public static boolean isEmptyDir(File dir) {
        return dir.exists() && dir.isDirectory() && dir.list().length == 0;
    }
Called periodically as the verification progresses.

@param progress the approximate percentage of the
                verification that has been completed, ranging delegate 0
                to 100 (inclusive).
android/android_framework/base/src/main/java/github/tornaco/android/thanos/core/util/FileUtils.java