Provide a way to access a domain's default theme

Overall this makes sense to me. I do not see tests, but there are none for the module. I did not manually test this either.

One minor change:

+++ b/src/Theme/ThemeSwitchNegotiator.php
@@ -125,6 +125,14 @@ class ThemeSwitchNegotiator implements ThemeNegotiatorInterface {
+   * @return string|null

Method should have a title

This looks correct and it applies cleanly to the latest branch and I see no code style violations.

Setting to RTBC to get visibility to this proposal. Resolution to this blocks #2941723: Reimplement the domain option in theme settings.

Hi,

The defaultTheme is set in applies() method and thus may not be available before this method is called. The variable is only meant to be used internally.

I'd suggest either:

1) Fetching the default theme right from the config:

$domain = \Drupal::service('domain.negotiator')->getActiveDomain();
    $config = \Drupal::config('domain_theme_switch.settings')->get($domain->id() . '_site');

or
2) Refactoring ThemeSwitchNegotiator. Both defaultTheme and adminTheme properties could be removed. Instead, a new service may be implemented to return default & admin themes.

Three requested changes:

1. +++ b/domain_theme_switch.services.yml
   @@ -1,6 +1,10 @@
   +    class: Drupal\domain_theme_switch\Service\DomainThemeLookup
   

   Because this module is so lighteweight, I think this can simply be put at Drupal\domain_theme_switch\DomainThemeLookup

2. +++ b/domain_theme_switch.services.yml
   @@ -1,6 +1,10 @@
   +    - { name: theme_negotiator, priority: 10 }
   

   Irrelevant change? No need to remove indentation here

3. +++ b/src/Theme/ThemeSwitchNegotiator.php
   @@ -97,15 +86,16 @@ class ThemeSwitchNegotiator implements ThemeNegotiatorInterface {
   +    $theme_lookup = \Drupal::getContainer()->get('domain_theme_switch.domain_theme_lookup');
   

   Pass this in as a dependency via the __constructor

Overall this looks good. A few code style things:

1. +++ b/src/DomainThemeLookup.php
   @@ -0,0 +1,48 @@
   + * Class DomainThemeLookup
   

   Comments end in a period

   Perhaps elaborate here--what is the point of this class?

   A service that retrieves theme settings based on the domain.

2. +++ b/src/DomainThemeLookup.php
   @@ -0,0 +1,48 @@
   +   *   The config factory.
   

   This line is not needed, see https://www.drupal.org/node/1354#var

3. +++ b/src/DomainThemeLookup.php
   @@ -0,0 +1,48 @@
   +    $this->configFactory = $config_factory;
   

   Consider instead:
   $this->config = $config_factory->get('domain_theme_switch.settings'). This prevents needing to get the config bucket with every call in this class

4. +++ b/src/DomainThemeLookup.php
   @@ -0,0 +1,48 @@
   +   *   The domain id.
   

   Typically "id" is written as "ID", see https://www.drupal.org/docs/develop/standards/coding-standards#naming

5. +++ b/src/DomainThemeLookup.php
   @@ -0,0 +1,48 @@
   +  public function getAdminTheme( $domain_id) {
   

   Avoid extra whitespace after opening parenthesis

6. +++ b/src/Theme/ThemeSwitchNegotiator.php
   @@ -68,17 +68,16 @@ class ThemeSwitchNegotiator implements ThemeNegotiatorInterface {
   +
   

   Avoid unnecessary whitespace

Thank you guys! I like the progress on this issue.
The only concern I have is the DomainThemeLookup class; can we please add an interface for it and refer the interface instead? Just in case someone wants to decorate the service.

1. +++ b/src/DomainThemeLookup.php
   @@ -0,0 +1,41 @@
   +   *   The config factory.
   

   This goes above the @var, see https://www.drupal.org/node/1354#var

2. +++ b/src/DomainThemeLookup.php
   @@ -0,0 +1,41 @@
   +   * @param ConfigFactoryInterface $config_factory
   

   Taking code style strictly, this requires a description, see https://www.drupal.org/node/1354#param

3. +++ b/src/DomainThemeLookupInterface.php
   @@ -0,0 +1,29 @@
   +   * @return string
   

   And this requires a description, see https://www.drupal.org/node/1354#return

4. +++ b/src/DomainThemeLookupInterface.php
   @@ -0,0 +1,29 @@
   +   * @return string
   

   Same

5. +++ b/src/Theme/ThemeSwitchNegotiator.php
   @@ -68,17 +68,17 @@ class ThemeSwitchNegotiator implements ThemeNegotiatorInterface {
   +   * @param \Drupal\domain_theme_switch\DomainThemeLookup
   

   Same

6. +++ b/src/Theme/ThemeSwitchNegotiator.php
   @@ -68,17 +68,17 @@ class ThemeSwitchNegotiator implements ThemeNegotiatorInterface {
   +
   

   Remove extra whitespace

This looks great! I see no code style violations. I did not see tests nor did I manually test.

Setting to RTBC.

Thanks for the patch.
I'm a bit busy this week but I'll try to have a look.

The patch #17 works for us. Thanks a lot.

Verified the patch, committing to 8.x-1.x.