[Gmail API] Label(Folder or Group) List

    // 원하는 코드 입력
	private static final String MAIL_SCOPE = "https://mail.google.com/";

Gmail에서는 메일 폴더들을 Label이라고 지칭.

메일 리스트를 가져오기위해선 먼저 Label들의 정보를 알아야 함.

	private static final String MAIL_SCOPE = "https://mail.google.com/";
    private static final HttpTransport HTTP_TRANSPORT = AndroidHttp.newCompatibleTransport();
    private static final JsonFactory JSON_FACTORY = JacksonFactory.getDefaultInstance();

    public static void loadLabels(Context context, Account account, GmailNetworkListener listener) {
        try {
            GoogleAccountCredential credential = GoogleAccountCredential.usingOAuth2(context, Collections.singleton(MAIL_SCOPE));
            credential.setSelectedAccount(account);

            Gmail service = new Gmail.Builder(HTTP_TRANSPORT, JSON_FACTORY, credential).build();
            new GetLabelsTask(account, service, listener).execute();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private static class GetLabelsTask extends AsyncTask<Void, Void, List<Label>> {

        private Account account;
        private Gmail service;
        private GmailNetworkListener listener;
        private List<Label> labelList;

        public GetLabelsTask(Account account, Gmail service, GmailNetworkListener listener) {
            this.account = account;
            this.service = service;
            this.listener = listener;
            this.labelList = new ArrayList<>();
        }

        final JsonBatchCallback<Label> callback = new JsonBatchCallback<Label>() {
            @Override
            public void onSuccess(Label label, HttpHeaders responseHeaders) throws IOException {
                labelList.add(label);
            }

            @Override
            public void onFailure(GoogleJsonError e, HttpHeaders responseHeaders) throws IOException {

            }
        };

        @Override
        protected List<Label> doInBackground(Void... voids) {
            if (listener == null) return null;

            try {
                ListLabelsResponse response = service.users().labels().list(account.name).execute();

                BatchRequest batch = service.batch();
                for (Label item : response.getLabels()) {
                    service.users().labels().get(account.name, item.getId()).queue(batch, callback);
                }
                batch.execute();

                return labelList;
            } catch (IOException e) {
                e.printStackTrace();
                listener.onFailure(e.getCause().getMessage());
            }
            return null;
        }

        @Override
        protected void onPostExecute(List<Label> result) {
            super.onPostExecute(result);
            if (listener != null) {
                listener.onSuccess(result);
            }
        }
    }