​
Using transactions to group changes

Read and write transactions present a context where multiple changes can be made then finally committed to the DB or rolled back. This ensures that either all the changes get persisted, or no change is made to the DB (in the case of a rollback or exception).

PowerSyncDatabase.writeTransaction(callback) automatically commits changes after the transaction callback is completed if tx.rollback() has not explicitly been called. If an exception is thrown in the callback then changes are automatically rolled back.

// ListsWidget.jsx
import {Alert, Button, FlatList, Text, View} from 'react-native';

export const ListsWidget = () => {
  // Populate lists with one of methods listed above
  const [lists, setLists] = React.useState([]);

  return (
    <View>
      <FlatList
        data={lists.map(list => ({key: list.id, ...list}))}
        renderItem={({item}) => (<View>
          <Text>{item.name}</Text>
           <Button
              title="Delete"
              onPress={async () => {
                  try {
                    await PowerSync.writeTransaction(async (tx) => {
                        // Delete the main list
                        await tx.execute(`DELETE FROM lists WHERE id = ?`, [item.id]);
                        // Delete any children of the list
                        await tx.execute(`DELETE FROM todos WHERE list_id = ?`, [item.id]);

                        // Transactions are automatically committed at the end of execution
                        // Transactions are automatically rolled back if an exception occurred
                      })
                    // Watched queries should automatically reload after mutation
                  } catch (ex) {
                    Alert.alert('Error', ex.message)
                  }
                }}
            />
        </View>)}
      />
      <Button
        title="Create List"
        color="#841584"
        onPress={async () => {
            try {
              await PowerSync.execute('INSERT INTO lists (id, created_at, name, owner_id) VALUES (uuid(), datetime(), ?, ?) RETURNING *', [
                'A list name',
                "[The user's uuid]"
              ])
              // Watched queries should automatically reload after mutation
            } catch (ex) {
              Alert.alert('Error', ex.message)
            }
          }}
      />
    </View>
    )
}

Also see PowerSyncDatabase.readTransaction(callback).

​
Subscribe to changes in data

Use PowerSyncDatabase.watch to watch for changes in source tables.

The watch method can be used with a AsyncIterable signature as follows:

async *attachmentIds(): AsyncIterable<string[]> {
  for await (const result of this.powersync.watch(
    `SELECT photo_id as id FROM ${TODO_TABLE} WHERE photo_id IS NOT NULL`,
    []
  )) {
    yield result.rows?._array.map((r) => r.id) ?? [];
  }
}

As of version 1.3.3 of the SDK, the watch method can also be used with a callback:

attachmentIds(onResult: (ids: string[]) => void): void {
  this.powersync.watch(
    `SELECT photo_id as id FROM ${TODO_TABLE} WHERE photo_id IS NOT NULL`,
    [],
    {
      onResult: (result) => {
        onResult(result.rows?._array.map((r) => r.id) ?? []);
      }
    }
  );
}

​
Insert, update, and delete data in the local database

Use PowerSyncDatabase.execute to run INSERT, UPDATE or DELETE queries.

const handleButtonClick = async () => {
  await db.execute(
    'INSERT INTO customers(id, name, email) VALUES(uuid(), ?, ?)',
    ['Fred', 'fred@example.org']
  );
};

return (
  <button onClick={handleButtonClick} title="+">
    <span>+</span>
    <i className="material-icons">add</i>
  </button>
);

​
Send changes in local data to your backend service

Override uploadData to send local updates to your backend service.

// Implement the uploadData method in your backend connector
async function uploadData(database) {
  const batch = await database.getCrudBatch();
  if (batch === null) return;

  for (const op of batch.crud) {
    switch (op.op) {
      case 'put':
        // Send the data to your backend service
        // replace `_myApi` with your own API client or service
        await _myApi.put(op.table, op.opData);
        break;
      default:
        // TODO: implement the other operations (patch, delete)
        break;
    }
  }

  await batch.complete();
}

​
Accessing PowerSync connection status information

Use PowerSyncDatabase.connected and register an event listener with PowerSyncDatabase.registerListener to listen for status changes to your PowerSync instance.

// Example of using connected status to show online or offline

// Tap into connected
const [connected, setConnected] = React.useState(powersync.connected);

React.useEffect(() => {
// Register listener for changes made to the powersync status
  return powersync.registerListener({
    statusChanged: (status) => {
      setConnected(status.connected);
    }
  });
}, [powersync]);

// Icon to show connected or not connected to powersync
// as well as the last synced time
<Icon
  name={connected ? 'wifi' : 'wifi-off'}
  type="material-community"
  color="black"
  size={20}
  style={{ padding: 5 }}
  onPress={() => {
    Alert.alert(
      'Status',
      `${connected ? 'Connected' : 'Disconnected'}. \nLast Synced at ${powersync.currentStatus?.lastSyncedAt.toISOString() ?? '-'
      }\nVersion: ${powersync.sdkVersion}`
    );
  }}
/>;

​
Wait for the initial sync to complete

Use the hasSynced property (available since version 1.4.1 of the SDK) and register an event listener with PowerSyncDatabase.registerListener to indicate to the user whether the initial sync is in progress.

// Example of using hasSynced to show whether the first sync has completed

// Tap into hasSynced
const [hasSynced, setHasSynced] = React.useState(powerSync.currentStatus?.hasSynced || false);

  React.useEffect(() => {
    // Register listener for changes made to the powersync status
    return powerSync.registerListener({
      statusChanged: (status) => {
        setHasSynced(!!status.hasSynced);
      }
    });
  }, [powerSync]);

return <Text>{hasSynced ? 'Initial sync completed!' : 'Busy with initial sync...'}</Text>;

For async use cases, see PowerSyncDatabase.waitForFirstSync, which returns a promise that resolves once the first full sync has completed (it queries the internal SQL ps_buckets table to determine if data has been synced).

​
Using logging to troubleshoot issues

You can enable logging to see what’s happening under the hood or to debug connection/authentication/sync issues. This SDK uses js-logger.

Enable JS Logger with your logging interface of choice or use the default console.

import Logger from 'js-logger';

// Log messages will be written to the window's console.
Logger.useDefaults();

Logger.setLevel(Logger.DEBUG);

​
Using Hooks

A separate powersync-react package is available containing React hooks for PowerSync:

npm: @powersync/react

See its README for example code.

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