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rules() {
   return 
       [
          'data',
          ArrayValidator::class,
          'subRules' => [
              [['foo', 'bar'], 'required'],
              [
                  'foo',                                         // associative array
                  ArrayValidator::class,
                  'subRules' => [
                      ['id', 'required'],
                      ['id', 'integer'],
                  ],
              ],
              ['bar', 'string'],
              [
                  'payments',                                     // non-associative array
                  CustomEachValidator::class,
                  'rule' => [
                      ArrayValidator::class,
                      'subRules' => [
                          [['type', 'sum'], 'required'],
                          ['type', 'in', 'range' => [1, 2, 3]],
                          ['sum', 'double'],
                      ],
                  ],
              ],
              [
                  'client',
                  ArrayValidator::class,
                  'subRules' => [                                // using closure inside 'when'. $client - internal validation model
                      ['email', 'required', 'message' => '[email] cannot be blank when [phone] is blank', 'when' => static function($client) {
                          return empty($client->phone);
                      }],
                      ['phone', 'required', 'message' => '[phone] cannot be blank when [email] is blank', 'when' => static function($client) {
                          return empty($client->email);
                      }],
                      ['email', 'email'],
                      ['phone', PhoneInputValidator::class],
                      ['email', function($attribute, $params) {  // Using a closure as a validator. $this - internal validation model
                          if ($this->$attribute === 'test@example.com') {
                              $this->addError("forbidden $attribute value was passed");
                          }
                      }],
                  ],
              ],
          ],
        ]
}