如何在Yii2中更改用户

I am using an API into a Yii2 app, and so Yii::$app->user is null.

Can I manually set Yii::$app->user?

All exmaples I have foun involved editing a user, not setting a user.

If you want log an user you could try getting and identity this way and log the user

  / find a user identity with the specified username.
  // note that you may want to check the password if needed

  $identity = User::find()-where(['username' => $username])->one();

  // logs in the user
  Yii::$app->user->login($identity);

http://www.yiiframework.com/doc-2.0/guide-security-authentication.html

You might need to configure the 'user' component in your API app config first. Like so:

'components'    => [
    // ...
    'user'      => [
        'class' => 'yii\web\User',
    ],
    // ...
],