public static Key obtain(Context context) {
        String packageName = context.getPackageName();
        Key result = PACKAGE_NAME_TO_KEY.get(packageName);
        if (result == null) {
            Key toAdd = obtainVersionSignature(context);
            result = PACKAGE_NAME_TO_KEY.putIfAbsent(packageName, toAdd);
            // There wasn't a previous mapping, so toAdd is now the Key.
            if (result == null) {
                result = toAdd;
            }
        }

        return result;
    }
Returns the signature {@link Key} for version code of the Application of the given
Context.
core/src/main/java/com/bumptech/glide/signature/ApplicationVersionSignature.java