added option --without-creating-snapshots (#18)

added option --without-creating-snapshots

README.md

@@ -76,3 +76,16 @@ The library currently supports the following wildcards:
 - DateTimeWildcard
 - DateTimeOrNullWildcard
 - StringWildcard
+
+### Usage in CI
+When running your tests in Continuous Integration you would possibly want to disable the creation of snapshots.
+
+By using the --without-creating-snapshots parameter, PHPUnit will fail if the snapshots don't exist.
+
+```
+> ./vendor/bin/phpunit -d --without-creating-snapshots
+
+1) ExampleTest::test_it_matches_a_string
+Snapshot "ExampleTest__test_it_matches_a_string__1.txt" does not exist. 
+You can automatically create it by removing `-d --without-creating-snapshots` of PHPUnit's CLI arguments.
+```

src/MatchesSnapshots.php

@@ -77,6 +77,18 @@ public function assertMatchesXmlSnapshot(string $actual, array $wildcards = [])
         $this->doSnapshotAssertion($actual, new XmlDriver(), $wildcards);
     }
 
+    /**
+     * Determines whether or not the snapshot should be created instead of
+     * matched.
+     *
+     * Override this method if you want to use a different flag or mechanism
+     * than `-d --without-creating-snapshots`.
+     */
+    protected function shouldCreateSnapshots() : bool
+    {
+        return ! in_array('--without-creating-snapshots', $_SERVER['argv'], true);
+    }
+
     /**
      * // phpcs:disable
      * @param bool $withDataSet
@@ -138,6 +150,8 @@ private function doSnapshotAssertion(string $actual, Driver $driver, array $wild
         );
 
         if (!$snapshotHandler->snapshotExists($snapshot)) {
+            $this->assertSnapshotShouldBeCreated($filename);
+
             $this->createSnapshotAndMarkTestIncomplete($snapshot, $actual);
         }
 
@@ -202,4 +216,17 @@ private function registerSnapshotChange(string $message): void
     {
         $this->snapshotChanges[] = $message;
     }
+
+    protected function assertSnapshotShouldBeCreated(string $snapshotFileName): void
+    {
+        if ($this->shouldCreateSnapshots()) {
+            return;
+        }
+
+        $this->fail(
+            "Snapshot \"$snapshotFileName\" does not exist.\n".
+            'You can automatically create it by removing '.
+            '`-d --without-creating-snapshots` of PHPUnit\'s CLI arguments.'
+        );
+    }
 }