WithForm.js

function defaultDirtyForForm(form) {
  return Object.keys(form).reduce((acc, key) => {
    acc[key] = false;
    return acc;
  }, {});
}

const WithForm = defaultForm => {
  const fields = Object.keys(defaultForm());

  return {
    defaultForm,

    props: {
      entity: {
        type: [Boolean, Object]
      }
    },

    data() {
      const form = this.$options.defaultForm();
      return {
        form,
        isDirty: defaultDirtyForForm(form)
      };
    },

    computed: {
      isEditing() {
        return !!this.entity;
      },

      isNew() {
        return !this.isEditing;
      },

      formDirty() {
        return Object.values(this.isDirty).some(value => value);
      },

      formValid() {
        return Object.keys(this.form).every(
          field => this[`${field}Validation`].state !== false
        );
      },

      ...fields.reduce((acc, field) => {
        acc[`${field}Validation`] = function validation() {
          if (!this.isDirty[field]) {
            return { state: null, messages: [] };
          }

          let state = true;
          const messages = [];

          const validator =
            this.$options.validateForm && this.$options.validateForm[field];
          if (validator && !validator(this.form.slug)) {
            state = false;
            messages.push("Inválido");
          }

          return { state, messages };
        };

        return acc;
      }, {})
    },

    methods: {
      resetForm() {
        const form = this.$options.defaultForm();
        const isDirty = defaultDirtyForForm(this.form);

        if (this.entity) {
          const cloned = JSON.parse(JSON.stringify(this.entity));
          for (let key of Object.keys(this.form)) {
            form[key] = cloned[key];
          }
        }

        this.$set(this, "form", form);
        this.$set(this, "isDirty", isDirty);
      },

      dirty(key) {
        this.isDirty[key] = true;
      },

      stopEditing() {
        this.$emit("stopEditing");
      }
    },

    watch: {
      entity() {
        this.resetForm();
      }
    }
  };
};

export default WithForm;