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React to Changes - Java SDK

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  • Auto-Refresh
  • Register a Realm Change Listener
  • Register a Collection Change Listener
  • Register an Object Change Listener
  • Unregister a Change Listener
  • Use Realm in System Apps on Custom ROMs
  • Change Notification Limits

Objects in Realm clients are live objects that update automatically to reflect data changes, including synced remote changes, and emit notification events that you can subscribe to whenever their underlying data changes.

Any modern app should be able to react when data changes, regardless of where that change originated. When a user adds a new item to a list, you may want to update the UI, show a notification, or log a message. When someone updates that item, you may want to change its visual state or fire off a network request. Finally, when someone deletes the item, you probably want to remove it from the UI. Realm's notification system allows you to watch for and react to changes in your data, independent of the writes that caused the changes.

Realm emits three kinds of notifications:

  • Realm notifications whenever a specific realm commits a write transaction.

  • Collection notifications whenever any Realm object in a collection changes, including inserts, updates, and deletes.

  • Object notifications whenever a specific Realm object changes, including updates and deletes.

Auto-Refresh

Realm objects accessed on a thread associated with a Looper automatically update periodically to reflect changes to underlying data.

The Android UI thread always contains a Looper instance. If you need to keep Realm objects around for long periods of time on any other thread, you should configure a Looper for that thread.

Warning

Always Close Realm Instances on Non-Looper Threads to Avoid Resource Leaks

Realms on a thread without a Looper do not automatically advance their version. This can increase the size of the realm in memory and on disk. Avoid using realm instances on non-Looper threads when possible. If you do open a realm on a non-Looper thread, close the realm when you're done using it.

Register a Realm Change Listener

You can register a notification handler on an entire realm. Realm calls the notification handler whenever any write transaction involving that realm is committed. The handler receives no information about the change.

This is useful when you want to know that there has been a change but do not care to know specifically what changed. For example, proof of concept apps often use this notification type and simply refresh the entire UI when anything changes. As the app becomes more sophisticated and performance-sensitive, the app developers shift to more granular notifications.

Example

Suppose you are writing a real-time collaborative app. To give the sense that your app is buzzing with collaborative activity, you want to have an indicator that lights up when any change is made. In that case, a realm notification handler would be a great way to drive the code that controls the indicator. The following code shows how to observe a realm for changes with with addChangeListener():

Important

Automatic Refresh

All threads that contain a Looper automatically refresh RealmObject and RealmResult instances when new changes are written to the realm. As a result, it isn't necessary to fetch those objects again when reacting to a RealmChangeListener, since those objects are already updated and ready to be redrawn to the screen.

Register a Collection Change Listener

You can register a notification handler on a specific collection within a realm. The handler receives a description of changes since the last notification. Specifically, this description consists of three lists of indices:

  • The indices of the objects that were deleted.

  • The indices of the objects that were inserted.

  • The indices of the objects that were modified.

Stop notification delivery by calling the removeChangeListener() or removeAllChangeListeners() methods. Notifications also stop if:

  • the object on which the listener is registered gets garbage collected.

  • the realm instance closes.

Keep a strong reference to the object you're listening to for as long as you need the notifications.

Important

Order Matters

In collection notification handlers, always apply changes in the following order: deletions, insertions, then modifications. Handling insertions before deletions may result in unexpected behavior.

Realm emits an initial notification after retrieving the collection. After that, Realm delivers collection notifications asynchronously whenever a write transaction adds, changes, or removes objects in the collection.

Unlike realm notifications, collection notifications contain detailed information about the change. This enables sophisticated and selective reactions to changes. Collection notifications provide all the information needed to manage a list or other view that represents the collection in the UI.

The following code shows how to observe a collection for changes with addChangeListener():

Register an Object Change Listener

You can register a notification handler on a specific object within a realm. Realm notifies your handler:

  • When the object is deleted.

  • When any of the object's properties change.

The handler receives information about what fields changed and whether the object was deleted.

Stop notification delivery by calling the removeChangeListener() or removeAllChangeListeners() methods. Notifications also stop if:

  • the object on which the listener is registered gets garbage collected.

  • the realm instance closes.

Keep a strong reference of the object you're listening to for as long as you need the notifications.

The following code shows how create a new instance of a class in a realm and observe that instance for changes with addChangeListener():

Unregister a Change Listener

You can unregister a change listener by passing your change listener to Realm.removeChangeListener(). You can unregister all change listeners currently subscribed to changes in a realm or any of its linked objects or collections with Realm.removeAllChangeListeners().

Use Realm in System Apps on Custom ROMs

Realm uses named pipes in order to support notifications and access to the realm file from multiple processes. While this is allowed by default for normal user apps, it is disallowed for system apps.

You can define a system apps by setting android:sharedUserId="android.uid.system" in the Android manifest. When working with a system app, you may see a security violation in Logcat that looks something like this:

05-24 14:08:08.984  6921  6921 W .realmsystemapp: type=1400 audit(0.0:99): avc: denied { write } for name="realm.testapp.com.realmsystemapp-Bfqpnjj4mUvxWtfMcOXBCA==" dev="vdc" ino=14660 scontext=u:r:system_app:s0 tcontext=u:object_r:apk_data_file:s0 tclass=dir permissive=0
05-24 14:08:08.984  6921  6921 W .realmsystemapp: type=1400 audit(0.0:100): avc: denied { write } for name="realm.testapp.com.realmsystemapp-Bfqpnjj4mUvxWtfMcOXBCA==" dev="vdc" ino=14660 scontext=u:r:system_app:s0 tcontext=u:object_r:apk_data_file:s0 tclass=dir permissive=0

In order to fix this you need to adjust the SELinux security rules in the ROM. This can be done by using the tool audit2allow, which ships as part of AOSP:

  1. Pull the current policy from the device:

    adb pull /sys/fs/selinux/policy

  2. Copy the SELinux error inside a text file called input.txt.

  3. Run the audit2allow tool:

    audit2allow -p policy -i input.txt

  4. The tool should output a rule you can add to your existing policy to enable the use of Realm.

An example of such a policy is provided below:

# Allow system_app to create named pipes required by Realm
# Credit: https://github.com/mikalackis/platform_vendor_ariel/blob/master_oreo/sepolicy/system_app.te
allow system_app fuse:fifo_file create;
allow system_app system_app_data_file:fifo_file create;
allow system_app system_app_data_file:fifo_file { read write };
allow system_app system_app_data_file:fifo_file open;

Tip

See also: audit2allow

audit2allow is produced when compiling AOSP/ROM and only runs on Linux. You can read more about it here.

Note

Changes in Android Oreo and Above

Since Android Oreo, Google changed the way it configures SELinux. The default security policies are now much more modularized. Read more about that here.

Change Notification Limits

Changes in nested documents deeper than four levels down do not trigger change notifications.

If you have a data structure where you need to listen for changes five levels down or deeper, workarounds include:

  • Refactor the schema to reduce nesting.

  • Add something like "push-to-refresh" to enable users to manually refresh data.

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