API Add withConfig method (#9011)

* With config functionality added.
* Update docs/en/02_Developer_Guides/04_Configuration/00_Configuration.md

docs/en/02_Developer_Guides/04_Configuration/00_Configuration.md

@@ -377,6 +377,48 @@ When you have more than one rule for a nested fragment, they're joined like
 That is, the fragment will be included if all Only rules match, except if all Except rules match.
 </div>
 
+## Unit tests
+
+Sometimes, it's necessary to change a configuration value in your unit tests.
+One way to do this is to use the `withConfig` method.
+This is especially handy when using data providers.
+Example below shows one unit test using a data provider.
+This unit test changes configuration before testing functionality.
+The test will run three times, each run with different configuration value.
+Note that the configuration change is active only within the callback function.
+
+```php
+/**
+ * @dataProvider testValuesProvider
+ * @param string $value
+ * @param string $expected
+ */
+public function testConfigValues($value, $expected)
+{
+    $result = Config::withConfig(function(MutableConfigCollectionInterface $config) use ($value) {
+        // update your config
+        $config->set(MyService::class, 'some_setting', $value);
+
+        // your test code goes here and it runs with your changed config
+        return MyService::singleton()->executeSomeFunction();
+    });
+
+    // your config change no longer applies here as it's outside of callback
+
+    // assertions can be done here but also inside the callback function
+    $this->assertEquals($expected, $result);
+}
+
+public function testValuesProvider(): array
+{
+    return [
+        ['test value 1', 'expected value 1'],
+        ['test value 2', 'expected value 2'],
+        ['test value 3', 'expected value 3'],
+    ];
+}
+```
+
 ## API Documentation
 
 * [Config](api:SilverStripe\Core\Config\Config)

src/Core/Config/Config.php

@@ -4,7 +4,6 @@
 
 use InvalidArgumentException;
 use SilverStripe\Config\Collections\ConfigCollectionInterface;
-use SilverStripe\Config\Collections\DeltaConfigCollection;
 use SilverStripe\Config\Collections\MutableConfigCollectionInterface;
 
 abstract class Config
@@ -119,4 +118,23 @@ public static function forClass($class)
     {
         return new Config_ForClass($class);
     }
+
+    /**
+     * Perform the given operation in an isolated config state.
+     * On return, the config state will be restored, so any modifications are temporary.
+     *
+     * @param callable $callback Callback to run. Will be passed the nested config state as a parameter
+     * @return mixed Result of callback
+     */
+    public static function withConfig($callback)
+    {
+        static::nest();
+        $config = static::modify();
+
+        try {
+            return $callback($config);
+        } finally {
+            static::unnest();
+        }
+    }
 }

tests/php/Core/Config/ConfigTest.php

@@ -2,6 +2,7 @@
 
 namespace SilverStripe\Core\Tests\Config;
 
+use SilverStripe\Config\Collections\MutableConfigCollectionInterface;
 use SilverStripe\Config\MergeStrategy\Priority;
 use SilverStripe\Core\Config\Config;
 use SilverStripe\Dev\SapphireTest;
@@ -322,4 +323,44 @@ public function testForClass()
         $this->assertTrue(empty($config->bar));
         $this->assertNull($config->bar);
     }
+
+    public function testWithConfig()
+    {
+        $oldValue = 'test_1';
+        $newValue1 = 'new value 1';
+        $newValue2 = 'new value 2';
+        $property = 'third';
+
+        $this->assertEquals(
+            $oldValue,
+            Config::inst()->get(ConfigTest\First::class, $property)
+        );
+
+        Config::withConfig(function (MutableConfigCollectionInterface $config) use ($newValue1, $newValue2, $property) {
+            $config->set(ConfigTest\First::class, $property, $newValue1);
+
+            $this->assertEquals(
+                $newValue1,
+                Config::inst()->get(ConfigTest\First::class, $property)
+            );
+
+            $resultValue = Config::withConfig(function (MutableConfigCollectionInterface $config) use ($newValue2, $property) {
+                $config->set(ConfigTest\First::class, $property, $newValue2);
+
+                return Config::inst()->get(ConfigTest\First::class, $property);
+            });
+
+            $this->assertEquals($newValue2, $resultValue);
+
+            $this->assertEquals(
+                $newValue1,
+                Config::inst()->get(ConfigTest\First::class, $property)
+            );
+        });
+
+        $this->assertEquals(
+            $oldValue,
+            Config::inst()->get(ConfigTest\First::class, $property)
+        );
+    }
 }