Either or validation for string and file yii2 - yii2

Question

I have two fields message and file where one is just plain string and file is an image.
I want to create validator which only allows user to send either one of those 2 fields.
I tried when validator but in when the field $model->file is always null so what is other method to do either or validation with file.
Here is my model code
class Message extends \yii\db\ActiveRecord
{
public $file;
/**
* #inheritdoc
*/
public static function tableName()
{
return 'message';
}
/**
* #inheritdoc
*/
public function rules()
{
return [
[['sender_id', 'receiver_id'], 'integer'],
[['message'], 'string'],
[['file'], 'file', 'extensions'=>'jpg, gif, png,jpeg'],
/*['file', 'required', 'when' => function($model) {
return $model->message == null;
}],
['message', 'required', 'when' => function($model) {
return $this->file->baseName == null;
}]*/
];
}
/**
* #inheritdoc
*/
public function attributeLabels()
{
return [
'id' => 'ID',
'sender_id' => 'Sender ID',
'receiver_id' => 'Receiver ID',
'message' => 'Message',
'file' => 'Image (jpg/png)',
'is_delivered' => 'Is Delivered',
'is_notified' => 'Is Notified',
'is_deleted' => 'Is Deleted',
'created_date' => 'Created Date',
'updated_date' => 'Updated Date',
'is_group' => 'Is Group',
];
}
}
Thank you

Answer 1

This may Help you..
Define Your rules as : -
public function rules()
{
return [
//Your Rules ......
['message' ,'string'],
['file' ,'file'],
['message', 'required', 'when' => function($model) {
return $model->file === null;
} ,'whenClient' => 'function (attribute, value) {
return $("#'. Html::getInputId($this ,'file') .'").val() === null;
}'],
['file', 'required', 'when' => function($model) {
return $model->message === null;
} , 'whenClient' => 'function (attribute, value) {
return $("#'. Html::getInputId($this ,'message') .'").val() == "";
}'],
];
}