Introduce new hook `Monitoring/CustomVarRenderer`

modules/monitoring/library/Monitoring/Hook/CustomVarRendererHook.php

@@ -0,0 +1,98 @@
+<?php
+/* Icinga Web 2 | (c) 2021 Icinga GmbH | GPLv2+ */
+
+namespace Icinga\Module\Monitoring\Hook;
+
+use Closure;
+use Exception;
+use Icinga\Application\Hook;
+use Icinga\Application\Logger;
+use Icinga\Module\Monitoring\Object\MonitoredObject;
+
+abstract class CustomVarRendererHook
+{
+    /**
+     * Prefetch the data the hook needs to render custom variables
+     *
+     * @param MonitoredObject $object The object for which they'll be rendered
+     *
+     * @return bool Return true if the hook can render variables for the given object, false otherwise
+     */
+    abstract public function prefetchForObject(MonitoredObject $object);
+
+    /**
+     * Render the given variable name
+     *
+     * @param string $key
+     *
+     * @return ?mixed
+     */
+    abstract public function renderCustomVarKey($key);
+
+    /**
+     * Render the given variable value
+     *
+     * @param string $key
+     * @param mixed $value
+     *
+     * @return ?mixed
+     */
+    abstract public function renderCustomVarValue($key, $value);
+
+    /**
+     * Return a group name for the given variable name
+     *
+     * @param string $key
+     *
+     * @return ?string
+     */
+    abstract public function identifyCustomVarGroup($key);
+
+    /**
+     * Prepare available hooks to render custom variables of the given object
+     *
+     * @param MonitoredObject $object
+     *
+     * @return Closure A callback ($key, $value) which returns an array [$newKey, $newValue, $group]
+     */
+    final public static function prepareForObject(MonitoredObject $object)
+    {
+        $hooks = [];
+        foreach (Hook::all('Monitoring/CustomVarRenderer') as $hook) {
+            /** @var self $hook */
+            try {
+                if ($hook->prefetchForObject($object)) {
+                    $hooks[] = $hook;
+                }
+            } catch (Exception $e) {
+                Logger::error('Failed to load hook %s:', get_class($hook), $e);
+            }
+        }
+
+        return function ($key, $value) use ($hooks, $object) {
+            $newKey = $key;
+            $newValue = $value;
+            $group = null;
+            foreach ($hooks as $hook) {
+                /** @var self $hook */
+
+                try {
+                    $renderedKey = $hook->renderCustomVarKey($key);
+                    $renderedValue = $hook->renderCustomVarValue($key, $value);
+                    $group = $hook->identifyCustomVarGroup($key);
+                } catch (Exception $e) {
+                    Logger::error('Failed to use hook %s:', get_class($hook), $e);
+                    continue;
+                }
+
+                if ($renderedKey !== null || $renderedValue !== null) {
+                    $newKey = $renderedKey !== null ? $renderedKey : $key;
+                    $newValue = $renderedValue !== null ? $renderedValue : $value;
+                    break;
+                }
+            }
+
+            return [$newKey, $newValue, $group];
+        };
+    }
+}