Add environment variable to activate snapshot writing

README.md

@@ -93,5 +93,21 @@ public class PlanetConverterTest {
 }
 ```
 
+## Activate snapshot writing
+Snapshots must be written at the same time as the tests.
+It is necessary to avoid writing them in CI environments.
+
+To activate snapshot writing, pass the VM option `-Dtest.snapshots.write`.
+
+### With Maven
+```bash
+mvn -Dtest.snapshots.write test
+```
+
+### With Intellij
+Set VM option inside test run configuration.
+
+![](./writesnapshot-intellij.png)
+
 ## Limitations
 - Only one snapshot assertion per test method

snapshot-matcher/src/main/java/com/zenika/snapshotmatcher/SnapshotMatcher.java

@@ -22,6 +22,9 @@
 import difflib.Patch;
 
 public class SnapshotMatcher<T> extends TypeSafeMatcher<T> {
+
+    private static final String SNAPSHOT_WRITE_VARIABLE = "test.snapshots.write";
+
     /**
      * Factory method to instantiate a snapshot matcher with the given type
      *
@@ -48,11 +51,14 @@ public boolean matchesSafely(T o) {
         if (Files.exists(snapshotPath)) {
             // File exists => Compare snapshot file to given object
             return compareSnapshot(o, snapshotPath);
-        } else {
+        } else if (isWriteSnapshotActivated()) {
             // File doesn't exist => Create snapshot file and return true
             createSnapshot(o, snapshotPath);
             return true;
         }
+        System.out.println("Snapshot writing is not activated in this environment.");
+        System.out.println("Activate snapshot writing by using -D" + SNAPSHOT_WRITE_VARIABLE);
+        return false;
     }
 
     /**
@@ -153,4 +159,7 @@ private StackTraceElement getCaller() {
                 .orElse(null);
     }
 
+    private boolean isWriteSnapshotActivated() {
+        return System.getProperty(SNAPSHOT_WRITE_VARIABLE) != null;
+    }
 }