Live Query — Working with Queries
Description — Couchbase Lite database data querying concepts — live queries
Related Content — Predictive Queries | Indexing | QueryBuilder
Activating a Live Query
A live query is a query that, once activated, remains active and monitors the database for changes; refreshing the result set whenever a change occurs. As such, it is a great way to build reactive user interfaces — especially table/list views — that keep themselves up to date.
So, a simple use case may be: A replicator running and pulling new data from a server, whilst a live-query-driven UI automatically updates to show the data without the user having to manually refresh. This helps your app feel quick and responsive.
To activate a LiveQuery just add a change listener to the query statement. It will be immediately active. When a change is detected the query automatically runs, and posts the new query result to any observers (change listeners).
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Kotlin
-
Java
val query = QueryBuilder
.select(SelectResult.all())
.from(DataSource.database(database)) (1)
// Adds a query change listener.
// Changes will be posted on the main queue.
val token = query.addChangeListener { change ->
change.results?.let {
for (result in it) {
Log.d(TAG, "results: ${result.keys}")
/* Update UI */
}
} (2)
}Query query = QueryBuilder
.select(SelectResult.all())
.from(DataSource.database(database)); (1)
// Adds a query change listener.
// Changes will be posted on the main queue.
ListenerToken token = query.addChangeListener(change -> { (2)
for (Result result : change.getResults()) {
Log.d(TAG, "results: " + result.getKeys());
/* Update UI */
}
});| 1 | Build the query statements |
| 2 | Activate the live query by attaching a listener. Save the token in order to detach the listener and stop the query later — se Example 2 |
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Kotlin
-
Java
query.removeChangeListener(token)query.removeChangeListener(token); (1)| 1 | Here we use the change lister token from Example 1 to remove the listener. Doing so stops the live query. |
Using Kotlin Flows and LiveData
Kotlin developers also have the option of using Flows and Live Data to feed query changes to the UI.
Define a live query as a Flow returning a LiveData object and activate an Observer in the View onCreate() function.
var liveQuery: LiveData<List<Any>?>? = null
@ExperimentalCoroutinesApi
fun watchQuery(query: Query): LiveData<List<Any>?> {
val queryFlow = query.queryChangeFlow()
.map {
val err = it.error
if (err != null) {
throw err
}
it.results?.allResults()?.flatMap { it.toList() }
}
.asLiveData()
liveQuery = queryFlow
return queryFlow
}