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/** * function used to migrate appspace data from one schema * to the next or previous. */export type MigrationFunction = () => Promise<void>;
/** * Describe a Migration step. */export type Migration = {	// TODO figure out how to document type alias object props	/**	 * Direction of migration. "up" means migrating to the next schema,	 * while "down" is the opposite.	 */	direction: "up"|"down",	/**	 * Schema is an integer representing the version of the data directory of the appspace.	 * - If direction is "up", schema is the version reached after the migration finishes.	 * - If the direction is down, schema is the version before the migration starts.	 * 	 * Put another way: for a given schema number, "down" reverses the changes made by "up".	 */	schema: number,	/**	 * The migration function.	 */	func: MigrationFunction}
/** * Migrations is the set of migrations that the app supports. */export type Migrations = Migration[];
/** * Class that helps create a Migrations object. */export class MigrationsBuilder {	migrations : Migrations = [];
	/**	 * Add an "up" migration.	 * @param schema (integer) schema version of the data directory	 * after the migration function has run.	 * @param func is the migration function.	 */	upTo(schema:number, func:MigrationFunction) :void {		schema = Math.round(schema);	// probably need more checks against schema.		this.migrations.push({			direction: "up",			schema,			func		});	}	/**	 * Add a "down" migration.	 * @param schema (integer) schema version of the data directory 	 * prior to running the migration function	 * @param func migration function	 */	downFrom(schema:number, func:MigrationFunction) :void {		schema = Math.round(schema);		this.migrations.push({			direction: "down",			schema,			func		});	}	// we'll need a check function to ensure migrations are consistent}