Mapping Events to Load from Database
In Open Event Android whenever App is started events are fetched for the location given by the user, since we have locally added isFavorite extra field for every event it is necessary to be updated for all the events returned in the API response before inserting it into our database otherwise all the favorite related information would be lost. This blog post will guide you on how its done in Open Event Android.
The sequence of steps followed
- Take the IDs of events being saved into the database
- Use DAO method which does “SELECT id from Event where favorite = 1 AND id in :eventIds and pass API returned eventIds to this function
- Set the old favorited on new event objects
- Save them in database
Let’s see how all of these steps are performed in greater details. Whenever user gives in a location following function is called
| fun getEventsByLocation(locationName: String): Single<List<Event>> { return eventApi.searchEvents(“name”, locationName).flatMap { apiList -> val eventIds = apiList.map { it.id }.toList() eventDao.getFavoriteEventWithinIds(eventIds).flatMap { favIds -> updateFavorites(apiList, favIds) } } } |
Here we are extracting all the Ids of events returned in the API response and then calling getFavoriteEventWithinIds on it. The latter takes the list of eventIds and return the Ids of events which are favorite out of them. This is then passed to the function updateFavorite along with the API returned Events. Following is the implementation of updateFavorite method.
| fun updateFavorites(apiEvents: List<Event>, favEventIds: List<Long>): Single<List<Event>> { apiEvents.map { if (favEventIds.contains(it.id)) it.favorite = true } eventDao.insertEvents(apiEvents) val eventIds = apiEvents.map { it.id }.toList() return eventDao.getEventWithIds(eventIds) } |
updateFavorite checks for all events in the list of events whether if its Id is present in the favorite event ids it sets favorite field of that event true. After the favorites for the list of events are updated they are inserted into the database using DAO method insertEvents. The last task is to take the fetch these events again from the database, to do this first we extract the Ids of the events we just inserted into the database and call the DAO method getEventsWithIds passing the extracted eventIds, getEventsWithids simply returns the Events with given Ids.
Given below are the implementations of the functions getEventWithIds and getFavoriteEventWithinIds
| @Query(“SELECT * from Event WHERE id in (:ids)”) fun getEventWithIds(ids: List<Long>): Single<List<Event>> @Query(“SELECT id from Event WHERE favorite = 1 AND id in (:ids)”) fun getFavoriteEventWithinIds(ids : List<Long>): Single<List<Long>> |
getEventWithIds simply makes a select query and checks for events whose ids lies in the ids passed to the method
getFavoriteEventWithinids returns the Ids of the favorite event out of the list of event id passed to the method.
Resources
- Android Developers Documetation for Room : https://developer.android.com/topic/libraries/architecture/room
- RxJava Documentation : https://github.com/ReactiveX/RxJava