TotpClock.java

/*
 * Copyright 2012 Google Inc. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package net.coding.program.login.auth;

import android.content.Context;
import android.content.SharedPreferences;
import android.preference.PreferenceManager;

/**
 * Clock input for the time-based OTPs (TOTP). The input is based on the current system time
 * and is adjusted by a persistently stored correction value (offset in minutes).
 *
 * @author klyubin@google.com (Alex Klyubin)
 */
public class TotpClock implements SharedPreferences.OnSharedPreferenceChangeListener {

    // @VisibleForTesting
    static final String PREFERENCE_KEY_OFFSET_MINUTES = "timeCorrectionMinutes";

    private final SharedPreferences mPreferences;

    private final Object mLock = new Object();

    /**
     * Cached value of time correction (in minutes) or {@code null} if not cached. The value is cached
     * because it's read very frequently (once every 100ms) and is modified very infrequently.
     *
     * @GuardedBy {@link #mLock}
     */
    private Integer mCachedCorrectionMinutes;

    public TotpClock(Context context) {
        mPreferences = PreferenceManager.getDefaultSharedPreferences(context);
        mPreferences.registerOnSharedPreferenceChangeListener(this);
    }

    /**
     * Gets the number of milliseconds since epoch.
     */
    public long currentTimeMillis() {
        return System.currentTimeMillis() + getTimeCorrectionMinutes() * Utilities.MINUTE_IN_MILLIS;
    }

    /**
     * Gets the currently used time correction value.
     *
     * @return number of minutes by which this device is behind the correct time.
     */
    public int getTimeCorrectionMinutes() {
        synchronized (mLock) {
            if (mCachedCorrectionMinutes == null) {
                try {
                    mCachedCorrectionMinutes = mPreferences.getInt(PREFERENCE_KEY_OFFSET_MINUTES, 0);
                } catch (ClassCastException e) {
                    mCachedCorrectionMinutes = Integer.valueOf(mPreferences.getString(PREFERENCE_KEY_OFFSET_MINUTES, "0"));
                }
            }
            return mCachedCorrectionMinutes;
        }
    }

    /**
     * Sets the currently used time correction value.
     *
     * @param minutes number of minutes by which this device is behind the correct time.
     */
    public void setTimeCorrectionMinutes(int minutes) {
        synchronized (mLock) {
            mPreferences.edit().putInt(PREFERENCE_KEY_OFFSET_MINUTES, minutes).commit();
            // Invalidate the cache to force reading actual settings from time to time
            mCachedCorrectionMinutes = null;
        }
    }

    public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
        if (key.equals(PREFERENCE_KEY_OFFSET_MINUTES)) {
            // Invalidate the cache
            mCachedCorrectionMinutes = null;
        }
    }
}