MAGETWO-38178: Check if Theme in use

 - Add missing description block
 - Test cases updated from CR feedback

Author: Fred Sung

app/code/Magento/Theme/Model/ThemeValidator.php

@@ -55,6 +55,7 @@ public function __construct(
     }
 
     /**
+     * Validate the theme if being in use in default, website, or store.
      *
      * @param string[] $themePaths
      * @return array
@@ -81,7 +82,7 @@ public function validateIsThemeInUse($themePaths)
                         . $this->storeManager->getWebsite($row['scope_id'])->getName();
                     break;
                 case ScopeInterface::SCOPE_STORES:
-                    $messages[] = $themesById[$row['value']] . ' is in use in website '
+                    $messages[] = $themesById[$row['value']] . ' is in use in store '
                         . $this->storeManager->getStore($row['scope_id'])->getName();
                     break;
             }

app/code/Magento/Theme/Test/Unit/Console/Command/ThemeUninstallCommandTest.php

@@ -341,8 +341,9 @@ public function testExecuteFailedThemeInUseCheck()
         $this->themeValidator
             ->expects($this->once())
             ->method('validateIsThemeInUse')
-            ->willReturn(['frontend/Magento/a']);
+            ->willReturn(['frontend/Magento/a is in use in default config']);
         $this->tester->execute(['theme' => ['frontend/Magento/a']]);
+        $this->assertEquals('frontend/Magento/a is in use in default config' . PHP_EOL, $this->tester->getDisplay());
     }
 
     public function testExecuteFailedDependencyCheck()

app/code/Magento/Theme/Test/Unit/Model/ThemeValidatorTest.php

@@ -12,7 +12,7 @@
 class ThemeValidatorTest extends \PHPUnit_Framework_TestCase
 {
     /**
-     * @var \Magento\Theme\Model\ThemeValidator|\PHPUnit_Framework_MockObject_MockObject
+     * @var \Magento\Theme\Model\ThemeValidator
      */
     protected $themeValidator;
 
@@ -72,10 +72,20 @@ public function testValidateIsThemeInUse()
             ->expects($this->at(2))
             ->method('addFieldToFilter')
             ->willReturn([$defaultEntity, $websitesEntity, $storesEntity]);
-        $website = $this->getMock('Magento\Store\Model\Website', [], [], '', false);
-        $store = $this->getMock('Magento\Store\Model\Store', [], [], '', false);
+        $website = $this->getMock('Magento\Store\Model\Website', ['getName'], [], '', false);
+        $website->expects($this->once())->method('getName')->willReturn('websiteA');
+        $store = $this->getMock('Magento\Store\Model\Store', ['getName'], [], '', false);
+        $store->expects($this->once())->method('getName')->willReturn('storeA');
         $this->storeManager->expects($this->once())->method('getWebsite')->willReturn($website);
         $this->storeManager->expects($this->once())->method('getStore')->willReturn($store);
-        $this->themeValidator->validateIsThemeInUse(['frontend/Magento/a']);
+        $result = $this->themeValidator->validateIsThemeInUse(['frontend/Magento/a']);
+        $this->assertEquals(
+            [
+                'frontend/Magento/a is in use in default config',
+                'frontend/Magento/a is in use in website websiteA',
+                'frontend/Magento/a is in use in store storeA'
+            ],
+            $result
+        );
     }
 }