how about change the usernameField expression as a Column expression

[liyadongcn]

change the expression like this :
....
'usernameField' => ['attribute' => 'username','label' => 'User Name'],
....

it will easy to change the user name label just like the extraColumns.

i used the theme to change the view of assigment. You have to change the code like this in the view.
...
if (is_array($usernameField)){
// usernameField is an array
$userName = $model->{$usernameField['attribute']};
}else{
// usernameField is a string
$userName = $model->{$usernameField};
}
....

[liyadongcn]

idea changed. change the 'extraColumns' parameter to 'columns' means you can show the field you want to. 'usernameFiled' can be showed or not if you put this field in the 'columns'. now the usernameField is showed in the simplest way , even you can not change the showing name on the table.