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Introduced in branch: 

After #1813762: Introduce unified interfaces, use dependency injection for interface translation and the fact that string translation itself is now a service, anyone can inject it to a class and use it directly instead of t(), thus making the class unit-testable.

The recommended/supported way to work with injected translation in Drupal 8 so that our string extractor can find them is by using a protected t() method inside the class.
FormBase and ControllerBase already provide one.
If your class is neither a controller or form, you can use the StringTranslationTrait

use Drupal\Core\StringTranslation\StringTranslationTrait;
use Drupal\Core\StringTranslation\TranslationInterface;
class MyClass {
  use StringTranslationTrait;

   * Constructs a MyClass object.
   * @param \Drupal\Core\StringTranslation\TranslationInterface $string_translation
   *   The string translation service.
  public function __construct(TranslationInterface $string_translation) {
    // You can skip injecting this service, the trait will fallback to \Drupal::translation()
    // but it is recommended to do so, for easier testability,
    $this->stringTranslation = $string_translation;

   * Does something.
  public function doSth() {
    // ...
    $string = $this->t('Something');
    // ...


Passing strings to anything else than t() or $this->t()
eg. $this->stringTranslation->translate('Something') won't work, it will not be possible to provide translation for that on

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