The Basics

为了备份应用数据，你需要实现一个 backup agent. 它用来为BackupManager 提供你想要备份的数据，在你重新安装应用时恢复数据。BackupManager 通过backup transport 处理和云存储相关的数据业务，通过你的backup agent 处理设备上的数据相关的业务。

BackupAgent实现步骤：

在 manifest 文件中声明 android:backupAgent 属性。
通过backup服务注册你的应用。谷歌为大多数 Android 设备提供了 Android Backup Service，你需要通过这个服务注册你的应用。其他的云服务也需要注册。
定义你的 backup agent:
- 扩展 BackupAgent
  重写 onBackup() onRestore()
- 扩展 BackupAgentHelper
  需要使用多个helper对象自动完成备份及恢复。
  会备份整个文件。

Declaring the Backup Agent in Your Manifest

在application标签中声明android:backupAgent.

android:restoreAnyVersion
默认值为false, 设为true 则会在该应用的任何版本恢复数据。

Registering for Android Backup Service

不同的设备对 backup 的支持不同。

...

Extending BackupAgent

一般情况下，因为BackupAgentHelper比BackupAgent更方便，所以尽可能使用前者。
然而，如果有以下需求，还是要通过BackupAgent来实现:

对数据进行版本控制
并不想备份整个数据文件
备份数据库中的数据

如果你想要备份SharedPreferences和内部存储中的整个文件，简单地使用BackupAgentHelper即可。

Performing backup

oldState
上一次备份的状态，主要是FileDescriptor

// Get the oldState input stream
FileInputStream instream = new FileInputStream(oldState.getFileDescriptor());
DataInputStream in = new DataInputStream(instream);

try {
    // Get the last modified timestamp from the state file and data file
    long stateModified = in.readLong();
    long fileModified = mDataFile.lastModified();

    if (stateModified != fileModified) {
        // The file has been modified, so do a backup
        // Or the time on the device changed, so be safe and do a backup
    } else {
        // Don't back up because the file hasn't changed
        return;
    }
} catch (IOException e) {
    // Unable to read state file... be safe and do a backup
}

data
要备份的数据，通过byte字节流的方式存储

// Create buffer stream and data output stream for our data
ByteArrayOutputStream bufStream = new ByteArrayOutputStream();
DataOutputStream outWriter = new DataOutputStream(bufStream);
// Write structured data
outWriter.writeUTF(mPlayerName);
outWriter.writeInt(mPlayerScore);
// Send the data to the Backup Manager via the BackupDataOutput
byte[] buffer = bufStream.toByteArray();
int len = buffer.length;
data.writeEntityHeader(TOPSCORE_BACKUP_KEY, len);
data.writeEntityData(buffer, len);

newState
本次备份的状态,作为下次备份时的oldState

Performing restore

data

用来恢复数据的 `BackupDataInput`.

appVersionCode

android:versionCode

newState
恢复后，需要把data里的ParcelFileDescriptor 写到newState中，它会以下次onBackup的oldState被用到。

@Override
public void onRestore(BackupDataInput data, int appVersionCode,
                      ParcelFileDescriptor newState) throws IOException {
    // There should be only one entity, but the safest
    // way to consume it is using a while loop
    while (data.readNextHeader()) {
        String key = data.getKey();
        int dataSize = data.getDataSize();

        // If the key is ours (for saving top score). Note this key was used when
        // we wrote the backup entity header
        if (TOPSCORE_BACKUP_KEY.equals(key)) {
            // Create an input stream for the BackupDataInput
            byte[] dataBuf = new byte[dataSize];
            data.readEntityData(dataBuf, 0, dataSize);
            ByteArrayInputStream baStream = new ByteArrayInputStream(dataBuf);
            DataInputStream in = new DataInputStream(baStream);

            // Read the player name and score from the backup data
            mPlayerName = in.readUTF();
            mPlayerScore = in.readInt();

            // Record the score on the device (to a file or something)
            recordScore(mPlayerName, mPlayerScore);
        } else {
            // We don't know this entity key. Skip it. (Shouldn't happen.)
            data.skipEntityData();
        }
    }

    // Finally, write to the state blob (newState) that describes the restored data
    FileOutputStream outstream = new FileOutputStream(newState.getFileDescriptor());
    DataOutputStream out = new DataOutputStream(outstream);
    out.writeUTF(mPlayerName);
    out.writeInt(mPlayerScore);
}

Extending BackupAgentHelper

SharedPreferencesBackupHelper
FileBackupHelper

Backing up SharedPreferences

public class MyPrefsBackupAgent extends BackupAgentHelper {
    // The name of the SharedPreferences file
    static final String PREFS = "user_preferences";

    // A key to uniquely identify the set of backup data
    static final String PREFS_BACKUP_KEY = "prefs";

    // Allocate a helper and add it to the backup agent
    @Override
    public void onCreate() {
        SharedPreferencesBackupHelper helper =
                new SharedPreferencesBackupHelper(this, PREFS);
        addHelper(PREFS_BACKUP_KEY, helper);
    }
}

Note: SharedPreferences are threadsafe, so you can safely read and write the shared preferences file from your backup agent and other activities.

Backing up other files

public class MyFileBackupAgent extends BackupAgentHelper {
    // The name of the file
    static final String TOP_SCORES = "scores";
    static final String PLAYER_STATS = "stats";

    // A key to uniquely identify the set of backup data
    static final String FILES_BACKUP_KEY = "myfiles";

    // Allocate a helper and add it to the backup agent
    @Override
    public void onCreate() {
        FileBackupHelper helper = new FileBackupHelper(this,
                TOP_SCORES, PLAYER_STATS);
        addHelper(FILES_BACKUP_KEY, helper);
    }
}

非线程安全，需要手动加锁

// Object for intrinsic lock
static final Object sDataLock = new Object();
try {
    synchronized (MyActivity.sDataLock) {
        File dataFile = new File(getFilesDir(), TOP_SCORES);
        RandomAccessFile raFile = new RandomAccessFile(dataFile, "rw");
        raFile.writeInt(score);
    }
} catch (IOException e) {
    Log.e(TAG, "Unable to write to file");
}

@Override
public void onBackup(ParcelFileDescriptor oldState, BackupDataOutput data,
          ParcelFileDescriptor newState) throws IOException {
    // Hold the lock while the FileBackupHelper performs backup
    synchronized (MyActivity.sDataLock) {
        super.onBackup(oldState, data, newState);
    }
}

@Override
public void onRestore(BackupDataInput data, int appVersionCode,
        ParcelFileDescriptor newState) throws IOException {
    // Hold the lock while the FileBackupHelper restores the file
    synchronized (MyActivity.sDataLock) {
        super.onRestore(data, appVersionCode, newState);
    }
}

Checking the Restore Data Version

android:versionCode
BackupManager恢复数据前会检查versionCode字段，如果待恢复的版本高于当前应用版本，BackupManager不会调用onRestore()来恢复数据。
android:restoreAnyVersion
设定trueorfalse指明你是否希望通过应用的版本来决定是否恢复数据。设为true，那么BackupManager将不会去检查android:versionCode,而是调用你的onRestore()方法。
你可以通过下面的方法来获取appVersionCode.

PackageInfo info;
try {
    String name = getPackageName();
    info = getPackageManager().getPackageInfo(name,0);
} catch (NameNotFoundException nnfe) {
    info = null;
}

int version;
if (info != null) {
    version = info.versionCode;
}

Requesting Backup

使用dataChanged()来请求备份，这个不会立即执行，会在未来某个合适的时间内执行
开发时使用bmgr工具来随时备份

Requesting Restore

自动检测备份数据
requestRestore()
bmgr工具

Testing Your Backup Agent

安装应用
确定备份功能可用
- 使用模拟器
  adb shell bmgr enable true
- If using a device, open the system Settings, select Backup & reset, then enable Back up my data and Automatic restore.
在用户改变数据的地方调用dataChanged()
然后执行:adb shell bmgr backup your.package.name

‘

初始化备份操作

adb shell bmgr run
卸载应用

adb uninstall your.package.name
重新安装

Android Data Backup