React to Changes - Flutter SDK
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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.
You can subscribe to changes on the following events:
Example
About the Examples on This Page
The examples in this page use two Realm object types, Character
and
Fellowship
:
()class _Character { () late ObjectId id; late String name; late String species; late int age; } ()class _Fellowship { () late ObjectId id; late String name; late List<_Character> members; }
The examples have this sample data:
final frodo = Character(ObjectId(), 'Frodo', 'Hobbit', 51); final samwise = Character(ObjectId(), 'Samwise', 'Hobbit', 39); final gollum = Character(ObjectId(), 'Gollum', 'Hobbit', 589); final aragorn = Character(ObjectId(), 'Aragorn', 'Human', 87); final legolas = Character(ObjectId(), 'Legolas', 'Elf', 2931); final gimli = Character(ObjectId(), 'Gimli', 'Dwarf', 140); final fellowshipOfTheRing = Fellowship( ObjectId(), 'Fellowship of the Ring', members: [frodo, samwise, aragorn, legolas, gimli]); final config = Configuration.local([Fellowship.schema, Character.schema]); final realm = Realm(config); realm.write(() { realm.add(fellowshipOfTheRing); realm.add(gollum); // not in fellowship });
Register a Query Change Listener
You can register a notification handler on any query within a Realm.
The handler receives a RealmResultsChanges object,
which includes description of changes since the last notification.
RealmResultsChanges
contains the following properties:
Property | Type | Description |
inserted | List<int> | Indexes in the new collection which were added in this version. |
modified | List<int> | Indexes of the objects in the new collection which were modified in this version. |
deleted | List<int> | Indexes in the previous version of the collection which have been removed from this one. |
newModified | List<int> | Indexes of modified objects after deletions and insertions are accounted for. |
moved | List<int> | Indexes of the objects in the collection which moved. |
results | RealmResults<T as RealmObject> | Results collection being monitored for changes. |
isCleared | bool | Deprecated in Realm Flutter SDK v1.1.0. Use RealmResultsChanges.results.isEmpty instead.
Returns true if the results collection is empty in the notification callback. |
// Listen for changes on whole collection final characters = realm.all<Character>(); final subscription = characters.changes.listen((changes) { changes.inserted; // indexes of inserted objects changes.modified; // indexes of modified objects changes.deleted; // indexes of deleted objects changes.newModified; // indexes of modified objects // after deletions and insertions are accounted for changes.moved; // indexes of moved objects changes.results; // the full List of objects }); // Listen for changes on RealmResults final hobbits = fellowshipOfTheRing.members.query('species == "Hobbit"'); final hobbitsSubscription = hobbits.changes.listen((changes) { // ... all the same data as above });
Register a RealmObject Change Listener
You can register a notification handler on a specific object within a realm.
Realm notifies your handler when any of the object's properties change.
The handler receives a RealmObjectChanges object,
which includes description of changes since the last notification.
RealmObjectChanges
contains the following properties:
Property | Type | Description |
isDeleted | bool | true if the object was deleted. |
object | RealmObject | Realm object being monitored for changes. |
properties | List<String> | Names of the Realm object's properties that have changed. |
final frodoSubscription = frodo.changes.listen((changes) { changes.isDeleted; // if the object has been deleted changes.object; // the RealmObject being listened to, `frodo` changes.properties; // the changed properties });
Register RealmList and RealmSet Change Listeners
You can register a notification handler on a list or set of any of the
supported data types within another RealmObject
.
Realm notifies your handler when any of the items in the set or list change.
The handler receives a RealmListChanges object
for RealmList
and RealmSetChanges object
for RealmSet
.
These objects include description of changes since the last notification.
RealmListChanges
contains the following properties:
Property | Type | Description |
inserted | List<int> | Indexes of items in the new list which were added in this version. |
modified | List<int> | Indexes of the items in the new list which were modified in this version. |
deleted | List<int> | Indexes of items in the previous version of the list which have been removed from this one. |
newModified | List<int> | Indexes of modified items after deletions and insertions are accounted for. |
moved | List<int> | Indexes of the items in the list which moved. |
list / set | RealmList<T> / RealmSet<T> | RealmList being monitored for changes if listening to a RealmList .
RealmSet being monitored for changes if listening to a RealmSet . |
isCleared | boolean | true when a collection has been cleared by calling its
RealmList.clear() or
RealmSet.clear() method. |
final fellowshipSubscription = fellowshipOfTheRing.members.changes.listen((changes) { changes.inserted; // indexes of inserted Realm objects changes.modified; // indexes of modified Realm objects changes.deleted; // indexes of deleted Realm objects changes.newModified; // indexes of modified Realm objects // after deletions and insertions are accounted for changes.moved; // indexes of moved Realm objects changes.list; // the full RealmList of Realm objects // `true` after call to fellowshipOfTheRing.members.clear(). // Otherwise false. changes.isCleared; });
Pause and Resume a Change Listener
Pause your subscription if you temporarily don't want to receive notifications. You can later resume listening.
subscription.pause(); // the changes.listen() method won't fire until the subscription is resumed subscription.resume();
Unsubscribe a Change Listener
Unsubscribe from your change listener when you no longer want to receive notifications on updates to the data it's watching.
await subscription.cancel();