Update.

test/fswatch/watch_file.wren

@@ -0,0 +1,34 @@
+// retoor <retoor@molodetz.nl>
+
+import "fswatch" for FileWatcher, FsEvent
+import "timer" for Timer
+import "io" for File, FileFlags
+
+var testFile = "/tmp/fswatch_test_%(Num.fromString("%(System.clock)".split(".")[1])).txt"
+
+var f = File.openWithFlags(testFile, FileFlags.writeOnly | FileFlags.create | FileFlags.truncate)
+f.writeBytes("initial", 0)
+f.close()
+
+var watcher = FileWatcher.new(testFile)
+var received = []
+
+watcher.start { |event|
+    received.add(event)
+}
+
+System.print(watcher.isActive) // expect: true
+
+Timer.immediate {
+    var f2 = File.openWithFlags(testFile, FileFlags.writeOnly | FileFlags.truncate)
+    f2.writeBytes("modified", 0)
+    f2.close()
+}
+
+Timer.sleep(100)
+
+System.print(received.count > 0) // expect: true
+System.print(received[0] is FsEvent) // expect: true
+
+watcher.stop()
+System.print(watcher.isActive) // expect: false