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Android Dev Blog

MVVM architecture with the data binding library

Model-View-Presenter (MVP) architecture is widely used in today’s Android apps to separate the view from the presentation logic and the model by introducing a presenter. Model-View-ViewModel (MVVM) is quite similar to MVP, with the view model acting as an enhanced presenter, using a data binder to keep the view model and the view in sync. By binding the view to view model properties, the data binder can handle view updates without the need to manually set changed data back to the view (e.g. no more setText() or setVisibility() on a TextView). As with the presenter in MVP, the view model can easily be unit tested. This article gives an introduction to both the data binding library and the MVVM architectural pattern and how they work together on Android.
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Using Retrofit with Realm and Parceler

Retrofit is a library, which is useful in most apps one can think of. If the app needs a backend, you can – and should – use Retrofit for RESTful services. It’s remarkably easy to setup – you’ll have networking up and running in no time, automatically getting the responses parsed into your model objects.

Often you also want to persist some of the data that is fetched from the backend. Realm is very popular, because you can work with “normal” objects, instead of having to use SQLite (even though many ORM libs exist). Also, RealmObjects are live objects, so you always get access to the newest data.

Another problem which often arises is that you may want to save only some of the fetched objects in your database, but you still need to persist the rest when the Activity gets killed by using onSaveInstanceState() (e.g. a list with locally cached bookmarked/favorited items). Parceler provides an easy solution for this problem, since it makes your objects Parcelable via an annotation processor.

Together these three libraries make working with data from a backend quick and easy. This blog post guides you through the setup process.

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Creating a watch face with a preview phone app

When creating a watch face for Android Wear devices, you may want to include a phone app for configuration. A preview of the watch face in the configuration app provides immediate feedback of the changes and makes it more comfortable for users to change settings.

To achieve this, you have to put the drawing code in a separate class, which can be reused in the phone app and the watch face service. First, create a separate Android Library module, where you put the common classes and resources for drawing the watch face. In this module, create an interface IWatchFaceConfig, which exposes your configuration options.

public interface IWatchFaceConfig {
    Calendar getCalendar();
    boolean isAmbient();
    boolean isLowBitAmbient();
    boolean isRound();

    // put your watch face options here
    boolean isLightTheme();
}

In your phone app and watch face service, you can now either choose to directly implement IWatchFaceConfig, or you can make a WatchFaceConfig class, if you prefer to have a separate configuration object. When you directly implement the interface, you have to be careful to avoid memory leaks (for example if your Activity implements the interface).

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Keeping your dependencies up to date

In your Android projects, you likely have many dependencies in your Gradle files. Updates often provide bugfixes or new features that you want to include in your app. However, checking for updates is not integrated by default. There is a solution though. Ben Manes wrote a Gradle Versions Plugin that does the work for you. To get started, add the buildscript dependency to your project’s gradle.build file and apply the plugin:

apply plugin: 'com.github.ben-manes.versions'

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:1.3.1'
        classpath 'com.github.ben-manes:gradle-versions-plugin:0.11.3'
    }
}

Simple as that. Now you have a new gradle task dependencyUpdates to check for updated dependencies in your project.

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Image Loading with Picasso

A picture is worth a thousand words. That’s also true for many apps out there. To make image loading easy and hassle-free, using a library is recommended. My personal favorite is Picasso by Square. From showing a placeholder while downloading pictures to resizing and caching images, everything is handled by the library. To use it, add a dependency to your module’s gradle.build file:

   compile 'com.squareup.picasso:picasso:2.5.2'

That’s it. You’re ready to go. Let’s load a picture into an ImageView:

Picasso.with(context.getApplicationContext())
  .load(url)
  .placeholder(R.drawable.img_placeholder)
  .error(R.drawable.img_error)
  .into(imageView);

With this call, a placeholder drawable is displayed while the image is downloaded. Afterwards the picture is faded in. If an error occurs while downloading, the error drawable is shown. This is the base use case of Picasso. But the library has many advanced features. With .fit(), Picasso automatically resizes the image to the dimensions of the ImageView, which is more memory efficient. Also, you can use .resize(width, height) to resize the picture manually – here you can also use .onlyScaleDown(). When resizing, .centerCrop() and .centerInside() are also provided.

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Design Support Library: NavigationView

The Android Design Support Library makes life a lot easier for developers wishing to comply to the Material Design Guidelines with their app. To use it, simply add a dependency to your module’s gradle.build file:

 compile 'com.android.support:design:23.0.1'

Before releasing this library, in many cases developers needed to write the code for implementing material design in their apps themselves or they had to rely on community-created libraries – the navigation drawer being one of these components. Now, the Design Support Library provides a NavigationView, which makes adding a pre-styled drawer straight forward.

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