feat: Implement `MultiChoicesQuestionViewModel`

[apomalyn]

Requirements

Implement the MultiChoicesQuestion model extending Question. It should be implemented using freezed

MultiChoicesQuestion schema

**Note**: the schema is represented using OpenAPI spec for simplicity. ```yaml MultipleChoiceQuestion: description: Representation of a multiple-choice question allOf: - $ref: "#/components/schemas/Question" - properties: rightAnswerIndex: description: Indicate the answer index in the options options: type: array minimum: 2 maximum: 4 items: $ref: "#/components/schemas/MultiChoiceOption" required: - rightAnswerIndex - options ```

Implement a MultiChoicesQuestionViewModel that:

• extends QuestionViewModel
• implement a method to validate the response

What needs to be done

• Implement the MultiChoicesQuestion model extending Question (format: multiple_choice) under src/practice/questions/models using freezed (documentation)
• Implement a MultiChoicesQuestionViewModel view-model, under src/practice/questions:
  • extends QuestionViewModel
  • constructor takes:
  • MultiChoiceQuestion question
  • everything from QuestionViewModel constructor
  • have a int _selectedOptionIndex = -1
  • have a getter MultiChoiceOption? selectedOption that give the current selected option based on _selectedOptionIndex
  • implement a onOptionPressed(int index) that can update the currently selected option
  • implement the onSubmitPressed method, it should:
  • if _selectedOptionIndex is -1, stop here
  • validate _selectedOptionIndex == question.rightAnswerIndex:
    • if true -> call _onSuccess(explanation: question.explanation)
    • if false
    • reduce question.knowledgeUpdates[0].grade by 1
    • call _onError(_determineAskAgain(), explanation: question.explanation) provide the explanation only if knowledge update is 1 or less

Dependencies

Child of #168

Depends on:

• 171

• 173

Dev notes

• Freezed (documentation) for union types