Add a ConfigOverlay class.

src/Config.php

@@ -36,12 +36,12 @@ public function has($key)
     /**
      * {@inheritdoc}
      */
-    public function get($key, $defaultOverride = null)
+    public function get($key, $defaultFallback = null)
     {
         if ($this->has($key)) {
             return $this->config->get($key);
         }
-        return $this->getDefault($key, $defaultOverride);
+        return $this->getDefault($key, $defaultFallback);
     }
 
     /**
@@ -84,9 +84,9 @@ public function hasDefault($key)
     /**
      * {@inheritdoc}
      */
-    public function getDefault($key, $defaultOverride = null)
+    public function getDefault($key, $defaultFallback = null)
     {
-        return $this->hasDefault($key) ? $this->defaults[$key] : $defaultOverride;
+        return $this->hasDefault($key) ? $this->defaults[$key] : $defaultFallback;
     }
 
     /**

src/ConfigInterface.php

@@ -12,11 +12,14 @@ public function has($key);
      * Fetch a configuration value
      *
      * @param string $key Which config item to look up
-     * @param string|null $defaultOverride Override usual default value with a different default. Deprecated; provide defaults to the config processor instead.
+     * @param string|null $defaultFallback Fallback default value to use when
+     *   configuration object has neither a value nor a default. Use is
+     *   discouraged; use default context in ConfigOverlay, or provide defaults
+     *   using a config processor.
      *
      * @return mixed
      */
-    public function get($key, $defaultOverride = null);
+    public function get($key, $defaultFallback = null);
 
     /**
      * Set a config value
@@ -55,11 +58,11 @@ public function hasDefault($key);
      * Return the default value for a given configuration item.
      *
      * @param string $key
-     * @param mixed $defaultOverride
+     * @param mixed $defaultFallback
      *
      * @return mixed
      */
-    public function getDefault($key, $defaultOverride = null);
+    public function getDefault($key, $defaultFallback = null);
 
     /**
      * Set the default value for a configuration setting. This allows us to

src/ConfigOverlay.php

@@ -0,0 +1,134 @@
+<?php
+namespace Consolidation\Config;
+
+/**
+ * Overlay different configuration objects that implement ConfigInterface
+ * to make a priority-based, merged configuration object.
+ *
+ * Note that using a ConfigOverlay hides the defaults stored in each
+ * individual configuration context. When using overlays, always call
+ * getDefault / setDefault on the ConfigOverlay object itself.
+ */
+class ConfigOverlay implements ConfigInterface
+{
+    protected $contexts = [];
+
+    public function __construct()
+    {
+        $this->contexts['default'] = new Config();
+        $this->contexts['process'] = new Config();
+    }
+
+    /**
+     * Add a named configuration object to the configuration overlay.
+     * Configuration objects added LAST have HIGHEST priority, with the
+     * exception of the fact that the process context always has the
+     * highest priority.
+     */
+    public function addContext($name, ConfigInterface $config)
+    {
+        $process = $this->contexts['process'];
+        unset($this->contexts['process']);
+        unset($this->contexts[$name]);
+        $this->contexts[$name] = $config;
+        $this->contexts['process'] = $process;
+    }
+
+    public function hasContext($name)
+    {
+        return isset($this->contexts[$name]);
+    }
+
+    public function getContext($name)
+    {
+        if ($this->hasContext($name)) {
+            return $this->contexts[$name];
+        }
+        return new Config();
+    }
+
+    /**
+     * Determine if a non-default config value exists.
+     */
+    public function findContext($key)
+    {
+        foreach (array_reverse($this->contexts) as $name => $config) {
+            if ($config->has($key)) {
+                return $config;
+            }
+        }
+        return false;
+    }
+
+    /**
+     * @inheritdoc
+     */
+    public function has($key)
+    {
+        return $this->findContext($key) != false;
+    }
+
+    /**
+     * @inheritdoc
+     */
+    public function get($key, $default = null)
+    {
+        $context = $this->findContext($key);
+        if ($context) {
+            return $context->get($key, $default);
+        }
+        return $default;
+    }
+
+    /**
+     * @inheritdoc
+     */
+    public function set($key, $value)
+    {
+        return $this->contexts['process']->set($key, $value);
+    }
+
+    /**
+     * @inheritdoc
+     */
+    public function import($data)
+    {
+        throw new \Exception('The method "import" is not supported for the ConfigOverlay class.');
+    }
+
+    /**
+     * @inheritdoc
+     */
+    public function export()
+    {
+        $export = [];
+        foreach ($this->contexts as $name => $config) {
+            $export = array_merge_recursive($export, $config->export());
+        }
+        return $export;
+    }
+
+    /**
+     * @inheritdoc
+     */
+    public function hasDefault($key)
+    {
+        return $this->contexts['default']->has($key);
+    }
+
+    /**
+     * @inheritdoc
+     */
+    public function getDefault($key, $default = null)
+    {
+        return $this->contexts['default']->get($key, $default);
+    }
+
+    /**
+     * @inheritdoc
+     */
+    public function setDefault($key, $value)
+    {
+        return $this->contexts['default']->set($key, $value);
+    }
+}

tests/ConfigOverlayTest.php

@@ -0,0 +1,114 @@
+<?php
+namespace Consolidation\Config;
+
+use Consolidation\Config\Loader\ConfigProcessor;
+use Consolidation\Config\Loader\YamlConfigLoader;
+
+class ConfigOverlayTest extends \PHPUnit_Framework_TestCase
+{
+    protected $overlay;
+
+    protected function setUp()
+    {
+        $aliasContext = new Config();
+        $aliasContext->import([
+            'hidden-by-a' => 'alias hidden-by-a',
+            'hidden-by-process' => 'alias hidden-by-process',
+            'options' =>[
+                'a-a' => 'alias-a',
+            ],
+            'command' => [
+                'foo' => [
+                    'bar' => [
+                        'command' => [
+                            'options' => [
+                                'a-b' => 'alias-b',
+                            ],
+                        ],
+                    ],
+                ],
+            ],
+        ]);
+
+        $configFileContext = new Config();
+        $configFileContext->import([
+            'hidden-by-cf' => 'config-file hidden-by-cf',
+            'hidden-by-a' => 'config-file hidden-by-a',
+            'hidden-by-process' => 'config-file hidden-by-process',
+            'options' =>[
+                'cf-a' => 'config-file-a',
+            ],
+            'command' => [
+                'foo' => [
+                    'bar' => [
+                        'command' => [
+                            'options' => [
+                                'cf-b' => 'config-file-b',
+                            ],
+                        ],
+                    ],
+                ],
+            ],
+        ]);
+
+        $this->overlay = new ConfigOverlay();
+        $this->overlay->set('hidden-by-process', 'process-h');
+        $this->overlay->addContext('cf', $configFileContext);
+        $this->overlay->addContext('a', $aliasContext);
+        $this->overlay->setDefault('df-a', 'default');
+        $this->overlay->setDefault('hidden-by-a', 'default hidden-by-a');
+        $this->overlay->setDefault('hidden-by-cf', 'default hidden-by-cf');
+        $this->overlay->setDefault('hidden-by-process', 'default hidden-by-process');
+    }
+
+    public function testGetPriority()
+    {
+        $this->assertEquals('process-h', $this->overlay->get('hidden-by-process'));
+        $this->assertEquals('config-file hidden-by-cf', $this->overlay->get('hidden-by-cf'));
+        $this->assertEquals('alias hidden-by-a', $this->overlay->get('hidden-by-a'));
+    }
+
+    public function testDefault()
+    {
+        $this->assertEquals('alias-a', $this->overlay->get('options.a-a'));
+        $this->assertEquals('alias-a', $this->overlay->get('options.a-a', 'ignored'));
+        $this->assertEquals('default', $this->overlay->getDefault('df-a', 'ignored'));
+        $this->assertEquals('nsv', $this->overlay->getDefault('a-a', 'nsv'));
+
+        $this->overlay->setDefault('df-a', 'new value');
+        $this->assertEquals('new value', $this->overlay->getDefault('df-a', 'ignored'));
+    }
+
+    public function testExport()
+    {
+        $data = $this->overlay->export();
+
+        $this->assertEquals('config-file-a', $data['options']['cf-a']);
+        $this->assertEquals('alias-a', $data['options']['a-a']);
+    }
+
+    /**
+     * @expectedException Exception
+     */
+    public function testImport()
+    {
+        $data = $this->overlay->import(['a' => 'value']);
+    }
+
+    public function testChangePriority()
+    {
+        // Get and re-add the 'cf' context. Now, it should have a higher
+        // priority than the 'alias' context, but should still have a lower
+        // priority than the 'process' context.
+        $configFileContext = $this->overlay->getContext('cf');
+        $this->overlay->addContext('cf', $configFileContext);
+
+        // These asserts are the same as in testGetPriority
+        $this->assertEquals('process-h', $this->overlay->get('hidden-by-process'));
+        $this->assertEquals('config-file hidden-by-cf', $this->overlay->get('hidden-by-cf'));
+
+        // This one has changed: the config-file value is now found instead
+        // of the alias value.
+        $this->assertEquals('config-file hidden-by-a', $this->overlay->get('hidden-by-a'));
+    }
+}