implement design interface

app/Services/PdfMaker/Designs/Plain.php

@@ -13,9 +13,10 @@
 namespace App\Services\PdfMaker\Designs;
 
 use App\Services\PdfMaker\Designs\Utilities\BuildTableHeader;
+use App\Services\PdfMaker\Designs\Utilities\DesignInterface;
 use App\Utils\Traits\MakesInvoiceValues;
 
-class Plain
+class Plain implements DesignInterface
 {
     use MakesInvoiceValues, BuildTableHeader;
 
@@ -65,7 +66,7 @@ class Plain
         ];
     }
 
-    public function productTable()
+    public function productTable(): array
     {
         return  [
             'id' => 'product-table',

app/Services/PdfMaker/Designs/Utilities/DesignInterface.php

@@ -0,0 +1,28 @@
+<?php
+
+/**
+ * Invoice Ninja (https://invoiceninja.com)
+ *
+ * @link https://github.com/invoiceninja/invoiceninja source repository
+ *
+ * @copyright Copyright (c) 2020. Invoice Ninja LLC (https://invoiceninja.com)
+ *
+ * @license https://opensource.org/licenses/AAL
+ */
+
+namespace App\Services\PdfMaker\Designs\Utilities;
+
+interface DesignInterface
+{
+    public function html(): ?string;
+
+    public function setup(): void;
+
+    public function elements(array $context): array;
+
+    public function productTable(): array;
+
+    public function buildTableHeader(): array;
+
+    public function buildTableBody(): array;
+}

resources/views/pdf-designs/plain.html

@@ -8,6 +8,7 @@
         />
         <meta http-equiv="x-ua-compatible" content="ie=edge" />
 
+        <link href="https://unpkg.com/tailwindcss@^1.0/dist/tailwind.min.css" rel="stylesheet">
         <link rel="stylesheet" href="$css" />
     </head>