This is an Android mobile app built in MVVM pattern supporting swipe gestures to like or dislike news. Within the app, users can:
- Swipe right to like (and save) news articles, swipe left to dislike
- View all saved articles in snippets
- Expand each article to see full details
- Search news articles by keywords
Three fragments are created to achieve the three main use cases. The HomeFragment implements SwipePlaceHolderView and presents a stack of newscards on top responsive to the swpie gesture. Both SaveFragment and SearchFragment implement RecyclerView and present a scrolling list of images or news snippets.
Within the RecyclerView, the Adapter tells the Recycler when to create view and provides a ViewHolder to wrap the view for reuse, and the LayoutManager positions the views in linear or grid format.
HomeViewModel,SearchViewModel and SaveViewModel are created corresponding to each of the fragments. The ViewModel classes are responsible for preparing data for the UI in a lifecycle conscious way.
NewsViewModelFactory manages the creation of ViewModels and handles their lifecycle behind the scene, such that when an activity or a fragment is re-created, it receives the same ViewModel instance that was created before. When the owner activity is finished, the instance is then cleared, freeing up resources.
NewsRepository specifies methods for both network and database operations. It responds to ViewModel calls to either get news articles from remote source or CRUD data objects.
RoomDB provides an abstraction layer over SQLite to persist data locally. Room caches relevant pieces of data even when the device cannot access the network. Any user-initiated content changes are then synced to the server after the device is back online.
RetrofitClient is configured to handle network requests. Retrofit significantly streamlines Android workflow for network requests by turning the HTTP API into a Java interface. Through simple annotations, Retrofit is able to generate the actual implementations for the specified requests.
The app is built with Android recommended architectural pattern MVVM (which stands for Model-View-ViewModel) that champions the separation of concerns. The View interacts with users' actions. The ViewModelexposes streams of events to which the View can bind to, and retrieves data for the View to consume. And the Model stores POJO ("plain old Java object") that abstracts the data source.
In this app, when HomeFragment is first created, it instantiates the HomeViewModel instance, which then instructs the NewsRepository to fetch news from the remote data source. The HomeViewModel also observes the swipe event binding to the view, and tells the NewsRepository to save liked articles in whenever the user swipes right.
Similarly, in SaveFragment and SearchFragment, respective ViewModels are created to call the repository for data operations and observe data changes from events.
The LiveData structure is incorporated in ViewModels. LiveData is an observable data holder class, meaning that any changes in the data will notify the observer, and in this case, the fragments.
LiveData is also lifecycle-aware. It only updates fragments that are in an active lifecycle state, at the same time, automatically cleans up references after they're no longer needed.
While View holds a reference to the ViewModel, the ViewModel does not, and should not know anything about the View. Just as the consumer knows about the producer, but the producer does not know, nor care about how the information is consumed.
Frequent network, disk and database access is a drain on the UI resources. Applications may stop responding to user inputs when it spends too much time on a query or computing task. For this reason, these heavy works should never be handled by the main thread, instead, they can be moved to a background thread and only send data back when the execution is complete.
The AsyncTask class enables proper and easy use of the UI thread. In the NewsRepostory, for example, an AsyncTask instance is created to save or delete favorite articles. When the main thread is cancelled, the AsyncTask should also be cancelled to prevent memory leak.
The Android Jetpack Navigation component is a collection of libraries, plugin and tooling for simplying workflow around navigation. It handles screen flow, back stack, fragment transactions and safeargs passing all in one place - the navigation graph.
The navigation graph is a xml file that centralizes all information about navigation. In addition to the <fragment> info, their available destinations are specified through the <action> tag, and allowable typesafe args, <argument>.
The Navigation component easily supports the bottom navigation view of defined fragments as seen in this app. Furthermore, the Articleobject is safely passed between SaveFragment and SavedNewsDetailFragment to allow expanding on any news snippet.
- News API: https://newsapi.org/
- Retrofit: a type-safe HTTP client for Android and Java https://square.github.io/retrofit/
- RoomDB: local database as an abstraction layer over SQLite https://developer.android.com/training/data-storage/room
- SwipePlaceHolderView: an implementation of views in stacked form responsive to swipe gestures https://blog.mindorks.com/android-tinder-swipe-view-example-3eca9b0d4794
- Picasso: image downloading and caching library https://square.github.io/picasso/