feat: instant preview of edited email template

src/Email/EmailTextRenderer.php

@@ -43,9 +43,30 @@ class EmailTextRenderer
         $definition = $this->catalog->get($key);
         $emailText = $this->emailTextRepository->findByKey($key);
 
-        $subject = $emailText?->getSubject() ?: $definition->defaultSubject;
-        $headline = $emailText?->getHeadline() ?? $definition->defaultHeadline;
-        $body = $emailText?->getBody() ?: $definition->defaultBody;
+        return $this->renderValues(
+            $key,
+            $emailText?->getSubject() ?: $definition->defaultSubject,
+            $emailText?->getHeadline() ?? $definition->defaultHeadline,
+            $emailText?->getBody() ?: $definition->defaultBody,
+            $placeholders
+        );
+    }
+
+    /**
+     * Renders wording that is not (or not yet) the saved one - the admin preview shows
+     * what is currently in the form, which is the whole point of previewing before saving.
+     * The values come straight from a half-filled form, so all three may be null.
+     *
+     * @param array<string, string|int|null> $placeholders
+     */
+    public function renderValues(
+        EmailTextKey $key,
+        ?string $subject,
+        ?string $headline,
+        ?string $body,
+        array $placeholders,
+    ): RenderedEmailTextDto {
+        $definition = $this->catalog->get($key);
 
         // Two token maps from the same values: the subject of a mail is plain text, the
         // body is HTML. Escaping happens here, before substitution, so that neither the
@@ -63,9 +84,9 @@ class EmailTextRenderer
         }
 
         return new RenderedEmailTextDto(
-            strtr($subject, $plainTokens),
-            $this->prepare($headline, $htmlTokens),
-            $this->renderBody($body, $htmlTokens),
+            strtr($subject ?? '', $plainTokens),
+            $this->prepare($headline ?? '', $htmlTokens),
+            $this->renderBody($body ?? '', $htmlTokens),
         );
     }