The favorite events feature in Open Event Android allows any user to favorite an event and those events can are available in the favorite events fragment which is easily accessible from the bottom navigation bar. This blog article will walk you through on how favorite works in Open Event Android. There are a couple of different ways to do it, in Open Event Android we are keeping track of favorite events using a favorite Boolean stored along with the information of the event ie we have added a favorite boolean field inside the event entity class. However, this method has its own pros and cons. The big plus point of this method is the fact that we can simply check the favorite field of the event object to check if that particular event is favorite or not. But since we have set onConflict strategy as replace in the event insert DAO method (shown below)
| @Insert(onConflict = REPLACE) fun insertEvents(events: List<Event>) |
This leads to a big problem when the same event is fetched or is present in the response while inserting the favorite field will be set to default value (false) and the favorite information would be lost. Hence extra care needs to be taken while updating events.
Given following is the event model class for serializing / deserializing JSON responses. Note the extra field favorite added here which helps us to provide user favorite feature locally.
| @Type(“event”) @JsonNaming(PropertyNamingStrategy.KebabCaseStrategy::class) @Entity data class Event( @Id(LongIdHandler::class) @PrimaryKey val id: Long, val name: String, val identifier: String, val isMapShown: Boolean = false, val favorite: Boolean = false ) |
Since we added a new field to the Event model class, to access the favorite events we will have to create DAO methods. To fetch all the favorite events we will have to create a Query method which returns all the events wherever favorite property is set. This can be done using a simple select statement which looks for events with favorite boolean set ie. 1, the method implementation is shown below.
| @Query(“SELECT * from Event WHERE favorite = 1”) fun getFavoriteEvents(): Flowable<List<Event>> |
To set an event favorite we will have to add one more method to the EventDao. We can create a generalized method which sets according to the boolean passed as a parameter. The function implementation is shown below, setFavorite takes in EventId, Id of the event which has to be updated and the boolean favorite which is what the new value for the favorite field is. setFavorite makes an SQL update query and matches for EventId and sets favorite for that event.
| @Query(“UPDATE Event SET favorite = :favorite WHERE id = :eventId”) fun setFavorite(eventId: Long, favorite: Boolean) |
In Open Event Android we use the service class which can be used to expose the DAO methods to the rest of the project. The idea behind creating service layer is the separation of concerns this service method is then called from the view model function (following the MVVM approach). Given following is the implementation of the get and set favorite methods.
| fun getFavoriteEvents(): Flowable<List<Event>> { return eventDao.getFavoriteEvents() } fun setFavorite(eventId: Long, favourite: Boolean): Completable { return Completable.fromAction { eventDao.setFavorite(eventId, favourite) } } |
The method getFavoriteEvents calls the DAO method to fetch the favorite evens and returns a flowable list of favorite events and setFavorite method generates a completable from the DAO action to favorite any event.
EventViewModel uses these methods and specifies where the observables should be observedOn and subscribedOn also defines what should happen when some error occurs. Given below is implementation for the two methods.
| fun setFavorite(eventId: Long, favourite: Boolean) { compositeDisposable.add(eventService.setFavorite(eventId, favourite) .subscribeOn(Schedulers.io()) .observeOn(AndroidSchedulers.mainThread()) .subscribe({ Timber.d(“Success”) }, { Timber.e(it, “Error”) error.value = “Error” })) } |
For every event, we have a floating action bar to toggle the favorite state of the same whenever user clicks on the FAB it checks the current state of the event and calls the view model function setFavorite passing the Event Id of the clicked event and the negation of current event.favorite state.
| val favouriteFabClickListener = object : FavoriteFabListener { override fun onClick(eventId: Long, isFavourite: Boolean) { eventsViewModel.setFavorite(eventId, !isFavourite) } } |
What we discussed in this blog post works well unless we are not inserting the events with the same event id, this will cause replace on the stored events and current favorite information will be lost. To overcome this limitation we follow the following procedure
- Fetch event id’s of all the events returned by the API response
- Use the DAO method to find out the id’s of favorite events out of these
- Set the favorite in the new events where IDs are in the ids returned in the second point
- Insert the events into the database
- Return the events from the database with ID’s as of point one