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DataMigration

interface DataMigration<T>
androidx.datastore.core.DataMigration

Interface for migrations to DataStore. Methods on this migration (shouldMigrate, migrate and cleanUp) may be called multiple times, so their implementations must be idempotent. These methods may be called multiple times if DataStore encounters issues when writing the newly migrated data to disk or if any migration installed in the same DataStore throws an Exception.

If you're migrating from SharedPreferences see SharedPreferencesMigration.

Summary

Public methods
abstract suspend Unit

Clean up any old state/data that was migrated into the DataStore.

abstract suspend T
migrate(currentData: T)

Perform the migration.

abstract suspend Boolean
shouldMigrate(currentData: T)

Return whether this migration needs to be performed.

Public methods

cleanUp

abstract suspend fun cleanUp(): Unit

Clean up any old state/data that was migrated into the DataStore. This will not be called if the migration fails. If cleanUp throws an exception, the exception will be propagated back to the DataStore call that triggered the migration and future calls to DataStore will result in DataMigrations being attempted again. This method may be run multiple times when any failure is encountered.

This is useful for cleaning up files or data outside of DataStore and accessing any data from DataStore directly from inside this function will result in deadlock, since DataStore doesn't return data until all migrations complete.

migrate

abstract suspend fun migrate(currentData: T): T

Perform the migration. Implementations should be idempotent since this may be called multiple times. If migrate fails, DataStore will not commit any data to disk, cleanUp will not be called, and the exception will be propagated back to the DataStore call that triggered the migration. Future calls to DataStore will result in DataMigrations being attempted again. This method may be run multiple times when any failure is encountered.

Note that this will always be called before a call to cleanUp.

Note that accessing any data from DataStore directly from inside this function will result in deadlock, since DataStore doesn't return data until all migrations complete.

Parameters
currentData: T the current data (it might be populated from other migrations or from manual changes before this migration was added to the app)
Return
The migrated data.

shouldMigrate

abstract suspend fun shouldMigrate(currentData: T): Boolean

Return whether this migration needs to be performed. If this returns false, no migration or cleanup will occur. Apps should do the cheapest possible check to determine if this migration should run, since this will be called every time the DataStore is initialized. This method may be run multiple times when any failure is encountered.

Note that this will always be called before each call to migrate.

Note that accessing any data from DataStore directly from inside this function will result in deadlock, since DataStore doesn't return data until all migrations complete.

Parameters
currentData: T the current data (which might already populated from previous runs of this or other migrations)