Live Queries

Description — Couchbase Lite JavaScript — Reactive Live Queries
Related Content — SQL++ for Mobile | Query Resultsets | Indexing

Overview

Live queries provide real-time updates when the results of a query change. This makes them perfect for building reactive user interfaces that automatically update when data changes.

When you create a live query, Couchbase Lite monitors the database and notifies you whenever documents that match the query are added, modified, or deleted.

Creating Live Queries

Create a live query by adding a change listener to your query:

Example 1. Basic live query

const query = database.createQuery(`
    SELECT title FROM tasks
    WHERE completed = false
    ORDER BY createdAt DESC
`);

// Add change listener - receives array of results
const token = query.addChangeListener((results) => {
    console.log(`Query results updated: ${results.length} tasks`);

    // Update UI with new results
    results.forEach(row => {
        console.log(`Task: ${row.title}`);
    });

    // Update your application UI
    updateTaskList(results);
});

// The listener is now active and will be called whenever results change

Removing Listeners

Always remove listeners when they’re no longer needed:

Example 2. Remove listener

const query = database.createQuery('SELECT * FROM tasks WHERE completed = false');
const token = query.addChangeListener(_ => {
    console.log('Results updated');
});

// Later, remove the listener
token.remove();
console.log('Listener removed');

Multiple Listeners

You can add multiple listeners to the same query:

Example 3. Multiple listeners

const query = database.createQuery('SELECT * FROM tasks WHERE priority >= 3');

// Listener 1: Update UI
const uiToken = query.addChangeListener(results => {
    updateTaskList(results);
});

// Listener 2: Log changes
const logToken = query.addChangeListener(results => {
    console.log(`High priority tasks: ${results.length}`);
});

// Listener 3: Send analytics
const analyticsToken = query.addChangeListener(results => {
    trackMetric('high_priority_tasks', results.length);
});

// Each listener receives independent notifications
// Remove listeners individually when done
uiToken.remove();
logToken.remove();
analyticsToken.remove();

Each listener receives independent notifications.

TypeScript Support

Type your live query results for type safety:

Example 4. Typed live queries

interface TaskResult {
    title: string;
    completed: boolean;
    priority: number;
    createdAt: string;
}

const query = database.createQuery(`
    SELECT title, completed, priority, createdAt
    FROM tasks
    WHERE completed = false
`);

// Add typed listener
const token = query.addChangeListener<TaskResult>((results) => {
    results.forEach(t => {
        const task = t as TaskResult;
        const title: string = task.title;  // Type-safe
        const priority: number = task.priority;

        console.log(`${title} - Priority: ${priority}`);
    });
});

Error Handling

Handle errors in live query listeners:

Example 5. Error handling

const query = database.createQuery('SELECT * FROM tasks');

query.addChangeListener(results => {
    try {
        // Process results
        updateUI(results);
    } catch (error) {
        console.error('Error updating UI:', error);
        // Handle error without crashing the listener
    }
});

// Wrap listener registration in try-catch for setup errors
try {
    query.addChangeListener(results => {
        processResults(results);
    });
} catch (error) {
    console.error('Failed to create live query listener:', error);
}

Live Query Lifecycle

Example 6. Lifecycle stages

class TaskManager {
    private query: Query | null = null;
    private database: Database | null = null;
    private listenerToken: ListenerToken | null = null;

    async start() {
        this.database = await Database.open({name: "mydb", version: 1, collections: { tasks: {} }});

        // Create query
        this.query = this.database.createQuery(`
            SELECT * FROM tasks
            WHERE assignedTo = $userId
            AND completed = false
        `);

        // Set parameters
        this.query.parameters = { userId: 'current-user' };

        // Start listening
        this.listenerToken = this.query.addChangeListener(results => {
            this.onResultsChanged(results);
        });

        console.log('Live query started');
    }

    onResultsChanged(results: QueryAliases[]) {
        console.log(`Tasks updated: ${results.length}`);
        this.updateUI(results);
    }

    updateParameters(userId: string) {
        // Changing parameters triggers listener with new results
        if (this.query) {
            this.query.parameters = { userId };
        }
    }

    stop() {
        // Clean up listener
        if (this.listenerToken) {
            this.listenerToken.remove();
            this.listenerToken = null;
            console.log('Live query stopped');
        }
    }

    private updateUI(_: QueryAliases[]) {
        // Update application UI
    }
}

// Usage
const manager = new TaskManager();
manager.start();  // Start listening

// Later: change parameters (triggers update)
manager.updateParameters('different-user');

// Clean up when component unmounts
manager.stop();

Limitations

Database Changes Only:

Live queries only respond to local database changes
They don’t poll external data sources
Replication changes trigger live query updates

Query Constraints:

All SQL++ query limitations apply (see query-n1ql-mobile.adoc#limitations)
Complex queries may have performance implications
Index availability affects live query performance

Browser Specific:

Live queries depend on browser staying active
Background tabs may throttle updates
Consider using Service Workers for background updates

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