Android Bytes is a simple app that showcases using Room and a Repository to create an offline cache. It fetches Android Dev Byte videos' information from the server to display and provides an intent that launches the corresponding video on Youtube.

In addition, it also uses WorkManager for scheduling a daily background data sync between local database and server.

Caching is copying data and storing it closer to the place it will be used for faster access in future uses. For example, browser caches every page web page visited on local file system, so that it is loaded faster the next time the page is visited.

We store each on the file system. Every Android app has access to a folder it can use to store files and data.

Cache invalidation means knowing when the cached data is no longer correct. For example, when the data on the server is itself changed, the cached data is of no use. Network requests over HTTP have features to deal with this and we can enable network caching using Retrofit. We can also create offline cache using SQL database.

Repository pattern is common pattern used to provide unified view of the data from several sources. For example, data coming from network and database are combined in the repository. The users of the repository don't have to care where the data comes from. Repository only provides APIs to the data sources.

Note: Repository is also responsible for managing data in cache.

Background works like uploading logs, process data, upload metrics or pre-fetch content may happen when app is in background or even not running. WorkManager is a Jetpack library that helps to perform tasks when the app is in background or even not running. It can also be used to schedule background work to be done periodically. We don't tell WorkManager how to do work, instead under what conditions to do the work. Constraints can be when the device is charging, device is idle or is connected to wifi.

We schedule the background work in the Application Class. It is the main object that the operating system uses to interact with the app. Application's onCreate() runs every time the app is launched and it has to run before any screen is shown. It lot of work is done in onCreate() it slows the app's loading time. We use Coroutines to move initialisation and schedule background work out of onCreate().

WorkManager schedules work with a work request. Work request can be periodic or one-time. To schedule the work we use enqueue method on WorkManager. Works are differentiated by WorkName, and we use ExistingPeriodicWorkPolicy to resolve when two works with same name are enqueued. We can keep the previous work and discard the newly arrived work request or replace the previous request with new one.