Backwards-compatible BackupAgent

Question

I am looking into using the new Backup API that available since Android 2.2, but need to maintain backwards compatibility (to 1.5 to be exact).

The docs state:

The backup service and the APIs you must use are available only on devices running API Level 8 (Android 2.2) or greater, so you should also set your android:minSdkVersion attribute to "8". However, if you implement proper backward compatibility in your application, you can support this feature for devices running API Level 8 or greater, while remaining compatible with older devices.

I indeed build against the level 8 targetSdkVersion with level 3 minSdkVersion and try to use a wrapper class (with reflection) to overcome the problem that the application will not run if you implement a class that extends an nonexisting class.

Here is the problem: since we don't make actual calls to the BackupHelper class ourselves, we can't check upfront if the class indeed exists. (As is explained in the Android Backwards Compatibility documentation with a checkAvailable() method.) The class will therefore be instantiated and cast to a BackupAgent. But since we use reflection, it doesn't actually override BackupAgent and an exception occurs at runtime when the backup is requested:

java.lang.RuntimeException: Unable to create BackupAgent org.transdroid.service.BackupAgent: java.lang.ClassCastException: org.transdroid.service.BackupAgent

Here is my approach to a backwards compatible BackupAgent: http://code.google.com/p/transdroid/source/browse/#svn/trunk/src/org/transdroid/service where the BackupAgent.java is the 'regular' BackupAgentHelper-extending class and BackupAgentHelperWrapper is the reflection-based wrapper class.

Anyone successfull in implementing a BackupAgent with backwards compatibility?

Answer 1

You need to set the minSDK version to the following:

<uses-sdk android:minSdkVersion="3" android:targetSdkVersion="8"/>

and setting the build target to sdk 8 (project properties in eclipse '.default.properties'):

# Project target.
target=android-8

Now to call new stuff added in SDK 8 you have to use reflection: http://developer.android.com/resources/articles/backward-compatibility.html

Answer 2

I ran into the same problem and here's what I did to work it out.

You don't extend BackupAgent with the wrapper, you extend it with the wrapped class. So you make your real backup class:

public class MyBackup extends BackupAgent {

@Override
public void onBackup(ParcelFileDescriptor oldState, BackupDataOutput data,
        ParcelFileDescriptor newState) throws IOException {
    // TODO Auto-generated method stub

}

@Override
public void onRestore(BackupDataInput data, int appVersionCode,
        ParcelFileDescriptor newState) throws IOException {
    // TODO Auto-generated method stub

}

Okay, and then you make a wrapper like the android developer backwards compatibility article said to do. Note that this class does not extend BackupAgent:

public class WrapMyBackup {
private MyBackup wb;

static {
    try {
        Class.forName("MyBackup");
    }
    catch (Exception ex) {
        throw new RuntimeException(ex);
    }
}

/** call this wrapped in a try/catch to see if we can instantiate **/
public static void checkAvailable() {}

public WrapMyBackup() {
    wb = new MyBackup();
}

public void onBackup(ParcelFileDescriptor oldState, BackupDataOutput data,
        ParcelFileDescriptor newState) throws IOException {
    wb.onBackup(oldState, data, newState);

}

public void onRestore(BackupDataInput data, int appVersionCode,
        ParcelFileDescriptor newState) throws IOException {
    wb.onRestore(data, appVersionCode, newState);

}

public void onCreate() {
    wb.onCreate();
}

public void onDestroy() {
    wb.onDestroy();
}

}

Finally, in your manifest, you declare the wrapper as your backup agent:

    <application 
    android:label="@string/app_name"
    android:icon="@drawable/ic_launch_scale"
    android:backupAgent="WrapMyBackup"
    >

Since your wrapper has the proper methods defined you won't run into a problem when the backup manager casts it to a BackupAgent. Since lower API levels won't have a BackupManager the code will never get called, so you won't run into any runtime exceptions there either.

Answer 3

I don't see why you run into this problem.

I have the same issue: I want to support backup with a app that supports also 1.5 (API 3).

There is no problem in creating my BackupAgentHelper class, since that class is never called from my own code, but from the BackupManager i.e. the system itself. Therefore I don't need to wrap it, and I don't see why you should be doing that:

 public class MyBackupAgentHelper extends BackupAgentHelper {
 @override onCreate()
 { 
       \\do something usefull
 }

However, you do want to get a backup running, to do that you need to call on BackupManager.dataChanged() whenever your data changes and you want to inform the system to backup it (using your BackupAgent or BackupAgentHelper).

You do need to wrap that class, since you call it from you application code.

public class WrapBackupManager {
private BackupManager wrappedInstance;

static 
{
    try
    {
        Class.forName("android.app.backup.BackupManager");
    }
    catch (Exception e)
    {
        throw new RuntimeException(e);
    }
}
public static void checkAvailable() {}

public void dataChanged()
{
    wrappedInstance.dataChanged();
}

public WrapBackupManager(Context context)
{
    wrappedInstance = new BackupManager(context);
}

}

You then call it from your code when you change a preference or save some data. Some code from my app:

private static Boolean backupManagerAvailable = null;

    private static void postCommitAction() {


        if (backupManagerAvailable == null) {
            try {
                WrapBackupManager.checkAvailable();
                backupManagerAvailable = true;
            } catch (Throwable t) {
                backupManagerAvailable = false;
            }
        }

        if (backupManagerAvailable == true) {
            Log.d("Fretter", "Backup Manager available, using it now.");
            WrapBackupManager wrapBackupManager = new WrapBackupManager(
                    FretterApplication.getApplication());
            wrapBackupManager.dataChanged();
        } else {
            Log.d("Fretter", "Backup Manager not available, not using it now.");
        }

So, hopefully this works for you!

(If you call adb shell bmgr run every time you want to emulate the actual system initiated backupprocess it should properly backup and restore when you reinstall the app.)