Offload schema validation to a trait+interface combo for easy re-use

src/SchemaValidatableElementInterface.php

@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace SimpleSAML\XML;
+
+use DOMDocument;
+
+/**
+ * interface class to be implemented by all the classes that can be validated against a schema
+ *
+ * @package simplesamlphp/xml-common
+ */
+interface SchemaValidatableElementInterface extends ElementInterface
+{
+    /**
+     * Validate the given DOMDocument against the schema set for this element
+     *
+     * @return void
+     * @throws \SimpleSAML\XML\Exception\SchemaViolationException
+     */
+    public static function schemaValidate(DOMDocument $document): DOMDocument;
+}

src/SchemaValidatableElementTrait.php

@@ -0,0 +1,68 @@
+<?php
+
+declare(strict_types=1);
+
+namespace SimpleSAML\XML;
+
+use DOMDocument;
+use SimpleSAML\Assert\Assert;
+use SimpleSAML\Exception\IOException;
+use SimpleSAML\Exception\SchemaViolationException;
+
+use function array_unique;
+use function defined;
+use function file_exists;
+use function implode;
+use function sprintf;
+use function trim;
+use function libxml_get_errors;
+
+/**
+ * trait class to be used by all the classes that implement the SchemaValidatableElementInterface
+ *
+ * @package simplesamlphp/xml-common
+ */
+trait SchemaValidatableElementTrait
+{
+    /**
+     * Validate the given DOMDocument against the schema set for this element
+     *
+     * @return void
+     * @throws \SimpleSAML\XML\Exception\SchemaViolationException
+     */
+    public static function schemaValidate(DOMDocument $document): DOMDocument
+    {
+        $schemaFile = self::getSchemaFile();
+        $result = $document->schemaValidate($schemaFile);
+
+        if ($result === false) {
+            $msgs = [];
+            foreach (libxml_get_errors() as $err) {
+                $msgs[] = trim($err->message) . ' on line ' . $err->line;
+            }
+
+            throw new SchemaViolationException(sprintf(
+                "XML schema validation errors:\n - %s",
+                implode("\n - ", array_unique($msgs)),
+            ));
+        }
+
+        return $document;
+    }
+
+
+    /**
+     * Get the schema file that can validate this element.
+     *
+     * @return string
+     */
+    public static function getSchemaFile(): string
+    {
+        if (defined('static::SCHEMA')) {
+            $schemaFile = static::SCHEMA;
+        }
+
+        Assert::true(file_exists($schemaFile), IOException::class);
+        return $schemaFile;
+    }
+}