Cover \Magento\GiftMessage\Observer\SalesEventQuoteMerge with Unit test

app/code/Magento/GiftMessage/Test/Unit/Observer/SalesEventQuoteMergeTest.php

@@ -0,0 +1,81 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+
+declare(strict_types=1);
+
+namespace Magento\GiftMessage\Test\Unit\Observer;
+
+use Magento\GiftMessage\Observer\SalesEventQuoteMerge;
+use Magento\Framework\TestFramework\Unit\Helper\ObjectManager;
+use Magento\Framework\Event\Observer;
+use Magento\Quote\Model\Quote;
+
+/**
+ *  SalesEventQuoteMergeTest
+ */
+class SalesEventQuoteMergeTest extends \PHPUnit\Framework\TestCase
+{
+
+    /**
+     * @var SalesEventQuoteMerge
+     */
+    private $salesEventQuoteMerge;
+
+    /**
+     * @return void
+     */
+    public function setUp(): void
+    {
+        $objectManger = new ObjectManager($this);
+        $this->salesEventQuoteMerge = $objectManger->getObject(SalesEventQuoteMerge::class);
+    }
+
+    /**
+     * @dataProvider dataProviderGiftMessageId
+     *
+     * @param null|int $giftMessageId
+     *
+     * @return void
+     */
+    public function testExecute($giftMessageId): void
+    {
+        $sourceQuoteMock = $this->createPartialMock(Quote::class, ['getGiftMessageId']);
+        $sourceQuoteMock->expects($this->once())
+            ->method('getGiftMessageId')
+            ->willReturn($giftMessageId);
+
+        $targetQuoteMock = $this->createPartialMock(Quote::class, ['setGiftMessageId']);
+
+        if ($giftMessageId) {
+            $targetQuoteMock->expects($this->once())
+                ->method('setGiftMessageId');
+        } else {
+            $targetQuoteMock->expects($this->never())
+                ->method('setGiftMessageId');
+        }
+
+        $observer = $this->createMock(Observer::class);
+        $observer->expects($this->exactly(2))
+            ->method('getData')
+            ->willReturnMap([
+                ['quote', null, $targetQuoteMock],
+                ['source', null, $sourceQuoteMock]
+            ]);
+
+        $this->salesEventQuoteMerge->execute($observer);
+    }
+
+    /**
+     * @return array
+     */
+    public function dataProviderGiftMessageId(): array
+    {
+        return [
+            [null],
+            [1]
+        ];
+    }
+}