameter 1 is the first dropdown answer’s value.

The question is how to get the first dropdown answer’s value dynamically.

the template is

<SurveyCreatorComponent :model="creator" />

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