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Trouble shoot your device. Check upgrade eligibility. To use Room, we need to define our local schema. First, we add the Entity annotation to our User data model class and a PrimaryKey annotation to the class's id field.
These annotations mark User as a table in our database and id as the table's primary key:. Then, we create a database class by implementing RoomDatabase for our app:. Notice that UserDatabase is abstract. Room automatically provides an implementation of it.
For details, see the Room documentation. We now need a way to insert user data into the database. For this task, we create a data access object DAO. Using Flow with Room allows you to get live updates. This means that every time there's a change in the user table, a new User will be emitted.
Now we can modify our UserRepository to incorporate the Room data source:. Notice that even though you changed where the data comes from in UserRepository , you didn't need to change UserProfileFragment. This small-scoped update demonstrates the flexibility that this app architecture provides.
It's also great for testing, because you can provide a mock instance of UserRepository and test your production UserProfileViewModel at the same time. If users wait a few days before returning to an app that uses this architecture, it's likely that they'll see out-of-date information until the repository can fetch updated information.
Depending on your use case, you may not want to show this out-of-date information. Instead, you can display placeholder data, which shows example values and indicates that your app is currently fetching and loading up-to-date information. For example, if our backend has another endpoint that returns a list of friends, the same user object could come from two different API endpoints, maybe even using different levels of granularity.
If the UserRepository were to return the response from the Webservice request as-is, without checking for consistency, our UIs could show confusing information because the version and format of data from the repository would depend on the endpoint most recently called. For this reason, our UserRepository implementation saves web service responses into the database.
Changes to the database then trigger callbacks on active LiveData objects. Using this model, the database serves as the single source of truth , and other parts of the app access it using our UserRepository. Regardless of whether you use a disk cache, we recommend that your repository designate a data source as the single source of truth for the rest of your app.
In some use cases, such as pull-to-refresh, it's important for the UI to show the user that there's currently a network operation in progress. It's good practice to separate the UI action from the actual data because the data might be updated for various reasons. From the UI's perspective, the fact that there's a request in flight is just another data point, similar to any other piece of data in the User object itself.
We can use one of the following strategies to display a consistent data-updating status in the UI, regardless of where the request to update the data came from:. In the separation of concerns section, we mentioned that one key benefit of following this principle is testability.
Because these instrumentation tests don't require any UI components, they run quickly. For each test, create an in-memory database to ensure that the test doesn't have any side effects, such as changing the database files on disk. This approach isn't recommended, however, because the SQLite version running on the device might differ from the SQLite version on your development machine. Webservice : In these tests, avoid making network calls to your backend. It's important for all tests, especially web-based ones, to be independent from the outside world.
Several libraries, including MockWebServer , can help you create a fake local server for these tests. Testing Artifacts : Architecture Components provides a maven artifact to control its background threads. The androidx.
Programming is a creative field, and building Android apps isn't an exception. There are many ways to solve a problem, be it communicating data between multiple activities or fragments, retrieving remote data and persisting it locally for offline mode, or any number of other common scenarios that nontrivial apps encounter.
Although the following recommendations aren't mandatory, it has been our experience that following them makes your code base more robust, testable, and maintainable in the long run:. Avoid designating your app's entry points—such as activities, services, and broadcast receivers—as sources of data.
Instead, they should only coordinate with other components to retrieve the subset of data that is relevant to that entry point.
Each app component is rather short-lived, depending on the user's interaction with their device and the overall current health of the system. Create well-defined boundaries of responsibility between various modules of your app. For example, don't spread the code that loads data from the network across multiple classes or packages in your code base. Similarly, don't define multiple unrelated responsibilities—such as data caching and data binding—into the same class. Don't be tempted to create "just that one" shortcut that exposes an internal implementation detail from one module.
You might gain a bit of time in the short term, but you then incur technical debt many times over as your codebase evolves. For example, having a well-defined API for fetching data from the network makes it easier to test the module that persists that data in a local database. If, instead, you mix the logic from these two modules in one place, or distribute your networking code across your entire code base, it becomes much more difficult—if not impossible—to test.
Don't reinvent the wheel by writing the same boilerplate code again and again. Instead, focus your time and energy on what makes your app unique, and let the Android Architecture Components and other recommended libraries handle the repetitive boilerplate. That way, users can enjoy your app's functionality even when their device is in offline mode.
Remember that not all of your users enjoy constant, high-speed connectivity. Whenever your app needs to access this piece of data, it should always originate from this single source of truth. In the recommended app architecture section above, we omitted network error and loading states to keep the code snippets simple.
This section demonstrates how to expose network status using a Resource class that encapsulate both the data and its state. The following code snippet provides a sample implementation of Resource :. Because it's common to load data from the network while showing the disk copy of that data, it's good to create a helper class that you can reuse in multiple places.
For this example, we create a class called NetworkBoundResource. The following diagram shows the decision tree for NetworkBoundResource :. It starts by observing the database for the resource.
When the entry is loaded from the database for the first time, NetworkBoundResource checks whether the result is good enough to be dispatched or that it should be re-fetched from the network. Note that both of these situations can happen at the same time, given that you probably want to show cached data while updating it from the network. If the network call completes successfully, it saves the response into the database and re-initializes the stream.
If network request fails, the NetworkBoundResource dispatches a failure directly. Note: After saving new data to disk, we re-initialize the stream from the database. We usually don't need to do that, however, because the database itself happens to dispatch the change.
Keep in mind that relying on the database to dispatch the change involves relying on the associated side effects, which isn't good because undefined behavior from these side effects could occur if the database ends up not dispatching changes because the data hasn't changed. Also, don't dispatch the result that arrived from the network because that would violate the single source of truth principle. After all, maybe the database includes triggers that change data values during a "save" operation.
The full implementation of the NetworkBoundResource class appears as part of the android-architecture-components GitHub project. After creating the NetworkBoundResource , we can use it to write our disk- and network-bound implementations of User in the UserRepository class:. Content and code samples on this page are subject to the licenses described in the Content License.
App Basics. Build your first app. App resources. Resource types. App manifest file. Device compatibility. Multiple APK support. Tablets, large screens, and foldables. Build responsive UIs. Build for foldables. Getting started. Handling data. User input. Watch Face Studio. Health services. Creating watch faces. Android TV. Build TV Apps.
Build TV playback apps. Help users find content on TV. Recommend TV content. Watch Next. Build TV games. Build TV input services. TV Accessibility. Android for Cars. Build media apps for cars. Build navigation, parking, and charging apps for cars. Android Things. Supported hardware. Advanced setup. Build apps. Create a Things app. Communicate with wireless devices. Configure devices. Interact with peripherals. Build user-space drivers. Manage devices. Create a build. Push an update.
Chrome OS devices. App architecture. Architecture Components. UI layer libraries. View binding. Data binding library. Lifecycle-aware components. Paging Library. Paging 2. Data layer libraries. How-To Guides. Advanced Concepts. Threading in WorkManager. App entry points. App shortcuts. App navigation. Navigation component. App links. Make it always about the problem, not your solution.
Host a contest. Tell your audience to complete a certain task and tag themselves with contest-specific hashtags on social media in order to follow the development. Reward the winners publically. Post a job. I am not proud of this one, but it has always served its purpose. People will discover your app and your company as they are looking for their next mission in life. Create a video channel on YouTube or Vimeo. Always add your main hashtag and descriptive hashtags to maximize exposure on social media.
Hire a PR firm. PR work can be pricey but very effective. Remember the Mailbox app and when it launched? It had half a million waitlist subscribers before it even came out! All the result of good PR. Book advertising space via www. Go offline and try guerrilla marketing. Remember that these campaigns are most effective in areas with a high concentration of your target customer.
The cornerstones of a guerrilla marketing campaign are: 1. The clarification must be the genius part of your campaign! Try AdWords, too. But only do so if you have your keywords down, and if you have some time each day to adjust your campaigns yes, multiple as needed. Communicate with your circle. Let your friends, family, colleagues, and classmates know what you are doing.
An occasional update will be enough, since friends and family are a lot less engaged in your activities than you might believe.
Offer discounts. If you are working with in-app purchases, offer them at cheaper prices on certain days, and let your users know via push notifications or your email newsletter. Grow your subscriber list. Talking about that newsletter, keep collecting email addresses and grow your list.
Give love to your critics. Focus on those negative reviews, and reach out to users and help them solve their problem with your app. You can use Apptentive. Link in your business cards. Add a short code or a scanable code to your business cards so that people can download your app faster.
Find a partner. Can you team up with another app? Then do so! These types of contests are hosted by several app discovery services. Give out swag. Think outside the box and deliver something that is relevant to your target audience.
We distributed them in co-working spaces around Miami. Encourage customer ratings. Ask your happy users to leave a rating on the app store —but keep it real!
Customers can smell fake reviews from a mile away these days. Put your app in the right spot. Choose your app keywords and your app categories very carefully.
They will determine your ranking in those areas. If applicable, create an invite system. Users who have the option between paying or inviting their friends are more likely to send out the invites. Remember Candy Crush? Monitor your performance. Keep checking your analytics via AppAnnie or Flurry to understand how your users interact with your app.
Draw new conclusions, prepare an action plan, execute, rinse, repeat! Get noticed. You might just have their next favorite app. Become a guest blogger. Contribute to other websites and share your expertise. Consider coupons. The best part is that you can track the outcome very easily through the coupon codes. Re-engage users who have already installed your app. Google has launched a new initiative where you can run ads to drive users to open your app. Facebook does a similar thing now, too.
Create your own stamps. Customize the postage stamps you use for mail. Answer questions.
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