Implement CryptAuthAccessTokenFetcher.

components/proximity_auth/cryptauth/cryptauth_account_token_fetcher.cc

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "components/proximity_auth/cryptauth/cryptauth_account_token_fetcher.h"
+
+namespace proximity_auth {
+
+namespace {
+
+OAuth2TokenService::ScopeSet GetScopes() {

[Ilya Sherman, 2014/12/11 02:53:21]

nit: Please add a brief docstring.

[Tim Song, 2014/12/12 20:48:44]

Done.

+  OAuth2TokenService::ScopeSet scopes;
+  scopes.insert("https://www.googleapis.com/auth/cryptauth");
+  return scopes;
+}
+
+}  // namespace
+
+// Inherits from OAuth2TokenService for the token caching.

[Ilya Sherman, 2014/12/11 02:53:21]

I'm not following what this comment means -- do you mean "inherits from
OAuth2TokenService::Consumer"?

[Tim Song, 2014/12/12 20:48:44]

Sorry, forgot to delete this comment.

+CryptAuthAccountTokenFetcher::CryptAuthAccountTokenFetcher(
+    OAuth2TokenService* token_service,
+    const std::string& account_id)
+    : OAuth2TokenService::Consumer("cryptauth_account_token_fetcher"),
+      token_service_(token_service),
+      account_id_(account_id),
+      fetch_started_(false) {
+}
+
+CryptAuthAccountTokenFetcher::~CryptAuthAccountTokenFetcher() {
+}
+
+void CryptAuthAccountTokenFetcher::FetchAccessToken(
+    const AccessTokenCallback& callback) {
+  if (fetch_started_) {
+    LOG(WARNING) << "Create an instance for each token fetched. Do not reuse.";
+    callback.Run(std::string());
+    return;
+  }
+
+  fetch_started_ = true;
+  callback_ = callback;
+  // The service will cache the access token, so we do not need to make a
+  // network roundtrip for each token request.

[Ilya Sherman, 2014/12/11 02:53:21]

I don't understand this comment -- where do we take advantage of the cached
token?  Is this just saying that StartRequest() might return a cached result,
rather than making a round-trip?  If so, I'd reword the comment a bit to be
clearer.

[Tim Song, 2014/12/12 20:48:44]

Done.

+  token_request_ = token_service_->StartRequest(account_id_, GetScopes(), this);
+}
+
+void CryptAuthAccountTokenFetcher::OnGetTokenSuccess(
+    const OAuth2TokenService::Request* request,
+    const std::string& access_token,
+    const base::Time& expiration_time) {
+  callback_.Run(access_token);
+}
+
+void CryptAuthAccountTokenFetcher::OnGetTokenFailure(
+    const OAuth2TokenService::Request* request,
+    const GoogleServiceAuthError& error) {
+  callback_.Run(std::string());
+}
+
+}  // namespace proximity_auth